We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project , to study gravity sensitive features as indicators of youth in brown dwarfs .
Low-resolution ( R \sim 2000 ) J -band and optical ( R \sim 1000 ) observations using NIRSPEC and LRIS at the W.M .
Keck Observatory reveal transitions of TiO , VO , K I , Na I , Cs I , Rb I , CaH , and FeH .
By comparing these features in late-type giants and in old field dwarfs we show that they are sensitive to the gravity ( g = GM / R ^ { 2 } ) of the object .
Using low-gravity spectral signatures as age indicators , we observed and analyzed J -band and optical spectra of two young brown dwarfs , G 196-3B ( 20-300 Myr ) and KPNO Tau-4 ( 1-2 Myr ) , and two possible low mass brown dwarfs in the \sigma Orionis cluster ( 3-7 Myr ) .
We report the identification of the \phi bands of TiO near 1.24 \mu m and the A-X band of VO near 1.18 \mu m together with extremely weak J -band lines of K I in KPNO-Tau4 .
This is the first detection of TiO and VO in the J -band in a sub-stellar mass object .
The optical spectrum of KPNO-Tau4 exhibits weak K I and Na I lines , weak absorption by CaH , and strong VO bands , also signatures of a lower gravity atmosphere .
G 196-3B shows absorption features in both wavelength regions like those of KPNO-Tau4 suggesting that its age and mass are at the lower end of published estimates .
Whereas \sigma Ori 51 appears to be consistent with a young sub-stellar object , \sigma Ori 47 shows signatures of high gravity most closely resembling an old L1.5/L0 , and can not be a member of the \sigma Orionis cluster .