Name/Affiliation: Michael Liu (University of Hawaii)
Ultracool Dwarfs in the Extended Solar Neighborhood as Exoplanet Analogs
Recent identification of young field brown dwarfs has opened a new opportunity to enrich our understanding of both free-floating substellar objects and directly imaged young gas-giant planets. However, studies to date have typically focused on individual discoveries. We present a large comprehensive study of this rare and intriguing population, comprising both previously known objects and our new discoveries from the latest wide-field sky surveys (Pan-STARRS-1 and WISE). With masses now extending down to ~5 Jupiter mssses, these objects directly overlap young gas-giant planets and thus are promising analogs for studying exoplanet atmospheres at high S/N and spectral resolution. We combine high-quality spectra and parallaxes to study their spectral energy distributions, luminosities, temperatures, and ages. We demonstrate that the peculiar IR fluxes of young gas-giant planets like those around HR 8799 do occur in some young brown dwarfs, but these properties do not have a simple correspondence with age. We find young field brown dwarfs can have unusally low temperatures, but they are not underluminous, as sometimes claimed. To help provide a reference for upcoming extreme-contrast planet imaging surveys, we establish a grid of spectral standards and benchmarks, in order to calibrate gravity (age/mass) and temperature diagnostics from near-IR spectroscopy. Finally, we use our data to critically examine the possibility that free-floating objects and substellar companions may have different evolutionary histories, thereby complicating the brown dwarf-exoplanet connection.