Characterizing Young Brown Dwarfs Using Low‐Resolution Near‐Infrared Spectra

Abstract

We present near-infrared (1.0-2.4 � m) spectra confirming the youth and cool temperatures of six brown dwarfs and low-mass stars with circumstellar disks toward the Chamaeleon II and Ophiuchus star-forming regions. The spectrum of one of our objects indicates a spectral type ofL1, making it one of the latest spectral type young brown dwarfs identified todate.Comparingspectraof youngbrown dwarfs,fielddwarfs, andgiant stars, wedefinea1.49-1.56 � m H2Oindexcapableofdeterminingspectraltypeto � 1subtype,independentofgravity.Wehavealsodefinedanindex basedonthe1.14� msodiumfeaturethatissensitivetogravity,butonlyweaklydependentonspectraltype.Our1.14 � m Na index can be used to distinguish youngcluster members (� P5Myr) from youngfield dwarfs, both of whichmay have the triangular H-band continuum shape that persists for at least tens of Myr. Using TeA values determined from thespectraltypes ofourobjectsalongwithluminosities derivedfromnearandmid-infrared photometry,weplaceour objectsontheH-Rdiagramandoverlayevolutionarymodelstoestimatethemassesandagesofouryoungsources.Three of our sources haveinferredages (� '10-30 Myr) that are significantly older than the median stellar age of their parent clouds (1-3 Myr). For these three objects, we derive masses � 3 times greater than expected for 1-3 Myr old brown dwarfs with thebolometric luminosities of our sources.The largediscrepancies inthe inferred masses and agesdeter- mined using two separate, yet reasonable, methods emphasize the need for caution when deriving or exploiting brown dwarf mass and age estimates. Subject headingg infrared: stars — stars: formation — stars: low-mass, brown dwarfs