Spectroscopy of Brown Dwarf Candidates in the ρ Ophiuchi Molecular Core

We present an analysis of low resolution infrared spectra for 20 brown dwarfcandidates in the core of the $\rho$ Ophiuchi molecular cloud. Fifteen of thesources display absorption-line spectra characteristic of late-type stars. Bycomparing the depths of water vapor absorption bands in our candidate objectswith a grid of M dwarf standards, we derive spectral types which areindependent of reddening. Optical spectroscopy of one brown dwarf candidateconfirms the spectral type derived from the water bands. Combining theirspectral types with published near-infrared photometry, effective temperaturesand bolometric stellar luminosities are derived enabling us to place our sampleon the Hertzsprung-Russell diagram. We compare the positions of the brown dwarfcandidates in this diagram with two sets of theoretical models in order toestimate their masses and ages. Considering uncertainties in placing thecandidates in the H-R diagram, six objects consistently lie in the brown dwarfregime and another five objects lie in the transition region between stellarand substellar objects. The ages inferred for the sample are consistent withthose derived for higher mass association members. Three of the newlyidentified brown dwarfs display infrared excesses at $\lambda$=2.2 $\mu$msuggesting that young brown dwarfs can have active accretion disks. Comparingour mass estimates of the brown dwarf candidates with those derived fromphotometric data alone suggests that spectroscopy is an essential component ofinvestigations of the mass functions of young clusters.Comment: Astronomical Journal, in press: 25 pages, latex, 5 tables and 6 figures (separate