Spectroscopy of brown dwarf candidates in the rho Ophiuchi molecular core

We present an analysis of low-resolution infrared spectra for 20 brown dwar f candidates in the core of the rho Ophiuchi molecular cloud. Fifteen of th e sources display absorption-line spectra characteristic of late-type stars . By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types that are independent of reddening. Optical spectroscopy of one brown dwarf candidat e confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatu res and bolometric stellar luminosities are derived, enabling us to place o ur sample on the Hertzsprung-Russell diagram. We compare the positions of t he brown dwarf candidates in this diagram with two sets of theoretical mode ls in order to estimate their masses and ages. Considering uncertainties in placing the candidates in the H-R diagram, six objects consistently lie in the brown dwarf regime and another five objects lie in the transition regi on between stellar and substellar objects. The ages inferred for the sample are consistent with those derived for higher mass association members. Thr ee of the newly identified brown dwarfs display infrared excesses at lambda = 2.2 mu m, suggesting that young brown dwarfs can have active accretion d isks. Comparing our mass estimates of the brown dwarf candidates with those derived from photometric data alone suggests that spectroscopy is an essen tial component of investigations of the mass functions of young clusters.