Poster Abstracts

Name/Affiliation:  Parke Loyd (Center for Astrophysics and Space Astronomy, University of Colorado at Boulder)

Title:  Fluctuations and Flares in Stellar UV Emission Observed by HST and GALEX with Implications for Exoplanet Transit Observations

The intensity of ultraviolet (UV) stellar continuum and line emission varies on timescales as short as minutes or even seconds. This variability can originate from a host of physical phenomena at play in the stellar atmosphere. Stochastically fluctuating and flaring stellar emission is important because (1) it reflects the degree of magnetic turmoil in the starís atmosphere and (2) it adds uncertainty to and may prohibit (spectro)photometric measurements of exoplanet transits. This latter is especially important at the wavelengths of strong resonant absorption by species in a planetís extended atmosphere. We present our recently published analysis of stochastic fluctuations and flares in the integrated C II ??1334,1335; Si III ?1206; Si IV ??1393,1402; and interspersed far-UV continuum emission of 38 F-M stars conducted with archival HST photon-event data binned to a 60 s cadence. To identify flares, we employed a cross-correlation algorithm that detected 116 events, over 50% lasting 4 min or less, occurring roughly once per 2.5 h. To quantify stochastic fluctuations, we computed the maximum-likelihood standard deviation of the lightcurve scatter above that attributable to Poisson noise, finding that it ranged from < 1% to 41%. We also present the preliminary results of an analysis of GALEX broadband far-UV and near-UV photon-event data for Kepler field stars. This analysis will ultimately include an expansive stellar sample enabling several important inquiries. Among these, we will quantify the generally accepted assumption that stellar activity traces the amplitude of UV time-variability. We will also search for evidence of star-planet interactions through correlations between the amplitude of a starís UV variability and the presence of closely orbiting planets.