17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Formation and Evolution of Cool Stars and Brown Dwarfs

Abstract
Direct detections of young exoplanets are now strengthening the link between the exoplanet and brown dwarf populations, enriching our understanding of both classes of objects. At the same time, recent exoplanet discoveries display puzzling spectrophotometric properties, with exceptionally red colors, significantly underluminous magnitudes, and peculiar near-IR spectra that are unlike most field brown dwarfs. A promising approach to understanding these objects is to identify their free-floating analogs in the solar neighborhood. Some possible examples are the rare late-M and L-type field dwarfs with very red, dusty photospheres and/or signs of low surface gravity (i.e. a young age) in their spectra. We analyze the physical properties (distances, temperatures, and ages) of the youngest field ultracool dwarfs, thanks to the first parallaxes for this intriguing class of objects. We report high precision distances for 30 objects with a median parallax RMS of 1.5 milliarcsec, including some of the best-known red, young, and/or dusty L dwarf archetypes. We demonstrate that the peculiar underluminous magnitudes and very red colors of the planetary-mass companions 2MASS~J1207$-$39b and HR 8799bcde do occur in some young brown dwarfs, but at the same time these properties do not have a simple correspondance with age. To make further progress, we derive accurate space motions for our sample and demonstrate that at least some are robustly asssociated with known young (10--100 Myr) moving groups, such as beta Pic, AB Dor, and Tuc-Hor. Such linkages allow us to assign precise ages to young brown dwarfs and thereby empirically calibrate the time-depdendence of low-gravity spectral features. These objects are also the first free-floating L dwarfs (~10 Mjup) found in young moving groups and thus serve as key age benchmarks for testing theoretical models. Finally, we discuss the degeneracies between low surface gravity, youth, and photospheric dust content that will be a challenge to interpreting young gas-giant exoplanets.