We present near-infrared ( 1.0–2.4 \mu m ) spectra confirming the youth and cool effective temperatures of 6 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 that it has a spectral type of \sim L1 , making it one of the latest spectral type young brown dwarfs identified to date .
Comparing spectra of young brown dwarfs , field dwarfs , and giant stars , we define a 1.49–1.56 \mu m H _ { 2 } O index capable of determining spectral type to \pm 1 sub-type , independent of gravity .
We have also defined an index based on the 1.14 \mu m sodium feature that is sensitive to gravity , but only weakly dependent on spectral type for field dwarfs .
Our 1.14 \mu m Na index can be used to distinguish young cluster members ( \tau \lesssim 5 Myr ) from young field dwarfs , both of which may have the triangular H-band continuum shape which persists for at least tens of Myr .
Using T _ { eff } ’ s determined from the spectral types of our objects along with luminosities derived from near and mid-infrared photometry , we place our objects on the H-R diagram and overlay evolutionary models to estimate the masses and ages of our young sources .
Three of our sources have inferred ages ( \tau \simeq 10–30 Myr ) significantly older than the median stellar age of their parent clouds ( 1–3 Myr ) .
For these three objects , we derive masses \sim 3 times greater than expected for 1–3 Myr old brown dwarfs with the bolometric luminosities of our sources .
The large discrepancies in the inferred masses and ages determined using two separate , yet reasonable methods , emphasize the need for caution when deriving or exploiting brown dwarf mass and age estimates .