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Research involving Lowell Observatory staff 2018
(Articles and chapters)

This is a work ever in progress.

(Pulled from ADS* by sel on 2023-05-29)

*We are grateful for all the effort that went into making The SAO/NASA Astrophysics Data System (ADS) possible. The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Agreement NNX16AC86A and can be found at: https://ui.adsabs.harvard.edu/

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Years: 2018 Bottom


  1. Trilling, D., Mommert, M., Hora, J., Farnocchia, D., Chodas, P., Giorgini, J., Smith, H., Carey, S., Lisse, C., Werner, M., McNeill, A., Chesley, S., Emery, J., Fazio, G., Fernandez, Y., Harris, A., Marengo, M., Mueller, M., Roegge, A., Smith, N., Weaver, H., Meech, K., Micheli, M., 2018, AJ, 156, 261, Spitzer Observations of Interstellar Object 1I/Oumuamua
    1I/Oumuamua is the first confirmed interstellar body in our solar system. Here we report on observations of Oumuamua made with the Spitzer Space Telescope on 2017 November 21-22 (UT). We integrated for 30.2 hr at 4.5 m (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 Jy (3). This implies an effective spherical diameter less than [98, 140, 440] m and albedo greater than [0.2, 0.1, 0.01] under the assumption of low, middle, or high thermal beaming parameter , respectively. With an aspect ratio for Oumuamua of 6:1, these results correspond to dimensions of [240:40, 341:57, 1080:180] m, respectively. We place upper limits on the amount of dust, CO, and CO2 coming from this object that are lower than previous results; we are unable to constrain the production of other gas species. Both our size and outgassing limits are important because Oumuamuas trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emission. We suggest that Oumuamua may have experienced low-level post-perihelion volatile emission that produced a fresh, bright, icy mantle. This model is consistent with the expected value and implied high-albedo value for this solution, but, given our strict limits on CO and CO2, requires another gas speciesprobably H2Oto explain the observed non-gravitational acceleration. Our results extend the mystery of Oumuamuas origin and evolution.
  2. Cauley, P., Shkolnik, E., Llama, J., Bourrier, V., Moutou, C., 2018, AJ, 156, 262, Evidence of Magnetic Star-Planet Interactions in the HD 189733 System from Orbitally Phased Ca II K Variations
    Magnetic star-planet interactions (SPI) provide a detection method and insight into exoplanet magnetic fields and, in turn, exoplanet interiors and atmospheric environments. These signatures can be sporadic and difficult to confirm for single-epoch observations of a system due to inhomogeneous stellar magnetospheres and periodic variability in stellar magnetism. Thus, an ideal SPI search consists of multiple epochs containing observations on consecutive nights spanning at least one complete planetary orbit. Such data sets are rare but do exist for some of the most intensely studied hot Jupiter systems. One such system is HD 189733 for which six suitable SPI data sets exist, the result of spectroscopic monitoring to perform some of the first SPI searches and also to study the stars magnetic field. Here we perform a uniform analysis of six archival Ca II K data sets for HD 189733, spanning 2006 June through 2015 July, in order to search for magnetic SPI signatures in the chromospheric line variations. We find significant evidence for modulations of Ca II K with a 2.29 0.04 day period in the 2013 August data, which is consistent with the planets orbital period. The peak in the orbital variations occurs at orb 0.9, which corresponds to the SPI emission leading the planet with a phase difference of 40 from the sub-planetary point. This is consistent with the phase-lead predictions of nonlinear force-free magnetic field SPI models. The stellar magnetic field strength at the planets orbit was greatest in 2013 August, which, due to the energy released in magnetic SPI scaling with B *, lends strength to the SPI interpretation.
  3. Massey, P., Levine, S., Neugent, K., Levesque, E., Morrell, N., Skiff, B., 2018, AJ, 156, 265, A Runaway Giant in the Galactic Halo
    New evidence provided by the Gaia satellite places the location of the runaway star J01020100-7122208 in the halo of the Milky Way (MW) rather than in the Small Magellanic Cloud (SMC) as previously thought. We conduct a reanalysis of the stars physical and kinematic properties, which indicates that the star may be an even more extraordinary find than previously reported. The star is a 180 Myr old 3-4 M G5-8 bright giant, with an effective temperature of 4800 100 K, a metallicity of Fe/H = -0.5, and a luminosity of {log}L/{L} =2.70+/- 0.20. A comparison with evolutionary tracks identifies the star as being in a giant or early asymptotic giant branch stage. The proper motion, combined with the previously known radial velocity, yields a total Galactocentric space velocity of 296 km s-1. The star is currently located 6.4 kpc below the plane of the MW, but our analysis of its orbit shows it passed through the disk 25 Myr ago. The stars metallicity and age argue against it being native to the halo, and we suggest that the star was likely ejected from the disk. We discuss several ejection mechanisms, and conclude that the most likely scenario is ejection by the MWs central black hole based upon our analysis of the stars orbit. The identification of the large radial velocity of J01020100-7122208 came about as a happenstance of it being seen in projection with the SMC, and we suggest that many similar objects may be revealed in Gaia data.

    This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  4. Aadland, E., Massey, P., Neugent, K., Drout, M., 2018, AJ, 156, 294, Shedding Light on the Isolation of Luminous Blue Variables
    In the standard view of massive star evolution, luminous blue variables (LBVs) are transitional objects between the most massive O-type stars and Wolf-Rayet (WR) stars. With short lifetimes, these stars should all be found near one another. A recent study of LBVs in the Large Magellanic Cloud (LMC) found instead that LBVs are considerably more isolated than either O-type stars or WRs, with a distribution intermediate between that of the WRs and red supergiants (RSGs). A similar study, using a more restricted sample of LBVs, reached the opposite conclusion. Both studies relied upon the distance to the nearest spectroscopically identified O-type star to define the degree of isolation. However, our knowledge of the spectroscopic content of the LMC is quite spotty. Here we re-examine the issue using carefully defined photometric criteria to select the highest-mass unevolved stars (bright blue stars, or BBSs), using spatially complete photometric catalogs of the LMC, M31, and M33. Our study finds that the LBVs are no more isolated than BBSs or WRs. This result holds no matter which sample of LBVs we employ. A statistical test shows that we can rule out the LBVs having the same distribution as the RSGs, which are about 2 more isolated. We demonstrate the robustness of our results using the second-closest neighbor. Furthermore, the majority of LBVs in the LMC are found in or near OB associations as are the BBS and WRs; the RSGs are not. We conclude that the spatial distribution of LBVs therefore is consistent with the standard picture of massive star evolution.
  5. Monnier, J., Kraus, S., Ireland, M., Baron, F., Bayo, A., Berger, J., Creech-Eakman, M., Dong, R., Duchene, G., Espaillat, C., Haniff, C., Honig, S., Isella, A., Juhasz, A., Labadie, L., Lacour, S., Leifer, S., Merand, A., Michael, E., Minardi, S., Mordasini, C., Mozurkewich, D., Olofsson, J., Paladini, C., Petrov, R., Pott, J., Ridgway, S., Rinehart, S., Stassun, K., Surdej, J., Brummelaar, T., Turner, N., Tuthill, P., Vahala, K., van Belle, G., Vasisht, G., Wishnow, E., Young, J., Zhu, Z., 2018, ExA, 46, 517, The planet formation imager
    The Planet Formation Imager (PFI, www.planetformationimager.org) is a next-generation infrared interferometer array with the primary goal of imaging the active phases of planet formation in nearby star forming regions. PFI will be sensitive to warm dust emission using mid-infrared capabilities made possible by precise fringe tracking in the near-infrared. An L/M band combiner will be especially sensitive to thermal emission from young exoplanets (and their disks) with a high spectral resolution mode to probe the kinematics of CO and H2O gas. In this paper, we give an overview of the main science goals of PFI, define a baseline PFI architecture that can achieve those goals, point at remaining technical challenges, and suggest activities today that will help make the Planet Formation Imager facility a reality.
  6. Leggett, S., Bergeron, P., Subasavage, J., Dahn, C., Harris, H., Munn, J., Ables, H., Canzian, B., Guetter, H., Henden, A., Levine, S., Luginbuhl, C., Monet, A., Monet, D., Pier, J., Stone, R., Vrba, F., Walker, R., Tilleman, T., Xu, S., Dufour, P., 2018, ApJS, 239, 26, Distant White Dwarfs in the US Naval Observatory Flagstaff Station Parallax Sample
    This paper presents new trigonometric parallaxes and proper motions for 214 stars. The measurements were made at the US Naval Observatory Flagstaff Station between 1989 and 2017, and the average uncertainty in the parallax values is 0.6 mas. We find good agreement with Gaia Data Release 2 measurements for the stars in common, although there may be a small systematic offset similar to what has been found by other investigators. The sample is matched to catalogs and the literature to create a photometric data set that spans the ultraviolet to the mid-infrared. New mid-infrared photometry is obtained for 19 stars from archived Spitzer mosaics. New optical spectroscopy is presented for seven systems and additional spectra were obtained from the literature. We identify a subsample of 179 white dwarfs (WDs) at distances of 25-200 pc. Their spectral energy distributions (SEDs) are analyzed using model atmospheres. The models reproduce the entire flux-calibrated SED very well and provide the atmospheric chemical composition, temperature, surface gravity, mass, and cooling age of each WD. Twenty-six WDs are newly classified, and 12 systems are presented as candidate unresolved binaries. We confirm one WD+red dwarf system and identify two WDs as candidate dust disk systems. Twelve old and high-velocity systems are identified as candidate thick disk or halo objects. The WDs in the sample generally have Galactic disk-like ages of <8 Gyr and masses close to the canonical 0.6 M .
  7. Jenniskens, P., Albers, J., Tillier, C., Edgington, S., Longenbaugh, R., Goodman, S., Rudlosky, S., Hildebrand, A., Hanton, L., Ciceri, F., Nowell, R., Lyytinen, E., Hladiuk, D., Free, D., Moskovitz, N., Bright, L., Johnston, C., Stern, E., 2018, M&PS, 53, 2445, Detection of meteoroid impacts by the Geostationary Lightning Mapper on the GOES-16 satellite
    Bolides are detected by the Geostationary Lightning Mapper onboard the GOES-16 weather satellite, which takes images of Earth at a rate of 500 Hz in a 1.1 nm wide pass band centered on 777.4 nm wavelength. Ten case studies are discussed. These initial results were obtained using the Level 0 data received during the nonoperational in-orbit postlaunch test period. GLM positions and timings are sufficiently accurate to assist in trajectory and orbit reconstruction. GLM samples the light curve nearly completely, unaffected by onboard and downlink processes tailored to lightning data. Sufficient data on the instantaneous background scene are provided to reconstruct the baseline drift in the brightest pixels. The agreement to within a factor of 2-3 between measured total radiated energy from GLM and that derived from other space-borne observations implies that during the bolide's peak brightness the GLM pass band is dominated by continuum emission, rather than O I line emission. The reported flux is corrected for angle-from-nadir shifts in the central wavelength of the pass band, which overestimates continuum flux by only up to 20% for most of the GLM field of view, but more so if the bolide is observed far from nadir. Assuming a 6000 K blackbody spectrum, GLM is able to detect bolides with peak visual magnitude (at a normalized 100 km distance) brighter than about -14 in nighttime, and slightly brighter in daytime.
  8. Ahrens, C., Grundy, W., Mandt, K., Cooper, P., Umurhan, O., Chevrier, V., 2018, SSRv, 214, 130, Recent Advancements and Motivations of Simulated Pluto Experiments
    This review of Pluto laboratory research presents some of the recent advancements and motivations in our understanding enabled by experimental simulations, the need for experiments to facilitate models, and predictions for future laboratory work. The spacecraft New Horizons at Pluto has given a large amount of scientific data already rising to preliminary results, spanning from the geology to the atmosphere. Different ice mixtures have now been detected, with the main components being nitrogen, methane, and carbon monoxide. Varying geology and atmospheric hazes, however, gives us several questions that need to be addressed to further our understanding. Our review summarizes the complexity of Pluto, the motivations and importance of laboratory simulations critical to understanding the low temperature and pressure environments of icy bodies such as Pluto, and the variability of instrumentation, challenges for research, and how simulations and modeling are complimentary.
  9. Thirouin, A., Moskovitz, N., Binzel, R., Christensen, E., DeMeo, F., Person, M., Polishook, D., Thomas, C., Trilling, D., Willman, M., Burt, B., Hinkle, M., Pugh, T., 2018, ApJS, 239, 4, The Mission Accessible Near-Earth Objects Survey: Four Years of Photometry
    Over 4.5 years, the Mission Accessible Near-Earth Object Survey assembled 228 near-Earth object (NEO) light curves. We report rotational light curves for 82 NEOs, constraints on amplitudes and periods for 21 NEOs, light curves with no detected variability within the image signal-to-noise and length of our observing block for 30 NEOs, and 10 tumblers. We uncovered two ultra-rapid rotators with periods below 20 s,2016 MA with a potential rotational periodicity of 18.4 s, and 2017 QG18 rotating in 11.9 sand estimated the fraction of fast/ultra-rapid rotators undetected in our project plus the percentage of NEOs with a moderate/long periodicity undetectable during our typical observing blocks. We summarize the findings of a simple model of synthetic NEOs to infer the objects morphology distribution using the measured distribution of light curve amplitudes. This model suggests that a uniform distribution of axis ratio can reproduce the observed sample. This suggests that the quantity of spherical NEOs (e.g., Bennu) is almost equivalent to the quantity of highly elongated objects (e.g., Itokawa), a result that can be directly tested thanks to shape models from Doppler delay radar imaging analysis. Finally, we fully characterized two NEOs2013 YS2 and 2014 FA7as appropriate targets for a potential robotic/human mission due to their moderate spin periods and low v.
  10. Hsieh, H., Ishiguro, M., Kim, Y., Knight, M., Lin, Z., Micheli, M., Moskovitz, N., Sheppard, S., Thirouin, A., Trujillo, C., 2018, AJ, 156, 223, The 2016 Reactivations of the Main-belt Comets 238P/Read and 288P/(300163) 2006 VW139
    We report observations of the reactivations of the main-belt comets (MBCs) 238P/Read and 288P/(300163) 2006 VW139 that also track the evolution of each objects activity over several months in 2016 and 2017. We additionally identify and analyze archival SDSS data showing 288P to have been active in 2000, meaning that both 238P and 288P have now each been confirmed to be active near perihelion on three separate occasions. From data obtained of 288P from 2012-2015 when it appeared inactive, we find best-fit R-band H, G phase function parameters of H R = 16.80 0.12 mag and G R = 0.18 0.11, corresponding to effective component radii of r c = 0.80 0.04 km, assuming a binary system with equally sized components. Fitting linear functions to ejected dust masses inferred for 238P and 288P soon after their observed reactivations in 2016, we find an initial average net dust production rate of {\dot{M}}d=0.7+/- 0.3 kg s-1 and a best-fit start date of 2016 March 11 (when the object was at a true anomaly of = -63) for 238P, and an initial average net dust production rate of {\dot{M}}d=5.6+/- 0.7 kg s-1 and a best-fit start date of 2016 August 5 (when the object was at = -27) for 288P. Applying similar analyses to archival data, we find similar start points for previous active episodes for both objects, suggesting that minimal mantle growth or ice recession occurred between the active episodes in question. Some changes in dust production rates between active episodes are detected, however. More detailed dust modeling is suggested to further clarify the process of activity evolution in MBCs.

    Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) (programs 12BH43, 15AT05, and 16BT05), which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de lUnivers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii, and at the Gemini Observatory (programs GN-2011B-Q-17, GN-2012A-Q-68, GN-2012B-Q-106, GN-2016B-LP-11, and GS-2016B-LP-11), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation (NSF) on behalf of the Gemini partnership: the NSF (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnologia e Innovacion Productiva (Argentina), and Ministerio da Ciencia, Tecnologia e Inovacao (Brazil).

  11. Neugent, K., Levesque, E., Massey, P., 2018, AJ, 156, 225, Binary Red Supergiants: A New Method for Detecting B-type Companions
    With the exception of a few well-known and studied systems, the binary population of red supergiants (RSGs) remains relatively uncharacterized. Famous systems such as VV Cep, 31 Cyg, and Aur contain RSG + B star binaries, and here we explore whether B stars are the main type of companion we expect from an evolutionary point of view. Using the Geneva evolutionary models, we find that this is indeed the case. However, few such systems are known, and we use model spectra to determine how easy such binaries would be to detect observationally. We find that it should be quite difficult to hide a B-type companion given a reasonable signal-to-noise in the optical/blue portion of the spectrum. We next examine spectra of Magellanic Cloud RSGs and newly acquired spectra of Galactic RSGs looking for new systems and refining our conclusions about what types of stars could be hidden in the spectra. Finally, we develop a set of photometric criteria that can help select likely binaries in the future without the overhead of large periodic or spectroscopic surveys.
  12. Docobo, J., Campo, P., Gomez, J., Horch, E., 2018, AJ, 156, 185, The Three-dimensional Orbit, Orbital Parallax, and Individual Masses of the Double-lined Spectroscopic Binaries HD 183255, HD 114882, and HD 30712
    Recent speckle observations performed at the Discovery Channel Telescope, the Gemini North Telescope, and the Special Astrophysical Observatory 6 m Telescope have permitted us to calculate the visual orbit of SB2 HD 114882 for the first time and to improve the visual orbits of two other SB2 systems, HD 30712 and HD 183255, using algorithms published by the authors of this research. Recently, new high-quality spectroscopic orbits have been obtained for these binaries by other authors. We determine their 3D orbits, individual masses, and orbital parallaxes, and present them in this paper. The parallaxes are compared with those available from the Gaia mission, and a comparison between the values confirms the precision of the results obtained here.
  13. Cauley, P., Kuckein, C., Redfield, S., Shkolnik, E., Denker, C., Llama, J., Verma, M., 2018, AJ, 156, 189, The Effects of Stellar Activity on Optical High-resolution Exoplanet Transmission Spectra
    Chromospherically sensitive atomic lines display different spectra in stellar active regions, spots, and the photosphere, raising the possibility that exoplanet transmission spectra are contaminated by the contrast between various portions of the stellar disk. To explore this effect, we performed transit simulations of G-type and K-type stars for the spectral lines Ca II K at 3933 A, Na I 5890 A, H I 6563 A (H), and He I 10830 A. We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for H, Ca II K, and Na I D, while spots and filaments are comparatively unimportant. The amount of contamination depends strongly on the location of the active regions and the intrinsic emission strength. In particular, active regions must be concentrated along the transit chord in order to produce a consistent in-transit signal. Mean absorption signatures in Na I and H, for example, can reach 0.2% and 0.3%, respectively, for transits of active latitudes with line emission similar in strength to moderate solar flares. Transmission spectra of planets transiting active stars, such as HD 189733, are likely contaminated by the contrast effect, although the tight constraints on active region geometry and emission strength make it unlikely that consistent in-transit signatures are due entirely to the contrast effect. He I 10830 A is not strongly affected and absorption signatures are likely diluted, rather than enhanced, by stellar activity. He I 10830 A should thus be considered a priority for probing extended atmospheres, even in the case of active stars.
  14. Cook, J., Ore, C., Protopapa, S., Binzel, R., Cartwright, R., Cruikshank, D., Earle, A., Grundy, W., Ennico, K., Howett, C., Jennings, D., Lunsford, A., Olkin, C., Parker, A., Philippe, S., Reuter, D., Schmitt, B., Stansberry, J., Alan Stern, S., Verbiscer, A., Weaver, H., Young, L., 2018, Icar, 315, 30, Composition of Pluto's small satellites: Analysis of New Horizons spectral images
    On July 14, 2015, NASA's New Horizons spacecraft encountered the Pluto-system. Using the near-infrared spectral imager, New Horizons obtained the first spectra of Nix, Hydra, and Kerberos and detected the 1.5 and 2.0 m bands of H2O-ice on all three satellites. On Nix and Hydra, New Horizons also detected bands at 1.65 and 2.21 m that indicate crystalline H2O-ice and an ammoniated species, respectively. A similar band linked to NH3-hydrate has been detected on Charon previously. However, we do not detect the 1.99 m band of NH3-hydrate. We consider NH4Cl (ammonium chloride), NH4NO3 (ammonium nitrate) and (NH4)2CO3 (ammonium carbonate) as potential candidates, but lack sufficient laboratory measurements of these and other ammoniated species to make a definitive conclusion. We use the observations of Nix and Hydra to estimate the surface temperature and crystalline H2O-ice fraction. We find surface temperatures < 20 K ( <70 K with 1- error) and 23 K ( < 150 K with 1- error) for Nix and Hydra, respectively. We find crystalline H2O-ice fractions of 78-22+12 % and > 30% for Nix an Hydra, respectively. New Horizons observed Nix and Hydra twice, about 2-3 hours apart, or 5 and 25% of their respective rotation periods. We find no evidence for rotational differences in the disk-averaged spectra between the two observations of Nix or Hydra. We perform a pixel-by-pixel analysis of Nix's disk-resolved spectra and find that the surface is consistent with a uniform crystalline H2O-ice fraction, and a 50% variation in the normalized band area of the 2.21 m band with a minimum associated with the red blotch seen in color images of Nix. Finally, we find evidence for bands on Nix and Hydra at 2.42 and possibly 2.45 m, which we cannot identify, and, if real, do not appear to be associated with the ammoniated species. We do not detect other ices, such as CO2, CH3OH and HCN.
  15. Schenk, P., Beyer, R., McKinnon, W., Moore, J., Spencer, J., White, O., Singer, K., Umurhan, O., Nimmo, F., Lauer, T., Grundy, W., Robbins, S., Stern, S., Weaver, H., Young, L., Smith, K., Olkin, C., New Horizons Geology, Geophysics Investigation Team, 2018, Icar, 315, 124, Breaking up is hard to do: Global cartography and topography of Pluto's mid-sized icy Moon Charon from New Horizons
    The 2015 New Horizons flyby through the Pluto system produced the first high-resolution topographic maps of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of the illuminated surface of Pluto's large icy moon Charon have been produced using both framing camera and line scan camera data (including four-color images at up to 1.47 km pixel scales), showing the best resolution data at all areas of the surface. Digital elevation models (DEMs) with vertical precisions of up to 0.1 km were constructed for 40% of Charon using stereo imagery. Local radii estimates for the surface were also determined from the cartographic control network solution for the LORRI framing camera data, which validate the stereo solutions. Charon is moderately cratered, the largest of which is 250-km across and 6 km deep. Charon has a topographic range over the observed hemisphere from lowest to highest of 19 km, the largest topographic amplitude of any mid-sized icy body (including Ceres) other than Iapetus. Unlike Saturn's icy moons whose topographic signature is dominated by global relaxation of topography and subsequent impact cratering, large-scale tectonics and regional resurfacing dominate Charon's topography. Most of Charon's encounter hemisphere north of the equator (Oz Terra) is broken into large polygonal blocks by a network of wide troughs with typically 3-6 km relief; the deepest of these occur near the illuminated pole and are up to 13 km deep with respect to the global mean radius, the deepest known surfaces on Charon. The edge of this terrain is defined by large tilted blocks sloping 5 or so, the crests of which rise to 5 or 6 km above Charon mean, the highest known points on Charon. The southern resurfaced plains, Vulcan Planitia, consist of rolling plains, locally fractured and pitted, that are depressed 1 km below the mean elevation of the disrupted northern terrains of Oz Terra that comprise much of the northern hemisphere (but 2-2.5 km below the surfaces of the blocks themselves). These plains roll downward gently to the south with a topographic range of 5 km. The outer margins of Vulcan Planitia along the boundary with Oz Terra form a 2-3-km-deep trough, suggesting viscous flow along the outer margins. Isolated massifs 2-4 km high, also flanked by annular moats, lie within the planitia itself. The plains may be formed from volcanic resurfacing of cryogenic fluids, but the tilted blocks along the outer margins and the isolated and tilted massifs within Vulcan Planitia also suggest that much of Charon has been broken into large blocks, some of which have been rotated and some of which have foundered into Charon's upper "mantle", now exposed as Vulcan Planitia, a history that may be most similar to the disrupted terrains of Ariel.
  16. Earle, A., Grundy, W., Howett, C., Olkin, C., Parker, A., Scipioni, F., Binzel, R., Beyer, R., Cook, J., Cruikshank, D., Dalle Ore, C., Ennico, K., Protopapa, S., Reuter, D., Schenk, P., Schmitt, B., Stern, S., Weaver, H., Young, L., New Horizons Surface Composition Theme Team, 2018, Icar, 314, 195, Methane distribution on Pluto as mapped by the New Horizons Ralph/MVIC instrument
    The data returned from NASA's New Horizons spacecraft have given us an unprecedented, detailed look at the Pluto system. New Horizons' Ralph/MVIC (Multispectral Visible Imaging Camera) is composed of 7 independent CCD arrays on a single substrate. Among these are a red channel (540-700 nm), near-infrared channel (780-975 nm), and narrow band methane channel (860-910 nm). By comparing the relative reflectance of these channels we are able to produce high-resolution methane "equivalent width" (based on the 890 nm absorption band) and spectral slope maps of Pluto's surface. From these maps we can then quantitatively study the relationships between methane distribution, redness, and other parameters like latitude and elevation. We find Pluto's surface to show a great diversity of terrains, particularly in the equatorial region between 30N and 30S latitude. Methane "equivalent width" also shows some dependence on elevation (while spectral slope shows very little).
  17. Grundy, W., Bertrand, T., Binzel, R., Buie, M., Buratti, B., Cheng, A., Cook, J., Cruikshank, D., Devins, S., Dalle Ore, C., Earle, A., Ennico, K., Forget, F., Gao, P., Gladstone, G., Howett, C., Jennings, D., Kammer, J., Lauer, T., Linscott, I., Lisse, C., Lunsford, A., McKinnon, W., Olkin, C., Parker, A., Protopapa, S., Quirico, E., Reuter, D., Schmitt, B., Singer, K., Spencer, J., Stern, S., Strobel, D., Summers, M., Weaver, H., Weigle, G., Wong, M., Young, E., Young, L., Zhang, X., 2018, Icar, 314, 232, Pluto's haze as a surface material
    Pluto's atmospheric haze settles out rapidly compared with geological timescales. It needs to be accounted for as a surface material, distinct from Pluto's icy bedrock and from the volatile ices that migrate via sublimation and condensation on seasonal timescales. This paper explores how a steady supply of atmospheric haze might affect three distinct provinces on Pluto. We pose the question of why they each look so different from one another if the same haze material is settling out onto all of them. Cthulhu is a more ancient region with comparatively little present-day geological activity, where the haze appears to simply accumulate over time. Sputnik Planitia is a very active region where glacial convection, as well as sublimation and condensation rapidly refresh the surface, hiding recently deposited haze from view. Lowell Regio is a region of intermediate age featuring very distinct coloration from the rest of Pluto. Using a simple model haze particle as a colorant, we are not able to match the colors in both Lowell Regio and Cthulhu. To account for their distinct colors, we propose that after arrival at Pluto's surface, haze particles may be less inert than might be supposed from the low surface temperatures. They must either interact with local materials and environments to produce distinct products in different regions, or else the supply of haze must be non-uniform in time and/or location, such that different products are delivered to different places.
  18. Schenk, P., Beyer, R., McKinnon, W., Moore, J., Spencer, J., White, O., Singer, K., Nimmo, F., Thomason, C., Lauer, T., Robbins, S., Umurhan, O., Grundy, W., Stern, S., Weaver, H., Young, L., Smith, K., Olkin, C., New Horizons Geology, Geophysics Investigation Team, 2018, Icar, 314, 400, Basins, fractures and volcanoes: Global cartography and topography of Pluto from New Horizons
    The 2015 New Horizons flyby has produced the first high-resolution maps of morphology and topography of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of Pluto were produced using both LORRI framing camera and MVIC line scan camera data, showing the best resolution data obtained for all areas of the illuminated surface, 78% of the body. A unique feature of the Pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze, allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness. MVIC 4-color data were combined with the panchromatic map to produce full color global maps. Digital elevation models (DEMs) over 42% of Pluto were produced using combinations of MVIC hemispheric scans and LORRI mosaics, from which slopes at scales of 1 km can be determined. Pluto can be divided into regions each with distinct topographic signatures, corresponding with major physiographic terrain types. Large areas of Pluto are comprised of low-relief moderately cratered plains units. Deeply pitted and glaciated plains east of Sputnik Planitia are elevated 0.7 km. The most dominant topographic feature on Pluto is the 1200-by-2000-km wide depression enclosing the bright Sputnik Planitia ice sheet, the surface of which is 2.5-to-3.5 km deep (relative to the rim) and 2 km deep relative to the mean radius. The partial ring of steep-sided massifs, several of which are more than 5 km high, along the western margins of Sputnik Planitia produce some of the locally highest and steepest relief on Pluto, with slopes of 40-50. The second major topographic feature is a complex, eroded, ridge-trough system 300-400 km wide and at least 3200 km long extending north-to-south along the 155 meridian. This enormous structure has several kilometers of relief. It may predate the large impact event forming the basin, though some post-Sputnik Planitia deformation is evident. The large depressed, partially walled plain, Hyecho Palus, lies due southwest of Sputnik Planitia. Near the center of Hyecho Palus lie the circular constructional edifices Wright and Piccard Montes. Wright Mons rises 4.5 km above these plains, with a central depression 4.5 km deep, whereas Piccard Mons, best observed in haze-light, rises 5.5 km above the plains but has a bowl-shaped central depression 5.5 km below the plains for a total relief of up to 11 km, the greatest observed on Pluto. Both of these features are interpreted as constructional (volcanic?) in nature. Additional prominent topographic features include a 2-3 km high and 600 km wide dome centered on the illuminated IAU pole and the amoeboidal plateaus of "bladed" terrains in the equatorial region, which rise 2-5 km above local terrains and are the highest standing geologic units on the encounter hemisphere. The mean elevations in the integrated DEM for the two radio occultation areas are consistent with the 5-6 km difference in elevation as determined independently by the radio experiment, and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area. Local relief of 3-5 km at massifs, troughs and pits supports conclusions that the icy shell of Pluto is relatively rigid. Numerous examples of topographic control of ice or frost deposition occur across Pluto, including the distinct coloration of the polar dome, the elevated terrains of eastern Tombaugh Regio, and along the ridge-trough system, where ridge tops and fossae rims are covered in different ices than at lower elevations. The topographic hypsogram of Pluto's encounter hemisphere is strongly bimodal due to the large Sputnik Planitia depression. Otherwise the topographic signature of Pluto is controlled by deviations from the otherwise dominant low plains, including elevated bladed terrain plateaus and the depressed volcanic province including Wright and Piccard Montes.
  19. Vilenius, E., Stansberry, J., Muller, T., Mueller, M., Kiss, C., Santos-Sanz, P., Mommert, M., Pal, A., Lellouch, E., Ortiz, J., Peixinho, N., Thirouin, A., Lykawka, P., Horner, J., Duffard, R., Fornasier, S., Delsanti, A., 2018, A&A, 618, A136, "TNOs are Cool": A survey of the trans-Neptunian region. XIV. Size/albedo characterization of the Haumea family observed with Herschel and Spitzer
    Context. A group of trans-Neptunian objects (TNOs) are dynamically related to the dwarf planet 136108 Haumea. Ten of them show strong indications of water ice on their surfaces, are assumed to have resulted from a collision, and are accepted as the only known TNO collisional family. Nineteen other dynamically similar objects lack water ice absorptions and are hypothesized to be dynamical interlopers.
    Aims: We have made observations to determine sizes and geometric albedos of six of the accepted Haumea family members and one dynamical interloper. Ten other dynamical interlopers have been measured by previous works. We compare the individual and statistical properties of the family members and interlopers, examining the size and albedo distributions of both groups. We also examine implications for the total mass of the family and their ejection velocities.
    Methods: We use far-infrared space-based telescopes to observe the target TNOs near their thermal peak and combine these data with optical magnitudes to derive sizes and albedos using radiometric techniques. Using measured and inferred sizes together with ejection velocities, we determine the power-law slope of ejection velocity as a function of effective diameter.
    Results: The detected Haumea family members have a diversity of geometric albedos 0.3-0.8, which are higher than geometric albedos of dynamically similar objects without water ice. The median geometric albedo for accepted family members is pV = 0.48-0.18+0.28, compared to 0.08-0.05+0.07 for the dynamical interlopers. In the size range D = 175-300 km, the slope of the cumulative size distribution is q = 3.2-0.4+0.7 for accepted family members, steeper than the q = 2.0 0.6 slope for the dynamical interlopers with D < 500 km. The total mass of Haumea's moons and family members is 2.4% of Haumea's mass. The ejection velocities required to emplace them on their current orbits show a dependence on diameter, with a power-law slope of 0.21-0.50.

    Herschel is an ESA space observatory with science instruments provided by a European-led Principal Investigator consortia and with important participation from NASA.

  20. Clarke, C., Tazzari, M., Juhasz, A., Rosotti, G., Booth, R., Facchini, S., Ilee, J., Johns-Krull, C., Kama, M., Meru, F., Prato, L., 2018, ApJL, 866, L6, High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au
    We present high-resolution millimeter continuum imaging of the disk surrounding the young star CI Tau, a system hosting the first hot Jupiter candidate in a protoplanetary disk system. The system has extended mm emission on which are superposed three prominent annular gaps at radii 13, 39, and 100 au. We argue that these gaps are most likely to be generated by massive planets so that, including the hot Jupiter, the system contains four gas giant planets at an age of only 2 Myr. Two of the new planets are similarly located to those inferred in the famous HL Tau protoplanetary disk; in CI Tau, additional observational data enables a more complete analysis of the system properties than was possible for HL Tau. Our dust and gas dynamical modeling satisfies every available observational constraint and points to the most massive ensemble of exoplanets ever detected at this age, with its four planets spanning a factor 1000 in orbital radius. Our results show that the association between hot Jupiters and gas giants on wider orbits, observed in older stars, is apparently in place at an early evolutionary stage.
  21. Hora, J., Siraj, A., Mommert, M., McNeill, A., Trilling, D., Gustafsson, A., Smith, H., Fazio, G., Chesley, S., Emery, J., Harris, A., Mueller, M., 2018, ApJS, 238, 22, Infrared Light Curves of Near-Earth Objects
    We present light curves and derive periods and amplitudes for a subset of 38 near-Earth objects (NEOs) observed at 4.5 m with the IRAC camera on the the Spitzer Space Telescope, many of them having no previously reported rotation periods. This subset was chosen from about 1800 IRAC NEO observations as having obvious periodicity and significant amplitude. For objects where the period observed did not sample the full rotational period, we derived lower limits to these parameters based on sinusoidal fits. Light curve durations ranged from 42 to 544 minutes, with derived periods from 16 to 270 minutes. We discuss the effects of light curve variations on the thermal modeling used to derive diameters and albedos from Spitzer photometry. We find that both diameters and albedos derived from the light curve maxima and minima agree with our previously published results, even for extreme objects, showing the conservative nature of the thermal model uncertainties. We also evaluate the NEO rotation rates, sizes, and their cohesive strengths.
  22. Bair, A., Schleicher, D., Knight, M., 2018, AJ, 156, 159, Coma Morphology, Numerical Modeling, and Production Rates for Comet C/Lulin (2007 N3)
    We report on narrowband photometry and extensive imaging observations of comet C/Lulin (2007 N3) obtained at Lowell Observatory during 2008 and 2009. Enhanced CN images revealed a double-corkscrew morphology with two near-polar jets oriented approximately east-west, and both CN and dust images showed nightly rotational variability and seasonal changes in bulk morphology. We determined a rotational pole direction of R.A./decl. = 81/+29 with an obliquity of 97 and a sidereal rotation period of 41.45 0.05 hr. Monte Carlo numerical modeling best replicated the observed CN features with an eastern source area at lat/long -80/125 and an 10 radius and a western source area at lat/long +77/245 and an 20 radius, 4 larger than the eastern source. An additional small, near-equatorial source was necessary to reproduce some dust features. Water morphology based on OH was quite different from that of the carbon-bearing species, implying a different driver for the polar jets such as CO or CO2. Ion tails were detected in decontaminated images from both the dust and NH filters, likely being H2O+ and OH+, respectively. We measured water production both before and after perihelion and extrapolated peak water production at perihelion to be about 1.0 1029 molecules s-1. We estimated an active fraction of only 4%-5% and a nucleus radius of up to 8 km. Our data suggest that Lulin, defined as dynamically new in a statistical sense, behaves more like a long-period comet due to its nearly asteroidal early appearance, isolated source regions, and dust properties.
  23. Borisov, G., Devogele, M., Cellino, A., Bagnulo, S., Christou, A., Bendjoya, P., Rivet, J., Abe, L., Vernet, D., Donchev, Z., Krugly, Y., Belskaya, I., Bonev, T., Steeghs, D., Galloway, D., Dhillon, V., O'Brien, P., Pollacco, D., Poshyachinda, S., Ramsay, G., Thrane, E., Ackley, K., Rol, E., Ulaczyk, K., Cutter, R., Dyer, M., 2018, MNRAS, 480, L131, Rotational variation of the linear polarization of the asteroid (3200) Phaethon as evidence for inhomogeneity in its surface properties
    Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in 2017 mid-December and the next one will be on 2026 October. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result, we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Earth. Phaethon has a spin period of 3.604 h, and we found a variation of the linear polarization with rotation. This seems to be a rare case in which such variation is unambiguously found, also a consequence of its fairly large amplitude. Combining this new information with the brightness and colour variation as well as previously reported results from Arecibo radar observations, we conclude that there is no variation of the mineralogy across the surface of Phaeton. However, the observed change in the linear polarization may be related to differences in the thickness of the surface regolith in different areas or local topographic features.
  24. Stern, S., Grundy, W., McKinnon, W., Weaver, H., Young, L., 2018, ARA&A, 56, 357, The Pluto System After New Horizons
    The New Horizons (NH) flyby of the Pluto-Charon binary planet and its system of four small surrounding satellites in mid-2015 revolutionized our knowledge of this distant planet and its moons. Beyond providing rich geo-logical, compositional, and atmospheric data sets, NH demonstrated that Pluto has been surprisingly geologically and climatologically active throughout the past 4+ Gyr and that the planet exhibits a remarkably complex range of atmospheric phenomenology and geological expressions that rival Mars in their richness. In contrast, Pluto's large, planet-sized satellite Charon, though also geologically complex, has no detected active surface volatiles, has no detectable atmosphere, has much more muted colors, has lower albedo, and exhibits only ancient terrains. Pluto's system of four small satellites orbiting outside of Charon is itself dynamically complex and geologically interesting. Here, we review both what was known about the Pluto system before NH and what it has taught us about the Pluto system specifically and, by inference, other small planets in the Kuiper Belt. We go on to examine the natural next steps in Kuiper Belt exploration.
  25. Mommert, M., McNeill, A., Trilling, D., Moskovitz, N., Delbo', M., 2018, AJ, 156, 139, The Main Belt Asteroid Shape Distribution from Gaia Data Release 2
    Gaia Data Release 2 includes observational data for 14099 pre-selected asteroids. From the sparsely sampled G-band photometry, we derive lower-limit light curve amplitudes for 11665 main belt asteroids (MBA) in order to provide constraints on the distribution of shapes in the asteroid main belt. Assuming a triaxial shape model for each asteroid, defined through the axial aspect ratios a > b and b = c, we find an average b/a = 0.80 0.04 for the ensemble, which is in agreement with previous results. By combining the Gaia data with asteroid properties from the literature, we investigate possible correlations of the aspect ratio with size, semimajor axis, geometric albedo, and intrinsic color. Based on our model simulations, we find that MBAs greater than 50 km in diameter on average have higher b/a aspect ratios (are rounder) than smaller asteroids. We furthermore find significant differences in the shape distribution of MBAs as a function of the other properties that do not affect the average aspect ratios. We conclude that a more detailed investigation of shape distribution correlations requires a larger data sample than is provided in Gaia Data Release 2.
  26. Devogele, M., Cellino, A., Borisov, G., Bendjoya, P., Rivet, J., Abe, L., Bagnulo, S., Christou, A., Vernet, D., Donchev, Z., Belskaya, I., Bonev, T., Krugly, Y., 2018, MNRAS, 479, 3498, The phase-polarization curve of asteroid (3200) Phaethon
    A multicolour phase-polarization curve of asteroid (3200) Phaethon has been obtained during the 2017 December apparition by merging measurements taken at the observing station of Calern (France) and at the Rozhen observatory (Bulgaria). All the observations were obtained in the positive polarization branch, the phase angle ranging from 36 to 116. The measured values of linear polarization are among the highest ever observed for a Solar system body. The covered interval of phase angle was not sufficiently extended to derive a firm determination of the Pmax parameter, but this appears to occur at a phase angle around 130 and reaches more than 45 per cent of linear polarization. Phaethon is the parent body of the Geminid meteor shower, and the real physical nature of this object (asteroid or comet) has been a long-debated subject. Our polarimetric measurements seem to support the asteroid hypothesis with a phase-polarization curve similar to the asteroid (2) Pallas, but further observations at smaller phase angles are needed to draw definitive conclusions.
  27. Moore, J., McKinnon, W., Cruikshank, D., Gladstone, G., Spencer, J., Stern, S., Weaver, H., Singer, K., Showalter, M., Grundy, W., Beyer, R., White, O., Binzel, R., Buie, M., Buratti, B., Cheng, A., Howett, C., Olkin, C., Parker, A., Porter, S., Schenk, P., Throop, H., Verbiscer, A., Young, L., Benecchi, S., Bray, V., Chavez, C., Dhingra, R., Howard, A., Lauer, T., Lisse, C., Robbins, S., Runyon, K., Umurhan, O., 2018, GeoRL, 45, 8111, Great Expectations: Plans and Predictions for New Horizons Encounter With Kuiper Belt Object 2014 MU69 ("Ultima Thule")
    The New Horizons encounter with the cold classical Kuiper Belt object 2014 MU69 (informally named "Ultima Thule," hereafter Ultima) on 1 January 2019 will be the first time a spacecraft has ever closely observed one of the free-orbiting small denizens of the Kuiper Belt. Related to but not thought to have formed in the same region of the solar system as the comets that been explored so far, it will also be the most distant, and most primitive body yet visited by spacecraft. In this letter we begin with a brief overview of cold classical Kuiper Belt objects, of which Ultima is a prime example. We give a short preview of our encounter plans. We note what is currently known about Ultima from Earth-based observations. We then review our expectations and capabilities to evaluate Ultima's composition, surface geology, structure, near space environment, small moons, rings, and the search for activity.
  28. Sheppard, S., Williams, G., Tholen, D., Trujillo, C., Brozovic, M., Thirouin, A., Devogele, M., Fohring, D., Jacobson, R., Moskovitz, N., 2018, RNAAS, 2, 155, New Jupiter Satellites and Moon-Moon Collisions
    We report the discovery of 12 new satellites of Jupiter, giving Jupiter 79 known satellites. The new finds are between 23rd-24th mag in the r-band and 1-3 km in diameter assuming dark albedos. Nine of the discoveries are in the distant retrograde satellite groupings. Two of the new satellites are in the closer Himalia prograde group near 28 degrees in inclination. S/2016 J2, nicknamed Valetudo, has an orbit unlike any other known outer satellite and is the most distant prograde satellite around any planet at 0.36 Hill radii. Numerical simulations show S/2016 J2 is very stable, with average and range of i=34.2+-3 deg, e=0.216+-0.125, and a=18.9+-0.7 million km over 100 Myrs. Our stability simulations show a S/2016 J2 like orbit would be stable out to a=21.8 million km or 0.41 Hill radii, but no further, unlike more distant and eccentric retrograde satellites. S/2016 J2's large semi-major axis means it significantly overlaps the orbits of the distant retrogrades. A prograde-retrograde moon-moon collision between outer satellites of Jupiter has likely happened over the age of the solar system.
  29. Neugent, K., Massey, P., Morrell, N., 2018, ApJ, 863, 181, A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds. IV. A Final Census
    We summarize the results of our 4 yr survey searching for Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud. Over the course of this survey we have discovered 15 new WR stars and 12 Of-type stars. In this last year we discovered two rare Of-type stars: an O6.5f?p and an O6nfp, in addition to the two new Of?p stars discovered in our first year and the three Onfp stars discovered in our second and third years. However, even more exciting was our discovery of a new type of WR star, ones we are calling WN3/O3s owing to their spectroscopic signatures. We describe the completeness limits of our survey and demonstrate that we are sensitive to weak-lined WRs several magnitudes fainter than any we have discovered, arguing that there is not a population of fainter WRs waiting to be discovered. We discuss the nature of the WN3/O3s, summarizing the results of our extensive spectroscopy and modeling. We also examine the important claim made by others that the WN3/O3s are isolated compared to other massive stars. We find that if we use a more complete sample of reference massive stars, the WN3/O3s show the same spatial distribution as other early WNs, consistent with a common origin. Finally, we use this opportunity to present the Fifth Catalog of LMC Wolf-Rayet Stars, which includes revised coordinates and updated spectral types for all 154 known LMC WRs.

    This paper includes data gathered with the 1 m Swope and 6.5 m Magellan telescopes located at Las Campanas Observatory, Chile.

  30. Davis, B., Ciardullo, R., Jacoby, G., Feldmeier, J., Indahl, B., 2018, ApJ, 863, 189, The True Luminosities of Planetary Nebulae in M31's Bulge: Massive Central Stars from an Old Stellar Population
    We measure the Balmer decrements of 23 of the brightest planetary nebulae (PNe) in the inner bulge (r 3) of M31 and deredden the bright end of the regions [O III] 5007 PN luminosity function. We show that the most luminous PNe produce 1200 L of power in their [O III] 5007 line, implying central star luminosities of at least 11,000 L . Even with the most recent accelerated-evolution post-AGB models, such luminosities require central star masses in excess of 0.66 M and main-sequence progenitors of at least 2.5 M . Since M31's bulge has very few intermediate-age stars, we conclude that conventional single-star evolution cannot be responsible for these extremely luminous objects. We also present the circumstellar extinctions for the regions bright PNe and demonstrate that the distribution is similar to that found for PNe in the Large Magellanic Cloud, with a median value of A 5007 = 0.71. Finally, we compare our results to extinction measurements made for PNe in the E6 elliptical NGC 4697 and the interacting lenticular NGC 5128. We show that such extinctions are not unusual and that the existence of very high-mass PN central stars is a general feature of old stellar populations. Our results suggest that single-star population synthesis models significantly underestimate the maximum luminosities and total integrated light of AGB stars.
  31. Agarwal, J., Mommert, M., 2018, A&A, 616, A54, Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1)
    Context. The dust emission from active asteroids is likely driven by collisions, fast rotation, sublimation of embedded ice, and combinations of these. Characterising these processes leads to a better understanding of their respective influence on the evolution of the asteroid population.
    Aims: We study the role of fast rotation in the active asteroid 358P (P 2012/T1).
    Methods: We obtained two nights of deep imaging of 358P with SOAR/Goodman and VLT/FORS2. We derived the rotational light curve from time-resolved photometry and searched for large fragments and debris >8 mm in a stacked, ultra-deep image.
    Results: The nucleus has an absolute magnitude of mR = 19.68, corresponding to a diameter of 530 m for standard assumptions on the albedo and phase function of a C-type asteroid. We do not detect fragments or debris that would require fast rotation to reduce surface gravity to facilitate their escape. The 10-h light curve does not show an unambiguous periodicity.
  32. Kammer, J., Becker, T., Retherford, K., Stern, S., Olkin, C., Buie, M., Spencer, J., Bosh, A., Wasserman, L., 2018, AJ, 156, 72, Probing the Hill Sphere of (486958) 2014 MU69: HST FGS Observations during the 2017 July 17 Stellar Occultation
    We observed the 2017 July 17 stellar occultation of HD 168233 by the Kuiper Belt Object (486958) 2014 MU69, the close flyby target of the extended New Horizons mission. Rather than capture a solid body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hubble Space Telescope Fine Guidance Sensors (HST FGS) instrument in TRANS F583W mode to collect 40 Hz time resolution photometry of the stellar occultation star for two HST orbits during this observation. We present the results of reduction and calibration of the HST FGS photometry, and set upper limits on rings or other dust opacity within the Hill sphere of (486958) 2014 MU69 at distances ranging from 20000 km to 75000 km from the main body.
  33. Flagg, L., Johns-Krull, C., Prato, L., Nofi, L., Llama, J., Sullivan, K., Jaffe, D., Mace, G., 2018, csss, 41, A Search for the Direct Detection of the 2 Myr Old Hot Jupiter Orbiting CI Tau
    Characterizing young exoplanets is critical for putting limits on planet formation scenarios. However, as of yet, only a few young exoplanet candidates have even been discovered, and no young planet with a model-independent mass has had its spectrum or brightness measured. A good candidate for such a detection is CI Tau b, an msini = 8.1 Mjup planet in a 9 day orbit around a 2 Myr old classical T Tauri star. We use high spectral resolution K band echelle spectroscopy to look for direct signatures of the planet itself, taking advantage of the large expected radial velocity variations of the planet as it orbits CI Tau. We report on our efforts to directly detect the spectrum of CI Tau b and present a tentative measurement of CO absorption in this young exoplanet. The properties of the planet determined from this CO detection are consistent with those described in the discovery paper and favor "hot start" formation models.
  34. Bertrand, T., Forget, F., Umurhan, O., Grundy, W., Schmitt, B., Protopapa, S., Zangari, A., White, O., Schenk, P., Singer, K., Stern, A., Weaver, H., Young, L., Ennico, K., Olkin, C., 2018, Icar, 309, 277, The nitrogen cycles on Pluto over seasonal and astronomical timescales
    Pluto's landscape is shaped by the endless condensation and sublimation cycles of the volatile ices covering its surface. In particular, the Sputnik Planitia ice sheet, which is thought to be the main reservoir of nitrogen ice, displays a large diversity of terrains, with bright and dark plains, small pits and troughs, topographic depressions and evidences of recent and past glacial flows. Outside Sputnik Planitia, New Horizons also revealed numerous nitrogen ice deposits, in the eastern side of Tombaugh Regio and at mid-northern latitudes.

    These observations suggest a complex history involving volatile and glacial processes occurring on different timescales. We present numerical simulations of volatile transport on Pluto performed with a model designed to simulate the nitrogen cycle over millions of years, taking into account the changes of obliquity, solar longitude of perihelion and eccentricity as experienced by Pluto. Using this model, we first explore how the volatile and glacial activity of nitrogen within Sputnik Planitia has been impacted by the diurnal, seasonal and astronomical cycles of Pluto. Results show that the obliquity dominates the N2 cycle and that over one obliquity cycle, the latitudes of Sputnik Planitia between 25S-30N are dominated by N2 condensation, while the northern regions between 30N and -50N are dominated by N2 sublimation. We find that a net amount of 1 km of ice has sublimed at the northern edge of Sputnik Planitia during the last 2 millions of years. It must have been compensated by a viscous flow of the thick ice sheet. By comparing these results with the observed geology of Sputnik Planitia, we can relate the formation of the small pits and the brightness of the ice at the center of Sputnik Planitia to the sublimation and condensation of ice occurring at the annual timescale, while the glacial flows at its eastern edge and the erosion of the water ice mountains all around the ice sheet are instead related to the astronomical timescale. We also perform simulations including a glacial flow scheme which shows that the Sputnik Planitia ice sheet is currently at its minimum extent at the northern and southern edges. We also explore the stability of N2 ice deposits outside the latitudes and longitudes of the Sputnik Planitia basin. Results show that N2 ice is not stable at the poles but rather in the equatorial regions, in particular in depressions, where thick deposits may persist over tens of millions of years, before being trapped in Sputnik Planitia. Finally, another key result is that the minimum and maximum surface pressures obtained over the simulated millions of years remain in the range of milli-Pascals and Pascals, respectively. This suggests that Pluto never encountered conditions allowing liquid nitrogen to flow directly on its surface. Instead, we suggest that the numerous geomorphological evidences of past liquid flow observed on Pluto's surface are the result of liquid nitrogen that flowed at the base of thick ancient nitrogen glaciers, which have since disappeared.

  35. van Belle, G., Armstrong, J., Benson, J., Baines, E., Bevilacqua, R., Buschmann, T., Clark, J., DeGroff, W., Hall, J., Hindsley, R., Jorgensen, A., Lindgren, A., Mozurkewich, D., Muterspaugh, M., Pooler, S., Restaino, S., Schmitt, H., Shankland, P., Zavala, R., 2018, SPIE, 10701, 1070105, Many interesting things are afoot at the Navy Precision Optical Interferometer
    The Navy Precision Optical Interferometer (NPOI) is currently undergoing a fundamental renaissance in its functionality and capabilities. Operationally, its fast delay line (FDL) infrastructure is completing its upgrade from a VME/VxWorks foundation to a modern PC/RTLinux core. The Classic beam combiner is being upgraded with the New Classic FPGA-based backend, and the VISION beam combiner has been upgraded over this past summer with low-noise EMCCD cameras, resulting in substantial gains in sensitivity. Building on those infrastructure improvements, substantial upgrades are also in progress. Three 1-meter PlaneWave CDK1000 telescopes are being delivered to the site, along with their relocatable enclosure-transporters, and stations are being commissioned for those telescopes with baselines ranging from 8 meters to 432 meters. Baseline-wavelength bootstrapping will be implemented on the facility back-end with a near-infrared beam combiner under development. Collectively, these improvements mark substantial progress in taking the facility towards realizing its full intrinsic potential.
  36. Armstrong, J., Schmitt, H., Restaino, S., Baines, E., van Belle, G., 2018, SPIE, 10701, 107010B, An infrared beam combiner for wavelength bootstrapping at the NPOI
    Since 1994, the Navy Precision Optical Interferometer (NPOI) has operated at visual wavelengths (450 to 850 nm). Its primary Classic backend is a pupil-plane combiner that disperses the light at a resolution R 50, uses avalanche photo-diodes as photon-counting detectors, and scans interference fringes by modulating the delay at 1 kHz. The newer NPOI image-plane combiner, VISION (Tennessee State University), which is similar to CHARA's MIRC and is currently being upgraded, dispenses with delay modulation. We are now developing a third backend to expand into the near infrared. Its primary purpose will be to stabilize the NPOI for high-resolution observations by bootstrapping from the infrared to visual wavelengths.
  37. Monnier, J., Ireland, M., Kraus, S., Alonso-Herrero, A., Bonsor, A., Baron, F., Bayo, A., Berger, J., Boyajian, T., Chiavassa, A., Ciardi, D., Creech-Eakman, M., de Wit, W., Defrere, D., Dong, R., Duchene, G., Espaillat, C., Gallenne, A., Gandhi, P., Gonzalez, J., Haniff, C., Hoenig, S., Ilee, J., Isella, A., Jensen, E., Juhasz, A., Kane, S., Kishimoto, M., Kley, W., Kral, Q., Kratter, K., Labadie, L., Lacour, S., Laughlin, G., Le Bouquin, J., Michael, E., Meru, F., Millan-Gabet, R., Millour, F., Minardi, S., Morbidelli, A., Mordasini, C., Morlok, A., Mozurkewich, D., Nelson, R., Olofsson, J., Oudmaijer, R., Packham, C., Paladini, C., Panic, O., Petrov, R., Pope, B., Pott, J., Quiroga-Nunez, L., Ramos Almeida, C., Raymond, S., Regaly, Z., Reynolds, M., Ridgway, S., Rinehart, S., Schreiber, M., Smith, M., Stassun, K., Surdej, J., ten Brummelaar, T., Tristram, K., Turner, N., Tuthill, P., van Belle, G., Vasisht, G., Wallace, A., Weigelt, G., Wishnow, E., Wittkowski, M., Wolf, S., Young, J., Zhao, M., Zhu, Z., Zuniga-Fernandez, S., 2018, SPIE, 10701, 1070118, Planet formation imager: project update
    The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4 x 8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry.
  38. Hahne, F., Horch, E., van Belle, G., Clark, C., Winters, J., Henry, T., 2018, SPIE, 10701, 107012A, Two-color speckle imaging of M-dwarfs with the Discovery Channel telescope
    Lowell Observatory and Southern Connecticut State University are currently involved in a joint project to determine the stellar multiplicity rates and the fundamental stellar parameters of M dwarf stars using the Differential Speckle Survey Instrument (DSSI) at Lowell's Discovery Channel Telescope (DCT). DSSI observes speckle patterns simultaneously at two separate wavelengths, allowing color measurements of the components of a binary system to be made in a single observation. This paper will describe the initial data gathering process, which began in 2016. Since then, over 1000 stars have been observed. We summarize the analysis on these objects so far, and discuss the relevance of these observations for existing and future space missions such as TESS, JWST, and Gaia.
  39. Mace, G., Sokal, K., Lee, J., Oh, H., Park, C., Lee, H., Good, J., MacQueen, P., Oh, J., Kaplan, K., Kidder, B., Chun, M., Yuk, I., Jeong, U., Pak, S., Kim, K., Nah, J., Lee, S., Yu, Y., Hwang, N., Park, B., Kim, H., Chinn, B., Peck, A., Diaz, R., Rutten, R., Prato, L., Jacoby, G., Cornelius, F., Hardesty, B., DeGroff, W., Dunham, E., Levine, S., Nofi, L., Lopez-Valdivia, R., Weinberger, A., Jaffe, D., 2018, SPIE, 10702, 107020Q, IGRINS at the Discovery Channel Telescope and Gemini South
    The Immersion GRating INfrared Spectrometer (IGRINS) was designed for high-throughput with the expectation of being a visitor instrument at progressively larger observing facilities. IGRINS achieves R45000 and > 20,000 resolution elements spanning the H and K bands (1.45-2.5m) by employing a silicon immersion grating as the primary disperser and volume-phase holographic gratings as cross-dispersers. After commissioning on the 2.7 meter Harlan J. Smith Telescope at McDonald Observatory, the instrument had more than 350 scheduled nights in the first two years. With a fixed format echellogram and no cryogenic mechanisms, spectra produced by IGRINS at different facilities have nearly identical formats. The first host facility for IGRINS was Lowell Observatory's 4.3-meter Discovery Channel Telescope (DCT). For the DCT a three-element fore-optic assembly was designed to be mounted in front of the cryostat window and convert the f/6.1 telescope beam to the f/8.8 beam required by the default IGRINS input optics. The larger collecting area and more reliable pointing and tracking of the DCT improved the faint limit of IGRINS, relative to the McDonald 2.7-meter, by 1 magnitude. The Gemini South 8.1-meter telescope was the second facility for IGRINS to visit. The focal ratio for Gemini is f/16, which required a swap of the four-element input optics assembly inside the IGRINS cryostat. At Gemini, observers have access to many southern-sky targets and an additional gain of 1.5 magnitudes compared to IGRINS at the DCT. Additional adjustments to IGRINS include instrument mounts for each facility, a glycol cooled electronics rack, and software modifications. Here we present instrument modifications, report on the success and challenges of being a visitor instrument, and highlight the science output of the instrument after four years and 699 nights on sky. The successful design and adaptation of IGRINS for various facilities make it a reliable forerunner for GMTNIRS, which we now anticipate commissioning on one of the 6.5 meter Magellan telescopes prior to the completion of the Giant Magellan Telescope.
  40. Dunham, E., Bida, T., Chylek, T., Cornelius, F., Gustafsson, A., Moskovitz, N., Roe, H., 2018, SPIE, 10702, 107023E, NIHTS: the near-infrared high throughput spectrograph for the Discovery Channel Telescope
    NIHTS is a first-generation instrument now in use on Lowell Observatory's Discovery Channel Telescope. It is a nearinfrared prism spectrograph of the BASS design featuring high throughput and low dispersion that is intended for observations of faint solar system and astrophysical objects over the YJHK spectral range. An unusual feature is its ability to observe simultaneously with the Large Monolithic Imager, an optical CCD camera, by means of a dichroic fold mirror. This is particularly valuable for time-variable targets such as Kuiper Belt Objects, asteroids, exoplanet transits, and brown dwarfs. We describe its design details and performance both in the lab and on the telescope.
  41. Scowen, P., Shkolnik, E., Ardila, D., Berman, T., Beasley, M., Bowman, J., Fitzgerald, M., Gorjian, V., Jacobs, D., Jewell, A., Llama, J., Meadows, V., Nikzad, S., Spittler, C., Swain, M., Zellem, R., 2018, SPIE, 10699, 106990F, Monitoring the high-energy radiation environment of exoplanets around low-mass stars with SPARCS (Star-Planet Activity Research CubeSat)
    Roughly 40 billion M dwarfs in our galaxy host at least one small planet in the habitable zone (HZ). The stellar ultraviolet (UV) radiation from M dwarfs is strong and highly variable, and impacts planetary atmospheric loss, composition and habitability. These effects are amplified by the extreme proximity of their HZs (0.1-0.4 AU). Knowing the UV environments of M dwarf planets will be crucial to understanding their atmospheric composition and a key parameter in discriminating between biological and abiotic sources for observed biosignatures. The Star-Planet Activity Research CubeSat (SPARCS) will be a 6U CubeSat devoted to photometric monitoring of M stars in the far-UV and near-UV, measuring the time-dependent spectral slope, intensity and evolution of low-mass star high-energy radiation.
  42. Levine, S., DeGroff, W., Bida, T., Dunham, E., Jacoby, G., 2018, SPIE, 10700, 107004P, Status and performance of Lowell Observatory's Discovery Channel telescope and its growing suite of instruments
    Lowell Observatory's Discovery Channel Telescope (DCT) is a 4.3-m telescope designed and constructed for optical and near infrared astronomical observation. The DCT is equipped with a cube at the RC focus capable of interfacing to five instruments along with the wave front sensing and guider systems at the f/6.1 RC focus. Over the period 2016 through mid-2018 the instrument cube ports were fully populated as several instruments new to the DCT were brought on-line (NIHTS, IGRINS, EXPRES). The primary and secondary mirrors of the telescope were re-aluminized, and the coating process modified. The facility operational modes have been refined to allow for greater flexibility and faster response to unexpected science opportunities. This report addresses operational methods, instrumentation integration, and the performance of the facility as determined from delivered science data, lessons learned, and plans for future work and additional instruments.
  43. Erasmus, N., McNeill, A., Mommert, M., Trilling, D., Sickafoose, A., van Gend, C., 2018, ApJS, 237, 19, Taxonomy and Light-curve Data of 1000 Serendipitously Observed Main-belt Asteroids
    We present VRI spectrophotometry of 1003 main-belt asteroids (MBAs) observed with the Sutherland, South Africa node of the Korea Microlensing Telescope Network (KMTNet). All of the observed MBAs were serendipitously captured in KMTNets large 2 2 field of view during a separate targeted near-Earth Asteroid study. Our broadband spectrophotometry is reliable enough to distinguish among four asteroid taxonomies and we confidently categorize 836 of the 1003 observed targets as either a S-, C-, X-, or D-type asteroid by means of a machine learning algorithm approach. Our data show that the ratio between S-type MBAs and (C+X+D)-type MBAs, with H magnitudes between 12 and 18 (12 km diameter 0.75 km), is almost exactly 1:1. Additionally, we report 0.5-3 hr (median: 1.3 hr) light-curve data for each MBA and we resolve the complete rotation periods and amplitudes for 59 targets. Of the 59 targets, 2 have rotation periods potentially below the theoretical zero-cohesion boundary limit of 2.2 hr. We report lower limits for the rotation periods and amplitudes for the remaining targets. Using the resolved and unresolved light curves we determine the shape distribution for this population using a Monte Carlo simulation. Our model suggests a population with an average elongation b/a = 0.74 0.07 and also shows that this is independent of asteroid size and taxonomy.
  44. Hsieh, H., Ishiguro, M., Knight, M., Micheli, M., Moskovitz, N., Sheppard, S., Trujillo, C., 2018, AJ, 156, 39, The Reactivation and Nucleus Characterization of Main-belt Comet 358P/PANSTARRS (P/2012 T1)
    We present observations of main-belt comet (MBC) 358P/PANSTARRS (P/2012 T1) obtained using the Gemini South telescope from 2017 July to December, as the object approached perihelion for the first time since its discovery. We find best-fit IAU phase function parameters of H R = 19.5 0.2 mag and G R = -0.22 0.13 for the nucleus, corresponding to an effective radius of r N = 0.32 0.03 km (assuming an albedo of p R = 0.05). The object appears significantly brighter (by 1 mag) than expected starting in 2017 November, while a faint dust tail oriented approximately in the antisolar direction is also observed on 2017 December 18. We conclude that 358P has become active again for the first time since its previously observed active period in 2012-2013. These observations make 358P the seventh MBC candidate confirmed to exhibit recurrent activity near perihelion with intervening inactivity away from perihelion, strongly indicating that its activity is sublimation-driven. Fitting a linear function to the ejected dust masses inferred for 358P in 2017 when it is apparently active, we find an average net dust production rate of \dot{M}=2.0+/- 0.6 kg s-1 (assuming a mean effective particle radius of {\bar{a}}d=1 mm) and an estimated activity start date of 2017 November 8 4 when the object was at a true anomaly of = 316 1 and a heliocentric distance of R = 2.54 au. Insufficient data is currently available to ascertain whether activity strength has changed between the objects 2012-2013 and 2017 active periods. Further observations are therefore highly encouraged during the objects upcoming observing window (2018 August through 2019 May).
  45. Hunter, D., Adamo, A., Elmegreen, B., Gallardo, S., Lee, J., Cook, D., Thilker, D., Kayitesi, B., Kim, H., Kahre, L., Ubeda, L., Bright, S., Ryon, J., Calzetti, D., Tosi, M., Grasha, K., Messa, M., Fumagalli, M., Dale, D., Sabbi, E., Cignoni, M., Smith, L., Gouliermis, D., Grebel, E., Aloisi, A., Whitmore, B., Chandar, R., Johnson, K., 2018, AJ, 156, 21, A Comparison of Young Star Properties with Local Galactic Environment for LEGUS/LITTLE THINGS Dwarf Irregular Galaxies
    We have explored the role environmental factors play in determining characteristics of young stellar objects in nearby dwarf irregular and blue compact dwarf galaxies. Star clusters are characterized by concentrations, masses, and formation rates; OB associations by mass and mass surface density; O stars by their numbers and near-ultraviolet absolute magnitudes; and H II regions by H surface brightnesses. These characteristics are compared to surrounding galactic pressure, stellar mass density, H I surface density, and star formation rate (SFR) surface density. We find no trend of cluster characteristics with environmental properties, implying that larger-scale effects are more important in determining cluster characteristics or that rapid dynamical evolution erases any memory of the initial conditions. On the other hand, the most massive OB associations are found at higher pressure and H I surface density, and there is a trend of higher H II region H surface brightness with higher pressure, suggesting that a higher concentration of massive stars and gas is found preferentially in regions of higher pressure. At low pressures we find massive stars but not bound clusters and OB associations. We do not find evidence for an increase of cluster formation efficiency as a function of SFR density. However, there is an increase in the ratio of the number of clusters to the number of O stars with increasing pressure, perhaps reflecting an increase in clustering properties with SFR.
  46. Cushing, M., Moskovitz, N., Gustafsson, A., 2018, RNAAS, 2, 50, Spectroscopic Confirmation That 2MASS J07414279-0506464 Is a Mid-type L Dwarf
    We present a low-resolution near-infrared spectrum of 2MASS J07414279-0506464, a mid-type L dwarf candidate recently identified by Scholz & Bell. The spectrum was obtained using the Near-Infrared High Throughput Spectrograph (NIHTS) on Lowell Observatory's 4.3 m Discovery Channel Telescope and indicates that 2MASS J07414279-0506464 has a spectral type of L5.
  47. Lucey, J., Smith, R., Schechter, P., Bosh, A., Levine, S., 2018, RNAAS, 2, 62, A New Quadruple-image Gravitational Lens in an Edge-on Disk Galaxy at z = 0.0956
    We report the serendipitous discovery of a quadruply-lensed source behind the z=0.095 edge-on disk galaxy 2MASXJ13170000-1405187, based on public imaging survey data from Pan-STARRS PS1 and the VISTA Hemisphere Survey. Follow-up imaging from Magellan/LDSS3 shows that the background source is spatially extended (i.e. not a QSO), and that two of the lensed images are observed through a prominent dust ring in the disk of the lens galaxy. We summarise results of preliminary modelling, which indicates an Einstein radius of 1.44 arcsec, and a K-band mass-to-light ratio of 0.5, relative to the solar value.
  48. Telfer, M., Parteli, E., Radebaugh, J., Beyer, R., Bertrand, T., Forget, F., Nimmo, F., Grundy, W., Moore, J., Stern, S., Spencer, J., Lauer, T., Earle, A., Binzel, R., Weaver, H., Olkin, C., Young, L., Ennico, K., Runyon, K., aff12, 2018, Sci, 360, 992, Dunes on Pluto
    The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (~0.4 to 1 kilometer) is best explained by the deposition of sand-sized (~200 to ~300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past.
  49. Thirouin, A., Sheppard, S., 2018, AJ, 155, 248, The Plutino Population: An Abundance of Contact Binaries
    We observed 12 Plutinos over two separated years with the 4.3 m Lowells Discovery Channel Telescope. Here, we present the first light-curve data for those objects. Three of them (2014 JL80, 2014 JO80, and 2014 JQ80) display a large light-curve amplitude explainable by a single elongated object, but they are most likely caused by a contact binary system due to their light-curve morphology. These potential contact binaries have rotational periods from 6.3 to 34.9 hr and peak-to-peak light-curve variability between 0.6 and 0.8 mag. We present partial light curves, allowing us to constrain the light-curve amplitude and the rotational period of another nine Plutinos. By merging our data with the literature, we estimate that up to 40% of the Plutinos could be contact binaries. Interestingly, we found that all of the suspected contact binaries in the 3:2 resonance are small with absolute magnitude H > 6 mag. Based on our sample and the literature, up to 50% of the small Plutinos are potential contact binaries.
  50. Harris, H., Dahn, C., Subasavage, J., Munn, J., Canzian, B., Levine, S., Monet, A., Pier, J., Stone, R., Tilleman, T., Hartkopf, W., 2018, AJ, 155, 252, Distances of Dwarf Carbon Stars
    Parallaxes are presented for a sample of 20 nearby dwarf carbon stars. The inferred luminosities cover almost two orders of magnitude. Their absolute magnitudes and tangential velocities confirm prior expectations that some originate in the Galactic disk, although more than half of this sample are halo stars. Three stars are found to be astrometric binaries, and orbital elements are determined; their semimajor axes are 1-3 au, consistent with the size of an AGB mass-transfer donor star.
  51. Trilling, D., Bellm, E., Malhotra, R., 2018, AJ, 155, 243, On the Detectability of Planet X with LSST
    Two planetary mass objects in the far outer solar systemcollectively referred to here as Planet X have recently been hypothesized to explain the orbital distribution of distant Kuiper Belt Objects. Neither planet is thought to be exceptionally faint, but the sky locations of these putative planets are poorly constrained. Therefore, a wide area survey is needed to detect these possible planets. The Large Synoptic Survey Telescope (LSST) will carry out an unbiased, large area (around 18000 deg2), deep (limiting magnitude of individual frames of 24.5) survey (the wide-fast-deep (WFD) survey) of the southern sky beginning in 2022, and it will therefore be an important tool in searching for these hypothesized planets. Here, we explore the effectiveness of LSST as a search platform for these possible planets. Assuming the current baseline cadence (which includes the WFD survey plus additional coverage), we estimate that LSST will confidently detect or rule out the existence of Planet X in 61% of the entire sky. At orbital distances up to 75 au, Planet X could simply be found in the normal nightly moving object processing; at larger distances, it will require custom data processing. We also discuss the implications of a nondetection of Planet X in LSST data.
  52. Grundy, W., Noll, K., Buie, M., Levison, H., 2018, Icar, 305, 198, The upcoming mutual event season for the Patroclus-Menoetius Trojan binary
    We present new Hubble Space Telescope and ground-based Keck observations and new Keplerian orbit solutions for the mutual orbit of binary Jupiter Trojan asteroid (617) Patroclus and Menoetius, targets of NASA's Lucy mission. We predict event times for the upcoming mutual event season, which is anticipated to run from late 2017 through mid 2019.
  53. Cauley, P., Shkolnik, E., Llama, J., 2018, RNAAS, 2, 23, Transit Time Derivation for Hot Planet Bow-shocks
    We present an analytical estimate of hot planet bow-shock transit times that will be useful for planning observations of such signatures.
  54. Bannister, M., Gladman, B., Kavelaars, J., Petit, J., Volk, K., Chen, Y., Alexandersen, M., Gwyn, S., Schwamb, M., Ashton, E., Benecchi, S., Cabral, N., Dawson, R., Delsanti, A., Fraser, W., Granvik, M., Greenstreet, S., Guilbert-Lepoutre, A., Ip, W., Jakubik, M., Jones, R., Kaib, N., Lacerda, P., Van Laerhoven, C., Lawler, S., Lehner, M., Lin, H., Lykawka, P., Marsset, M., Murray-Clay, R., Pike, R., Rousselot, P., Shankman, C., Thirouin, A., Vernazza, P., Wang, S., 2018, ApJS, 236, 18, OSSOS. VII. 800+ Trans-Neptunian ObjectsThe Complete Data Release
    The Outer Solar System Origins Survey (OSSOS), a wide-field imaging program in 2013-2017 with the Canada-France-Hawaii Telescope, surveyed 155 deg2 of sky to depths of m r = 24.1-25.2. We present 838 outer solar system discoveries that are entirely free of ephemeris bias. This increases the inventory of trans-Neptunian objects (TNOs) with accurately known orbits by nearly 50%. Each minor planet has 20-60 Gaia/Pan-STARRS-calibrated astrometric measurements made over 2-5 oppositions, which allows accurate classification of their orbits within the trans-Neptunian dynamical populations. The populations orbiting in mean-motion resonance with Neptune are key to understanding Neptunes early migration. Our 313 resonant TNOs, including 132 plutinos, triple the available characterized sample and include new occupancy of distant resonances out to semimajor axis a 130 au. OSSOS doubles the known population of the nonresonant Kuiper Belt, providing 436 TNOs in this region, all with exceptionally high-quality orbits of a uncertainty a 0.1% they show that the belt exists from a 37 au, with a lower perihelion bound of 35 au. We confirm the presence of a concentrated low-inclination a 44 au kernel population and a dynamically cold population extending beyond the 2:1 resonance. We finely quantify the surveys observational biases. Our survey simulator provides a straightforward way to impose these biases on models of the trans-Neptunian orbit distributions, allowing statistical comparison to the discoveries. The OSSOS TNOs, unprecedented in their orbital precision for the size of the sample, are ideal for testing concepts of the history of giant planet migration in the solar system.
  55. Neugent, K., Massey, P., Morrell, N., Skiff, B., Georgy, C., 2018, AJ, 155, 207, A Runaway Yellow Supergiant Star in the Small Magellanic Cloud
    We recently discovered a yellow supergiant (YSG) in the Small Magellanic Cloud (SMC) with a heliocentric radial velocity of 300 km s-1, which is much larger than expected for a star at its location in the SMC. This is the first runaway YSG ever discovered and only the second evolved runaway star discovered in a galaxy other than the Milky Way. We classify the star as G5-8 I and use de-reddened broad-band colors with model atmospheres to determine an effective temperature of 4700 250 K, consistent with what is expected from its spectral type. The stars luminosity is then log L/L 4.2 0.1, consistent with it being a 30 Myr 9 M star according to the Geneva evolution models. The star is currently located in the outer portion of the SMCs body, but if the stars transverse peculiar velocity is similar to its peculiar radial velocity, in 10 Myr the star would have moved 1.6 across the disk of the SMC and could easily have been born in one of the SMCs star-forming regions. Based on its large radial velocity, we suggest it originated in a binary system where the primary exploded as a supernovae, thus flinging the runaway star out into space. Such stars may provide an important mechanism for the dispersal of heavier elements in galaxies given the large percentage of massive stars that are runaways. In the future, we hope to look into additional evolved runaway stars that were discovered as part of our other past surveys.

    This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  56. Jenniskens, P., Baggaley, J., Crumpton, I., Aldous, P., Pokorny, P., Janches, D., Gural, P., Samuels, D., Albers, J., Howell, A., Johannink, C., Breukers, M., Odeh, M., Moskovitz, N., Collison, J., Ganju, S., 2018, P&SS, 154, 21, A survey of southern hemisphere meteor showers
    Results are presented from a video-based meteoroid orbit survey conducted in New Zealand between Sept. 2014 and Dec. 2016, which netted 24,906 orbits from +5 to -5 magnitude meteors. 44 new southern hemisphere meteor showers are identified after combining this data with that of other video-based networks. Results are compared to showers reported from recent radar-based surveys. We find that video cameras and radar often see different showers and sometimes measure different semi-major axis distributions for the same meteoroid stream. For identifying showers in sparse daily orbit data, a shower look-up table of radiant position and speed as a function of time was created. This can replace the commonly used method of identifying showers from a set of mean orbital elements by using a discriminant criterion, which does not fully describe the distribution of meteor shower radiants over time.
  57. Thompson, S., Coughlin, J., Hoffman, K., Mullally, F., Christiansen, J., Burke, C., Bryson, S., Batalha, N., Haas, M., Catanzarite, J., Rowe, J., Barentsen, G., Caldwell, D., Clarke, B., Jenkins, J., Li, J., Latham, D., Lissauer, J., Mathur, S., Morris, R., Seader, S., Smith, J., Klaus, T., Twicken, J., Van Cleve, J., Wohler, B., Akeson, R., Ciardi, D., Cochran, W., Henze, C., Howell, S., Huber, D., Prsa, A., Ramirez, S., Morton, T., Barclay, T., Campbell, J., Chaplin, W., Charbonneau, D., Christensen-Dalsgaard, J., Dotson, J., Doyle, L., Dunham, E., Dupree, A., Ford, E., Geary, J., Girouard, F., Isaacson, H., Kjeldsen, H., Quintana, E., Ragozzine, D., Shabram, M., Shporer, A., Silva Aguirre, V., Steffen, J., Still, M., Tenenbaum, P., Welsh, W., Wolfgang, A., Zamudio, K., Koch, D., Borucki, W., 2018, ApJS, 235, 38, Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog with Measured Completeness and Reliability Based on Data Release 25
    We present the Kepler Object of Interest (KOI) catalog of transiting exoplanets based on searching 4 yr of Kepler time series photometry (Data Release 25, Q1-Q17). The catalog contains 8054 KOIs, of which 4034 are planet candidates with periods between 0.25 and 632 days. Of these candidates, 219 are new, including two in multiplanet systems (KOI-82.06 and KOI-2926.05) and 10 high-reliability, terrestrial-size, habitable zone candidates. This catalog was created using a tool called the Robovetter, which automatically vets the DR25 threshold crossing events (TCEs). The Robovetter also vetted simulated data sets and measured how well it was able to separate TCEs caused by noise from those caused by low signal-to-noise transits. We discuss the Robovetter and the metrics it uses to sort TCEs. For orbital periods less than 100 days the Robovetter completeness (the fraction of simulated transits that are determined to be planet candidates) across all observed stars is greater than 85%. For the same period range, the catalog reliability (the fraction of candidates that are not due to instrumental or stellar noise) is greater than 98%. However, for low signal-to-noise candidates between 200 and 500 days around FGK-dwarf stars, the Robovetter is 76.7% complete and the catalog is 50.5% reliable. The KOI catalog, the transit fits, and all of the simulated data used to characterize this catalog are available at the NASA Exoplanet Archive.
  58. Sacchi, E., Cignoni, M., Aloisi, A., Tosi, M., Calzetti, D., Lee, J., Adamo, A., Annibali, F., Dale, D., Elmegreen, B., Gouliermis, D., Grasha, K., Grebel, E., Hunter, D., Sabbi, E., Smith, L., Thilker, D., Ubeda, L., Whitmore, B., 2018, ApJ, 857, 63, Star Formation Histories of the LEGUS Dwarf Galaxies. II. Spatially Resolved Star Formation History of the Magellanic Irregular NGC 4449
    We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity (D = 3.82 0.27 Mpc), we reach stars 3 mag fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history (SFH) spans the whole Hubble time, but due to the age-metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e., 3 Gyr. The most recent peak of star formation (SF) is around 10 Myr ago. The average surface density SF rate over the whole galaxy lifetime is 0.01 M yr-1 kpc-2. From our study, it emerges that NGC 4449 has experienced a fairly continuous SF regime in the last 1 Gyr, with peaks and dips whose SF rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its SFH does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.

    Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555.

  59. Earle, A., Binzel, R., Young, L., Stern, S., Ennico, K., Grundy, W., Olkin, C., Weaver, H., New Horizons Surface Composition Theme, 2018, Icar, 303, 1, Albedo matters: Understanding runaway albedo variations on Pluto
    The data returned from NASA's New Horizons reconnaissance of the Pluto system show striking albedo variations from polar to equatorial latitudes as well as sharp longitudinal boundaries. Pluto has a high obliquity (currently 119) that varies by 23 over a period of less than 3 million years. This variation, combined with its regressing longitude of perihelion (360 over 3.7 million years), creates epochs of "Super Seasons" where one pole is pointed at the Sun at perihelion, thereby experiencing a short, relatively warm summer followed by its longest possible period of winter darkness. In contrast, the other pole experiences a much longer, less intense summer and a short winter season. We use a simple volatile sublimation and deposition model to explore the relationship between albedo variations, latitude, and volatile sublimation and deposition for the current epoch as well as historical epochs during which Pluto experienced these "Super Seasons." Our investigation quantitatively shows that Pluto's geometry creates the potential for runaway albedo and volatile variations, particularly in the equatorial region, which can sustain stark longitudinal contrasts like the ones we see between Tombaugh Regio and the informally named Cthulhu Regio.
  60. Cignoni, M., Sacchi, E., Aloisi, A., Tosi, M., Calzetti, D., Lee, J., Sabbi, E., Adamo, A., Cook, D., Dale, D., Elmegreen, B., Gallagher, J., Gouliermis, D., Grasha, K., Grebel, E., Hunter, D., Johnson, K., Messa, M., Smith, L., Thilker, D., Ubeda, L., Whitmore, B., 2018, ApJ, 856, 62, Star Formation Histories of the LEGUS Dwarf Galaxies. I. Recent History of NGC 1705, NGC 4449, and Holmberg II
    We use Hubble Space Telescope observations from the Legacy Extragalactic UV Survey to reconstruct the recent star formation histories (SFHs) of three actively star-forming dwarf galaxies, NGC 4449, Holmberg II, and NGC 1705, from their UV color-magnitude diagrams (CMDs). We apply a CMD fitting technique using two independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the uncertainties related to stellar evolution modeling. Irrespective of the adopted stellar models, all three dwarfs are found to have had almost constant star formation rates (SFRs) in the last 100-200 Myr, with modest enhancements (a factor of 2) above the 100 Myr averaged SFR. Significant differences among the three dwarfs are found in terms of the overall SFR, the timing of the most recent peak, and the SFR/area. The initial mass function of NGC 1705 and Holmberg II is consistent with a Salpeter slope down to 5 M , whereas it is slightly flatter, s = -2.0, in NGC 4449. The SFHs derived with the two different sets of stellar models are consistent with each other, except for some quantitative details, attributable to their input assumptions. They also share the drawback that all synthetic diagrams predict a clear separation in color between the upper main-sequence and helium-burning stars, which is not apparent in the data. Since neither differential reddening, which is significant in NGC 4449, nor unresolved binaries appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks.

    Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA Contract NAS 5-26555.

  61. Kahre, L., Walterbos, R., Kim, H., Thilker, D., Calzetti, D., Lee, J., Sabbi, E., Ubeda, L., Aloisi, A., Cignoni, M., Cook, D., Dale, D., Elmegreen, B., Elmegreen, D., Fumagalli, M., Gallagher, J., Gouliermis, D., Grasha, K., Grebel, E., Hunter, D., Sacchi, E., Smith, L., Tosi, M., Adamo, A., Andrews, J., Ashworth, G., Bright, S., Brown, T., Chandar, R., Christian, C., de Mink, S., Dobbs, C., Evans, A., Herrero, A., Johnson, K., Kennicutt, R., Krumholz, M., Messa, M., Nair, P., Nota, A., Pellerin, A., Ryon, J., Schaerer, D., Shabani, F., Van Dyk, S., Whitmore, B., Wofford, A., 2018, ApJ, 855, 133, Extinction Maps and Dust-to-gas Ratios in Nearby Galaxies with LEGUS
    We present a study of the dust-to-gas ratios in five nearby galaxies: NGC 628 (M74), NGC 6503, NGC 7793, UGC 5139 (Holmberg I), and UGC 4305 (Holmberg II). Using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury program Legacy ExtraGalactic UV Survey (LEGUS) combined with archival HST/Advanced Camera for Surveys data, we correct thousands of individual stars for extinction across these five galaxies using an isochrone-matching (reddening-free Q) method. We generate extinction maps for each galaxy from the individual stellar extinctions using both adaptive and fixed resolution techniques and correlate these maps with neutral H I and CO gas maps from the literature, including the H I Nearby Galaxy Survey and the HERA CO-Line Extragalactic Survey. We calculate dust-to-gas ratios and investigate variations in the dust-to-gas ratio with galaxy metallicity. We find a power-law relationship between dust-to-gas ratio and metallicity, consistent with other studies of dust-to-gas ratio compared to metallicity. We find a change in the relation when H2 is not included. This implies that underestimation of {N}{{{H}}2} in low-metallicity dwarfs from a too-low CO-to-H2 conversion factor X CO could have produced too low a slope in the derived relationship between dust-to-gas ratio and metallicity. We also compare our extinctions to those derived from fitting the spectral energy distribution (SED) using the Bayesian Extinction and Stellar Tool for NGC 7793 and find systematically lower extinctions from SED fitting as compared to isochrone matching.
  62. Radick, R., Lockwood, G., Henry, G., Hall, J., Pevtsov, A., 2018, ApJ, 855, 75, Patterns of Variation for the Sun and Sun-like Stars
    We compare patterns of variation for the Sun and 72 Sun-like stars by combining total and spectral solar irradiance measurements between 2003 and 2017 from the SORCE satellite, Stromgren b, y stellar photometry between 1993 and 2017 from Fairborn Observatory, and solar and stellar chromospheric Ca II H+K emission observations between 1992 and 2016 from Lowell Observatory. The new data and their analysis strengthen the relationships found previously between chromospheric and brightness variability on the decadal timescale of the solar activity cycle. Both chromospheric H+K and photometric b, y variability among Sun-like stars are related to average chromospheric activity by power laws on this timescale. Young active stars become fainter as their H+K emission increases, and older, less active, more Sun-age stars tend to show a pattern of direct correlation between photometric and chromospheric emission variations. The directly correlated pattern between total solar irradiance and chromospheric Ca II emission variations shown by the Sun appears to extend also to variations in the Stromgren b, y portion of the solar spectrum. Although the Sun does not differ strongly from its stellar age and spectral class mates in the activity and variability characteristics that we have now studied for over three decades, it may be somewhat unusual in two respects: (1) its comparatively smooth, regular activity cycle, and (2) its rather low photometric brightness variation relative to its chromospheric activity level and variation, perhaps indicating that facular emission and sunspot darkening are especially well-balanced on the Sun.
  63. Sabbi, E., Calzetti, D., Ubeda, L., Adamo, A., Cignoni, M., Thilker, D., Aloisi, A., Elmegreen, B., Elmegreen, D., Gouliermis, D., Grebel, E., Messa, M., Smith, L., Tosi, M., Dolphin, A., Andrews, J., Ashworth, G., Bright, S., Brown, T., Chandar, R., Christian, C., Clayton, G., Cook, D., Dale, D., de Mink, S., Dobbs, C., Evans, A., Fumagalli, M., Gallagher, J., Grasha, K., Herrero, A., Hunter, D., Johnson, K., Kahre, L., Kennicutt, R., Kim, H., Krumholz, M., Lee, J., Lennon, D., Martin, C., Nair, P., Nota, A., Ostlin, G., Pellerin, A., Prieto, J., Regan, M., Ryon, J., Sacchi, E., Schaerer, D., Schiminovich, D., Shabani, F., Van Dyk, S., Walterbos, R., Whitmore, B., Wofford, A., 2018, ApJS, 235, 23, The Resolved Stellar Populations in the LEGUS Galaxies1
    The Legacy ExtraGalactic UV Survey (LEGUS) is a multiwavelength Cycle 21 Treasury program on the Hubble Space Telescope. It studied 50 nearby star-forming galaxies in 5 bands from the near-UV to the I-band, combining new Wide Field Camera 3 observations with archival Advanced Camera for Surveys data. LEGUS was designed to investigate how star formation occurs and develops on both small and large scales, and how it relates to the galactic environments. In this paper we present the photometric catalogs for all the apparently single stars identified in the 50 LEGUS galaxies. Photometric catalogs and mosaicked images for all filters are available for download. We present optical and near-UV color-magnitude diagrams for all the galaxies. For each galaxy we derived the distance from the tip of the red giant branch. We then used the NUV color-magnitude diagrams to identify stars more massive than 14 M , and compared their number with the number of massive stars expected from the GALEX FUV luminosity. Our analysis shows that the fraction of massive stars forming in star clusters and stellar associations is about constant with the star formation rate. This lack of a relation suggests that the timescale for evaporation of unbound structures is comparable or longer than 10 Myr. At low star formation rates this translates to an excess of mass in clustered environments as compared to model predictions of cluster evolution, suggesting that a significant fraction of stars form in unbound systems.

    Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc., under NASA contract NAS 5-26555.

  64. Hunter, D., Gallardo, S., Zhang, H., Adamo, A., Cook, D., Oh, S., Elmegreen, B., Kim, H., Kahre, L., Ubeda, L., Bright, S., Ryon, J., Fumagalli, M., Sacchi, E., Kennicutt, R., Tosi, M., Dale, D., Cignoni, M., Messa, M., Grebel, E., Gouliermis, D., Sabbi, E., Grasha, K., Gallagher, J., Calzetti, D., Lee, J., 2018, ApJ, 855, 7, A Study of Two Dwarf Irregular Galaxies with Asymmetrical Star Formation Distributions
    Two dwarf irregular galaxies, DDO 187 and NGC 3738, exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the H I distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate halves. We find that the pressure and gas density are higher on the HSF sides by 30%-70%. In addition we find in both galaxies that the H I velocity fields exhibit significant deviations from ordered rotation and there are large regions of high-velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there.
  65. Schaefer, G., Prato, L., Simon, M., 2018, AJ, 155, 109, Orbital Motion of Young Binaries in Ophiuchus and Upper Centaurus-Lupus
    We present measurements of the orbital positions and flux ratios of 17 binary and triple systems in the Ophiuchus star-forming region and the Upper Centaurus-Lupus cluster based on adaptive optics imaging at the Keck Observatory. We report the detection of visual companions in MML 50 and MML 53 for the first time, as well as the possible detection of a third component in WSB 21. For six systems in our sample, our measurements provide a second orbital position following their initial discoveries over a decade ago. For eight systems with sufficient orbital coverage, we analyze the range of orbital solutions that fit the data. Ultimately, these observations will help provide the groundwork toward measuring precise masses for these pre-main-sequence stars and understanding the distribution of orbital parameters in young multiple systems.
  66. Le Corre, L., Sanchez, J., Reddy, V., Takir, D., Cloutis, E., Thirouin, A., Becker, K., Li, J., Sugita, S., Tatsumi, E., 2018, MNRAS, 475, 614, Ground-based characterization of Hayabusa2 mission target asteroid 162173 Ryugu: constraining mineralogical composition in preparation for spacecraft operations
    Asteroids that are targets of spacecraft missions are interesting because they present us with an opportunity to validate ground-based spectral observations. One such object is near-Earth asteroid (NEA) (162173) Ryugu, which is the target of the Japanese Space Agency's (JAXA) Hayabusa2 sample return mission. We observed Ryugu using the 3-m NASA Infrared Telescope Facility on Mauna Kea, Hawaii, on 2016 July 13 to constrain the object's surface composition, meteorite analogues, and link to other asteroids in the main belt and NEA populations. We also modelled its photometric properties using archival data. Using the Lommel-Seeliger model we computed the predicted flux for Ryugu at a wide range of viewing geometries as well as albedo quantities such as geometric albedo, phase integral, and spherical Bond albedo. Our computed albedo quantities are consistent with results from Ishiguro et al. Our spectral analysis has found a near-perfect match between our spectrum of Ryugu and those of NEA (85275) 1994 LY and Mars-crossing asteroid (316720) 1998 BE7, suggesting that their surface regoliths have similar composition. We compared Ryugu's spectrum with that of main belt asteroid (302) Clarissa, the largest asteroid in the Clarissa asteroid family, suggested as a possible source of Ryugu by Campins et al. We found that the spectrum of Clarissa shows significant differences with our spectrum of Ryugu, but it is similar to the spectrum obtained by Moskovitz et al. The best possible meteorite analogues for our spectrum of Ryugu are two CM2 carbonaceous chondrites, Mighei and ALH83100.
  67. Hindson, L., Kitchener, G., Brinks, E., Heesen, V., Westcott, J., Hunter, D., Zhang, H., Rupen, M., Rau, U., 2018, ApJS, 234, 29, A Radio Continuum Study of Dwarf Galaxies: 6 cm Imaging of LITTLE THINGS
    In this paper, we examine to what extent the radio continuum can be used as an extinction-free probe of star formation in dwarf galaxies. To that aim, we observe 40 nearby dwarf galaxies with the Very Large Array at 6 cm (4-8 GHz) in C-configuration. We obtained images with 3-8 resolution and noise levels of 3-15 Jy beam-1. We detected emission associated with 22 of the 40 dwarf galaxies, eight of which are new detections. The general picture is that of an interstellar medium largely devoid of radio continuum emission, interspersed by isolated pockets of emission associated with star formation. We find an average thermal fraction of 50%-70% and an average magnetic field strength of 5-8 G, only slightly lower than that found in larger, spiral galaxies. At 100 pc scales, we find surprisingly high values for the average magnetic field strength of up to 50 G. We find that dwarf galaxies follow the theoretical predictions of the radio continuum-star formation rate relation within regions of significant radio continuum emission but that the nonthermal radio continuum is suppressed relative to the star formation rate when considering the entire optical disk. We examine the far-infrared-star formation rate relation for our sample and find that the far-infrared is suppressed compared to the expected star formation rate. We discuss explanations for these observed relations and the impact of our findings on the radio continuum-far-infrared relation. We conclude that radio continuum emission at centimeter wavelengths has the promise of being a largely extinction-free star formation rate indicator. We find that star formation rates of gas-rich, low-mass galaxies can be estimated with an uncertainty of 0.2 dex between the values of 2 10-4 and 0.1 M yr-1.
  68. Oszkiewicz, D., Skiff, B., Moskovitz, N., Kankiewicz, P., Marciniak, A., Licandro, J., Galiazzo, M., Zeilinger, W., 2018, A&A, 610, C3, Non-Vestoid candidate asteroids in the inner main belt (Corrigendum)
  69. Davis, B., Ciardullo, R., Feldmeier, J., Jacoby, G., 2018, RNAAS, 2, 32, The Planetary Nebula Luminosity Function (PNLF): Contamination from Supernova Remnants
    The planetary nebula luminosity function (PNLF) has been used as an extragalactic distance indicator since the 1980's, but there are still unsolved problems associated with its use. One of the most serious involves PNLF distances beyond ~ 10 Mpc, which tend to be slightly smaller than those of other methods. We consider the implications of previous spectroscopic investigations that found that several of the brightest planetary nebula (PN) candidates in M74 are actually compact supernova remnants (SNRs). Using narrow-band imaging data from the KPNO 4-m telescope, we measure the [O III] $\lambda$5007 and H$\alpha$ fluxes of all the known SNRs in M31 and M33, and test whether those objects could be misidentified as bright PNe at distances beyond ~ 10 Mpc. Our results suggest that compact SNRs are not an important source of contamination in photometric surveys for extragalactic PNe.
  70. Mace, G., Mann, A., Skiff, B., Sneden, C., Kirkpatrick, J., Schneider, A., Kidder, B., Gosnell, N., Kim, H., Mulligan, B., Prato, L., Jaffe, D., 2018, ApJ, 854, 145, Wolf 1130: A Nearby Triple System Containing a Cool, Ultramassive White Dwarf
    Following the discovery of the T8 subdwarf WISE J200520.38+542433.9 (Wolf 1130C), which has a proper motion in common with a binary (Wolf 1130AB) consisting of an M subdwarf and a white dwarf, we set out to learn more about the old binary in the system. We find that the A and B components of Wolf 1130 are tidally locked, which is revealed by the coherence of more than a year of V-band photometry phase-folded to the derived orbital period of 0.4967 days. Forty new high-resolution, near-infrared spectra obtained with the Immersion Grating Infrared Spectrometer provide radial velocities and a projected rotational velocity (v sin i) of 14.7 0.7 {km} {{{s}}}-1 for the M subdwarf. In tandem with a Gaia parallax-derived radius and verified tidal locking, we calculate an inclination of i = 29 2. From the single-lined orbital solution and the inclination we derive an absolute mass for the unseen primary ({1.24}-0.15+0.19 M ). Its non-detection between 0.2 and 2.5 m implies that it is an old (>3.7 Gyr) and cool (T eff < 7000 K) ONe white dwarf. This is the first ultramassive white dwarf within 25 pc. The evolution of Wolf 1130AB into a cataclysmic variable is inevitable, making it a potential SN Ia progenitor. The formation of a triple system with a primary mass >100 times the tertiary mass and the survival of the system through the common-envelope phase, where 80% of the system mass was lost, is remarkable. Our analysis of Wolf 1130 allows us to infer its formation and evolutionary history, which has unique implications for understanding low-mass star and brown dwarf formation around intermediate-mass stars.
  71. Marciniak, A., Bartczak, P., Muller, T., Sanabria, J., Ali-Lagoa, V., Antonini, P., Behrend, R., Bernasconi, L., Bronikowska, M., Butkiewicz-Bak, M., Cikota, A., Crippa, R., Ditteon, R., Dudzinski, G., Duffard, R., Dziadura, K., Fauvaud, S., Geier, S., Hirsch, R., Horbowicz, J., Hren, M., Jerosimic, L., Kaminski, K., Kankiewicz, P., Konstanciak, I., Korlevic, P., Kosturkiewicz, E., Kudak, V., Manzini, F., Morales, N., Murawiecka, M., Ogoza, W., Oszkiewicz, D., Pilcher, F., Polakis, T., Poncy, R., Santana-Ros, T., Siwak, M., Skiff, B., Sobkowiak, K., Stoss, R., Zejmo, M., Zukowski, K., 2018, A&A, 610, A7, Photometric survey, modelling, and scaling of long-period and low-amplitude asteroids
    Context. The available set of spin and shape modelled asteroids is strongly biased against slowly rotating targets and those with low lightcurve amplitudes. This is due to the observing selection effects. As a consequence, the current picture of asteroid spin axis distribution, rotation rates, radiometric properties, or aspects related to the object's internal structure might be affected too.
    Aims: To counteract these selection effects, we are running a photometric campaign of a large sample of main belt asteroids omitted in most previous studies. Using least chi-squared fitting we determined synodic rotation periods and verified previous determinations. When a dataset for a given target was sufficiently large and varied, we performed spin and shape modelling with two different methods to compare their performance.
    Methods: We used the convex inversion method and the non-convex SAGE algorithm, applied on the same datasets of dense lightcurves. Both methods search for the lowest deviations between observed and modelled lightcurves, though using different approaches. Unlike convex inversion, the SAGE method allows for the existence of valleys and indentations on the shapes based only on lightcurves.
    Results: We obtain detailed spin and shape models for the first five targets of our sample: (159) Aemilia, (227) Philosophia, (329) Svea, (478) Tergeste, and (487) Venetia. When compared to stellar occultation chords, our models obtained an absolute size scale and major topographic features of the shape models were also confirmed. When applied to thermophysical modelling (TPM), they provided a very good fit to the infrared data and allowed their size, albedo, and thermal inertia to be determined.
    Conclusions: Convex and non-convex shape models provide comparable fits to lightcurves. However, some non-convex models fit notably better to stellar occultation chords and to infrared data in sophisticated thermophysical modelling (TPM). In some cases TPM showed strong preference for one of the spin and shape solutions. Also, we confirmed that slowly rotating asteroids tend to have higher-than-average values of thermal inertia, which might be caused by properties of the surface layers underlying the skin depth.

    The photometric data is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr ( or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/610/A7

  72. Scholz, A., Llama, J., Muzic, K., Faller, S., Froebrich, D., Stelzer, B., 2018, RNAAS, 2, 27, Discovery of a Magnetic White Dwarf with Unusual Short-period Variability
    We report the discovery of a magnetic white dwarf which shows periodic variability with P=110 min, color-dependent amplitudes and a transient phase shift in the blue compared to the red lightcurve - a previously unknown type of variability for this type of object. We attribute the variations either to a close ultracool (thus far undetected) companion or, more likely, to magnetic spots with unusual temperature structure.
  73. Biddle, L., Johns-Krull, C., Llama, J., Prato, L., Skiff, B., 2018, ApJL, 853, L34, K2 Reveals Pulsed Accretion Driven by the 2 Myr Old Hot Jupiter CI Tau b
    CI Tau is a young (2 Myr) classical T Tauri star located in the Taurus star-forming region. Radial velocity observations indicate it hosts a Jupiter-sized planet with an orbital period of approximately 9 days. In this work, we analyze time series of CI Taus photometric variability as seen by K2. The light curve reveals the stellar rotation period to be 6.6 days. Although there is no evidence that CI Tau b transits the host star, a 9 day signature is also present in the light curve. We believe this is most likely caused by planet-disk interactions that perturb the accretion flow onto the star, resulting in a periodic modulation of the brightness with the 9 day period of the planets orbit.
  74. Llama, J., Jardine, M., Wood, K., Hallinan, G., Morin, J., 2018, ApJ, 854, 7, Simulating Radio Emission from Low-mass Stars
    Understanding the origins of stellar radio emission can provide invaluable insight into the strength and geometry of stellar magnetic fields and the resultant space weather environment experienced by exoplanets. Here, we present the first model capable of predicting radio emission through the electron cyclotron maser instability using observed stellar magnetic maps of low-mass stars. We determine the structure of the coronal magnetic field and plasma using spectropolarimetric observations of the surface magnetic fields and the X-ray emission measure. We then model the emission of photons from the locations within the corona that satisfy the conditions for electron cyclotron maser emission. Our model predicts the frequency and intensity of radio photons from within the stellar corona. We have benchmarked our model against the low-mass star V374 Peg. This star has both radio observations from the Very Large Array and a nearly simultaneous magnetic map. Using our model we are able to fit the radio observations of V374 Peg, providing additional evidence that the radio emission observed from low-mass stars may originate from the electron cyclotron maser instability. Our model can now be extended to all stars with observed magnetic maps to predict the expected frequency and variability of stellar radio emission in an effort to understand and guide future radio observations of low-mass stars.
  75. Jones, G., Knight, M., Battams, K., Boice, D., Brown, J., Giordano, S., Raymond, J., Snodgrass, C., Steckloff, J., Weissman, P., Fitzsimmons, A., Lisse, C., Opitom, C., Birkett, K., Bzowski, M., Decock, A., Mann, I., Ramanjooloo, Y., McCauley, P., 2018, SSRv, 214, 20, The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
    This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun's centre, equal to half of Mercury's perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics.
  76. Baines, E., Armstrong, J., Schmitt, H., Zavala, R., Benson, J., Hutter, D., Tycner, C., van Belle, G., 2018, AJ, 155, 30, Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer
    We present the fundamental properties of 87 stars based on angular diameter measurements from the Navy Precision Optical Interferometer, 36 of which have not been measured previously using interferometry. Our sample consists of 5 dwarfs, 3 subgiants, 69 giants, 3 bright giants, and 7 supergiants, and span a wide range of spectral classes from B to M. We combined our angular diameters with photometric and distance information from the literature to determine each stars physical radius, effective temperature, bolometric flux, luminosity, mass, and age.
  77. Robbins, S., Runyon, K., Singer, K., Bray, V., Beyer, R., Schenk, P., McKinnon, W., Grundy, W., Nimmo, F., Moore, J., Spencer, J., White, O., Binzel, R., Buie, M., Buratti, B., Cheng, A., Linscott, I., Reitsema, H., Reuter, D., Showalter, M., Tyler, G., Young, L., Olkin, C., Ennico, K., Weaver, H., Stern, S., 2018, JGRE, 123, 20, Investigation of Charon's Craters With Abrupt Terminus Ejecta, Comparisons With Other Icy Bodies, and Formation Implications
    On the moon and other airless bodies, ballistically emplaced ejecta transitions from a thinning, continuous inner deposit to become discontinuous beyond approximately one crater radius from the crater rim and can further break into discrete rays and secondary craters. In contrast, on Mars, ejecta often form continuous, distinct, and sometimes thick deposits that transition to a low ridge or escarpment that may be circular or lobate. The Martian ejecta type has been variously termed pancake, rampart, lobate, or layered, and in this work we refer to it as "abrupt termini" ejecta (ATE). Two main formation mechanisms have been proposed, one requiring interaction of the ejecta with the atmosphere and the other mobilization of near-surface volatiles. ATE morphologies are also unambiguously seen on Ganymede, Europa, Dione, and Tethys, but they are not as common as on Mars. We have identified up to 38 craters on Charon that show signs of ATE, including possible distal ramparts and lobate margins. These ejecta show morphologic and morphometric similarities with other moons in the solar system, which are a subset of the properties observed on Mars. From comparison of these ejecta on Charon and other solar system bodies, we find the strongest support for subsurface volatile mobilization and ejecta fluidization as the main formation mechanism for the ATE, at least on airless, icy worlds. This conclusion comes from the bodies on which they are found, an apparent preference for certain terrains, and the observation that craters with ATE can be near to similarly sized craters that only have gradational ejecta.
  78. Boyajian, T., Alonso, R., Ammerman, A., Armstrong, D., Asensio Ramos, A., Barkaoui, K., Beatty, T., Benkhaldoun, Z., Benni, P., Bentley, R., Berdyugin, A., Berdyugina, S., Bergeron, S., Bieryla, A., Blain, M., Capetillo Blanco, A., Bodman, E., Boucher, A., Bradley, M., Brincat, S., Brink, T., Briol, J., Brown, D., Budaj, J., Burdanov, A., Cale, B., Aznar Carbo, M., Castillo Garcia, R., Clark, W., Clayton, G., Clem, J., Coker, P., Cook, E., Copperwheat, C., Curtis, J., Cutri, R., Cseh, B., Cynamon, C., Daniels, A., Davenport, J., Deeg, H., De Lorenzo, R., de Jaeger, T., Desrosiers, J., Dolan, J., Dowhos, D., Dubois, F., Durkee, R., Dvorak, S., Easley, L., Edwards, N., Ellis, T., Erdelyi, E., Ertel, S., Farfan, R., Farihi, J., Filippenko, A., Foxell, E., Gandolfi, D., Garcia, F., Giddens, F., Gillon, M., Gonzalez-Carballo, J., Gonzalez-Fernandez, C., Gonzalez Hernandez, J., Graham, K., Greene, K., Gregorio, J., Hallakoun, N., Hanyecz, O., Harp, G., Henry, G., Herrero, E., Hildbold, C., Hinzel, D., Holgado, G., Ignacz, B., Ilyin, I., Ivanov, V., Jehin, E., Jermak, H., Johnston, S., Kafka, S., Kalup, C., Kardasis, E., Kaspi, S., Kennedy, G., Kiefer, F., Kielty, C., Kessler, D., Kiiskinen, H., Killestein, T., King, R., Kollar, V., Korhonen, H., Kotnik, C., Konyves-Toth, R., Kriskovics, L., Krumm, N., Krushinsky, V., Kundra, E., Lachapelle, F., LaCourse, D., Lake, P., Lam, K., Lamb, G., Lane, D., Lau, M., Lewin, P., Lintott, C., Lisse, C., Logie, L., Longeard, N., Lopez Villanueva, M., Whit Ludington, E., Mainzer, A., Malo, L., Maloney, C., Mann, A., Mantero, A., Marengo, M., Marchant, J., Martinez Gonzalez, M., Masiero, J., Mauerhan, J., McCormac, J., McNeely, A., Meng, H., Miller, M., Molnar, L., Morales, J., Morris, B., Muterspaugh, M., Nespral, D., Nugent, C., Nugent, K., Odasso, A., O'Keeffe, D., Oksanen, A., O'Meara, J., Ordasi, A., Osborn, H., Ott, J., Parks, J., Rodriguez Perez, D., Petriew, V., Pickard, R., Pal, A., Plavchan, P., Pollacco, D., Pozo Nunez, F., Pozuelos, F., Rau, S., Redfield, S., Relles, H., Ribas, I., Richards, J., Saario, J., Safron, E., Sallai, J., Sarneczky, K., Schaefer, B., Schumer, C., Schwartzendruber, M., Siegel, M., Siemion, A., Simmons, B., Simon, J., Simon-Diaz, S., Sitko, M., Socas-Navarro, H., Sodor, A., Starkey, D., Steele, I., Stone, G., Strassmeier, K., Street, R., Sullivan, T., Suomela, J., Swift, J., Szabo, G., Szabo, R., Szakats, R., Szalai, T., Tanner, A., Toledo-Padron, B., Tordai, T., Triaud, A., Turner, J., Ulowetz, J., Urbanik, M., Vanaverbeke, S., Vanderburg, A., Vida, K., Vietje, B., Vinko, J., von Braun, K., Waagen, E., Walsh, D., Watson, C., Weir, R., Wenzel, K., Westendorp Plaza, C., Williamson, M., Wright, J., Wyatt, M., Zheng, W., Zsidi, G., 2018, ApJL, 853, L8, The First Post-Kepler Brightness Dips of KIC 8462852
    We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in 2015 October, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1%-2.5% dips, named Elsie, Celeste, Skara Brae, and Angkor, which persist on timescales from several days to weeks. Our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale 1 m, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term secular dimming, which may be caused by independent processes, or probe different regimes of a single process.
  79. Hunter, D., Melton, C., Leshin, S., Wong, A., Clark, M., Kamienski, J., Moriya, N., Packwood, B., Birket, B., Edwards, W., Millward, M., Wheelband, I., 2018, ApJ, 852, 114, Wide-field Imaging of the Environments of LITTLE THINGS Dwarf Irregular Galaxies
    We have obtained wide-field images of 36 of the 41 LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H I Nearby Galaxy Survey) nearby (<10.3 Mpc) dwarf irregular and blue compact dwarf galaxies. Although the LITTLE THINGS galaxies were chosen to be non-interacting and no companions were found in H I imaging, the purpose of this imaging was to search for optical companion galaxies that had been missed in imaging with smaller fields of view and that might indicate an external factor in ongoing star formation. The limiting magnitudes of the images range from 19.7 to 28.3 mag arcsec-2, with a median value of 25.9 mag arcsec-2. We did not find any unknown companions. Two of the LITTLE THINGS galaxies, NGC 4163 and NGC 4214, and the fainter dwarf, UGCA 276, lie potentially within 100 kpc of each other, but our imaging does not reveal any stellar bridge between the galaxies. This project was part of the Lowell Amateur Research Initiative.
  80. Rector, T., Shuping, R., Prato, L., Schweiker, H., 2018, ApJ, 852, 13, The Discovery of Herbig-Haro Objects in LDN 673
    We report the discovery of 12 faint Herbig-Haro (HH) objects in LDN 673 found using a novel color-composite imaging method that reveals faint H emission in complex environments. Follow-up observations in [S II] confirmed their classification as HH objects. Potential driving sources are identified from the Spitzer c2d Legacy Program catalog and other infrared observations. The 12 new HH objects can be divided into three groups: four are likely associated with a cluster of eight young stellar object class I/II IR sources that lie between them; five are colinear with the T Tauri multiple star system AS 353, and are likely driven by the same source as HH 32 and HH 332 and three are bisected by a very red source that coincides with an infrared dark cloud. We also provide updated coordinates for the three components of HH 332. Inaccurate numbers were given for this object in the discovery paper. The discovery of HH objects and associated driving sources in this region provides new evidence for star formation in the Aquila clouds, implying a much larger T Tauri population in a seldom-studied region.
  81. Prato, L., Ruiz-Rodriguez, D., Wasserman, L., 2018, ApJ, 852, 38, Orbital Solution for the Spectroscopic Binary in the GW Ori Hierarchical Triple
    We present the first double-lined orbital solution for the close binary in the GW Ori triple system. Using 12 epochs of infrared spectroscopy, we detected the lines of both stars in the inner pair, previously known as single-lined only. Our preliminary infrared orbital solution has an eccentricity of e = 0.21 0.10, a period of P = 241.15 0.72 days, and a mass ratio of q = 0.66 0.13. We find a larger semi-amplitude for the primary star, K1 = 6.57 1.00 km s-1, with an infrared-only solution compared to K1 = 4.41 0.33 km s-1 with optical data from the literature, likely the result of line blending and veiling in the optical. The component spectral types correspond to G3 and K0 stars, with v\sin i values of 43 km s-1 and 50 km s-1, respectively. We obtained a flux ratio of = 0.58 0.14 in the H-band, allowing us to estimate individual masses of 3.2 and 2.7 M for the primary and secondary, respectively, using evolutionary tracks. The tracks also yield a coeval age of 1 Myr for both components to within 1. GW Ori is surrounded by a circumbinary/circumtriple disk. A tertiary component has been detected in previous studies; however, we did not detect this component in our near-infrared spectra, probably the result of its relative faintness and blending in the absorption lines of these rapidly rotating stars. With these results, GW Ori joins the small number of classical T Tauri, double-lined spectroscopic binaries.
  82. Adams, A., Boyajian, T., von Braun, K., 2018, MNRAS, 473, 3608, Predicting stellar angular diameters from V, IC, H and K photometry
    Determining the physical properties of microlensing events depends on having accurate angular sizes of the source star. Using long baseline optical interferometry, we are able to measure the angular sizes of nearby stars with uncertainties 2 per cent. We present empirically derived relations of angular diameters which are calibrated using both a sample of dwarfs/subgiants and a sample of giant stars. These relations are functions of five colour indices in the visible and near-infrared, and have uncertainties of 1.8-6.5 per cent depending on the colour used. We find that a combined sample of both main-sequence and evolved stars of A-K spectral types is well fitted by a single relation for each colour considered. We find that in the colours considered, metallicity does not play a statistically significant role in predicting stellar size, leading to a means of predicting observed sizes of stars from colour alone.
  83. Dalle Ore, C., Protopapa, S., Cook, J., Grundy, W., Cruikshank, D., Verbiscer, A., Ennico, K., Olkin, C., Stern, S., Weaver, H., Young, L., New Horizons Science Team, 2018, Icar, 300, 21, Ices on Charon: Distribution of H2O and NH3 from New Horizons LEISA observations
    Charon, the largest moon of Pluto, appeared as a fairly homogeneous, gray, icy world to New Horizons during closest approach on July 14th, 2015. Charon's sub-Pluto hemisphere was scanned by the Ralph/LEISA near-IR spectrograph providing an unprecedented opportunity to measure its surface composition. We apply a statistical clustering tool to identify spectrally distinct terrains and a radiative transfer approach to study the variations of the 2.0-m H2O ice band. We map the distribution of the ices previously reported to be present on Charon's surface, namely H2O and the products of NH3 in H2O. We find that H2O ice is mostly in the crystalline phase, confirming previous studies. The regions with the darkest albedos show the strongest signature of amorphous-phase ice, although the crystalline component is still strong. The brighter albedo regions, often corresponding to crater ejecta blankets, are characterized by larger H2O grains, possibly an indication of a younger age. We observe two different behaviors for the two absorption bands representing NH3 in H2O. The 2.21-m band tends to cluster more in the northern areas compared to the 2.01-m band. Both bands are present in the brighter crater rays, but not all craters show both bands. The 2.21-m band is also clearly present on the smaller moons Hydra and Nix. These results hint that different physical conditions may determine the appearance or absence of these two different forms of NH3 in H2O ice in the Pluto system. We also investigate the blue slope affecting the spectrum at wavelengths longer than 1.8 m previously reported by several authors. We find that the slope is common among the objects in the Pluto system, Charon, the smaller moons Nix and Hydra, and the darkest terrains on Pluto. It also characterizes the analog ice tholin obtained from irradiation of Pluto-specific materials (a mixture of N2, CH4, and CO ices) in the laboratory. Our modeling results show that Pluto ice tholins are widespread almost uniformly on Charon suggesting a common distribution possibly part of the original reservoir of materials that made up Charon. This was irradiated over the years to yield the gray color characteristic of Charon today. On top of the 'primordial' Pluto ice tholin there is the redder component produced by irradiation of the CH4 provided by Pluto's atmospheric contribution as illustrated by Grundy et al. (2016a).
  84. Moore, J., Howard, A., Umurhan, O., White, O., Schenk, P., Beyer, R., McKinnon, W., Spencer, J., Singer, K., Grundy, W., Earle, A., Schmitt, B., Protopapa, S., Nimmo, F., Cruikshank, D., Hinson, D., Young, L., Stern, S., Weaver, H., Olkin, C., Ennico, K., Collins, G., Bertrand, T., Forget, F., Scipioni, F., New Horizons Science Team, 2018, Icar, 300, 129, Bladed Terrain on Pluto: Possible origins and evolution
    Bladed Terrain on Pluto consists of deposits of massive CH4, which are observed to occur within latitudes 30 of the equator and are found almost exclusively at the highest elevations (> 2 km above the mean radius). Our analysis indicates that these deposits of CH4 preferentially precipitate at low latitudes where net annual solar energy input is lowest. CH4 and N2 will both precipitate at low elevations. However, since there is much more N2 in the atmosphere than CH4, the N2 ice will dominate at these low elevations. At high elevations the atmosphere is too warm for N2 to precipitate so only CH4 can do so. We conclude that following the time of massive CH4 emplacement; there have been sufficient excursions in Pluto's climate to partially erode these deposits via sublimation into the blades we see today. Blades composed of massive CH4 ice implies that the mechanical behavior of CH4 can support at least several hundred meters of relief at Pluto surface conditions. Bladed Terrain deposits may be widespread in the low latitudes of the poorly seen sub-Charon hemisphere, based on spectral observations. If these locations are indeed Bladed Terrain deposits, they may mark heretofore unrecognized regions of high elevation.
  85. Harper, D., Runyan, M., Dowell, C., Wirth, C., Amato, M., Ames, T., Amiri, M., Banks, S., Bartels, A., Benford, D., Berthoud, M., Buchanan, E., Casey, S., Chapman, N., Chuss, D., Cook, B., Derro, R., Dotson, J., Evans, R., Fixsen, D., Gatley, I., Guerra, J., Halpern, M., Hamilton, R., Hamlin, L., Hansen, C., Heimsath, S., Hermida, A., Hilton, G., Hirsch, R., Hollister, M., Hostetter, C., Irwin, K., Jhabvala, C., Jhabvala, M., Kastner, J., Kovacs, A., Lin, S., Loewenstein, R., Looney, L., Lopez-Rodriguez, E., Maher, S., Michail, J., Miller, T., Moseley, S., Novak, G., Pernic, R., Rennick, T., Rhody, H., Sandberg, E., Sandford, D., Santos, F., Shafer, R., Sharp, E., Shirron, P., Siah, J., Silverberg, R., Sparr, L., Spotz, R., Staguhn, J., Toorian, A., Towey, S., Tuttle, J., Vaillancourt, J., Voellmer, G., Volpert, C., Wang, S., Wollack, E., 2018, JAI, 7, 1840008-1025, HAWC+, the Far-Infrared Camera and Polarimeter for SOFIA
    High-resolution Airborne Wide-band Camera (HAWC+) is the facility far-infrared imager and polarimeter for SOFIA, NASAs Stratospheric Observatory for Infrared Astronomy. It is designed to cover the portion of the infrared spectrum that is completely inaccessible to ground-based observatories and which is essential for studies of astronomical sources with temperatures between tens and hundreds of degrees Kelvin. Its ability to make polarimetric measurements of aligned dust grains provides a unique new capability for studying interstellar magnetic fields. HAWC+ began commissioning flights in April 2016 and was accepted as a facility instrument in early 2018. In this paper, we describe the instrument, its operational procedures, and its performance on the observatory.
  86. Shkolnik, E., Llama, J., 2018, haex, 20, Signatures of Star-Planet Interactions
    Planets interact with their host stars through gravity, radiation, and magnetic fields, and for those giant planets that orbit their stars within 10 stellar radii (0.1 AU for a sun-like star), star-planet interactions (SPI) are observable with a wide variety of photometric, spectroscopic, and spectropolarimetric studies. At such close distances, the planet orbits within the sub-Alfvenic radius of the star in which the transfer of energy and angular momentum between the two bodies is particularly efficient. The magnetic interactions appear as enhanced stellar activity modulated by the planet as it orbits the star rather than only by stellar rotation. These SPI effects are informative for the study of the internal dynamics and atmospheric evolution of exoplanets. The nature of magnetic SPI is modeled to be strongly affected by both the stellar and planetary magnetic fields, possibly influencing the magnetic activity of both, as well as affecting the irradiation and even the migration of the planet and rotational evolution of the star. As phase-resolved observational techniques are applied to a large statistical sample of hot Jupiter systems, extensions to other tightly orbiting stellar systems, such as smaller planets close to M dwarfs become possible. In these systems, star-planet separations of tens of stellar radii begin to coincide with the radiative habitable zone where planetary magnetic fields are likely a necessary condition for surface habitability.
  87. Dunham, E., 2018, haex, 170, Discovery of the First Transiting Planets
    Early thinking about detecting extrasolar planets was largely circumscribed by the expectation that other solar systems would be similar to our own, the only known example at the time. Given this mind-set, transit detections were expected to be exceedingly difficult for small planets and rarely seen for larger ones. The discovery of 51 Peg and subsequent hot Jupiters by the radial velocity method completely upended our thinking - transits were suddenly practical, perhaps even easy! This immediately led to follow-up searches for transits in systems discovered by the radial velocity technique and, conversely, to wide-field ground-based transit search programs with radial velocity follow-up observations. As is usually the case, transit work turned out to be harder than initially expected but was still possible and productive. This chapter reviews the circumstances leading to the first transit observations of HD 209458, the early OGLE exoplanets, and TrES-1 and TrES-2, as well as some of the frustrations and difficulties encountered along the way.
  88. Olkin, C., Grundy, W., 2018, oss, 3, A Survey of Pluto's Surface Composition
  89. 88 publications and 2067 citations in 2018.

88 publications and 2067 citations total.