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

We present near-infrared (1.0-2.4 micron) spectra confirming the youth andcool effective temperatures of 6 brown dwarfs and low mass stars withcircumstellar disks toward the Chamaeleon II and Ophiuchus star formingregions. The spectrum of one of our objects indicates that it has a spectraltype of ~L1, making it one of the latest spectral type young brown dwarfsidentified to date. Comparing spectra of young brown dwarfs, field dwarfs, andgiant stars, we define a 1.49-1.56 micron H2O index capable of determiningspectral type to within 1 sub-type, independent of gravity. We have alsodefined an index based on the 1.14 micron sodium feature that is sensitive togravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14micron Na index can be used to distinguish young cluster members (t <~ 5 Myr)from young field dwarfs, both of which may have the triangular H-band continuumshape which persists for at least tens of Myr. Using effective temperaturesdetermined from the spectral types of our objects along with luminositiesderived from near and mid-infrared photometry, we place our objects on the H-Rdiagram and overlay evolutionary models to estimate the masses and ages of ouryoung sources. Three of our sources have inferred ages (t ~= 10-30 Myr)significantly older than the median stellar age of their parent clouds (1-3Myr). For these three objects, we derive masses ~3 times greater than expectedfor 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources.The large discrepancies in the inferred masses and ages determined using twoseparate, yet reasonable methods, emphasize the need for caution when derivingor exploiting brown dwarf mass and age estimates.Comment: 11 pages, Accepted to Ap