H- and K-band spectra of brown dwarf candidates in the core of the ρ Ophiuchi molecular cloud complex

We present low-resolution H- and K-band spectra of 14 brown dwarf candidates located in the core of the rho Ophiuchi molecular cloud complex. Half of the spectra exhibit broad water absorption bands characteristic of M stars, while half of the spectra are featureless. We derive spectral types and effective temperatures for the seven candidates whose spectra exhibit water absorption bands by comparing the strength of these bands with those of standard M dwarf stars. We estimate their masses by first combining the spectral types with published near-infrared photometry to estimate bolometric luminosities and then by comparing their positions in a Hertzsprung-Russell diagram with theoretical evolutionary tracks and isochrones. The masses of four of the candidates lie below the substellar limit when the errors in their effective temperatures and bolometric luminosities are considered, while the remaining three candidates have masses on the stellar-substellar border. We find our mass estimates to be consistent with those determined using near-infrared photometry alone. The derived ages of three candidates are considerably older than the previously determined age of the embedded cluster. If these objects are members of the cluster, then they may belong to an older and previously unrecognized population of the rho Ophiuchi molecular cloud complex.