Substellar Objects in Nearby Young Clusters (SONYC) V: New brown dwarfs
  in rho Ophiuchi
  SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to
investigate the frequency and properties of substellar objects with masses down
to a few times that of Jupiter in nearby star-forming regions. For the ~1Myr
old rho Ophiuchi cluster, in our earlier paper we reported deep, wide-field
optical and near-infrared imaging using Subaru, combined with 2MASS and Spitzer
photometry, as well as follow-up spectroscopy confirming three likely cluster
members, including a new brown dwarf with a mass close to the deuterium-burning
limit. Here we present the results of extensive new spectroscopy targeting a
total of ~100 candidates in rho Oph, with FMOS at the Subaru Telescope and
SINFONI at the ESO's Very Large Telescope. We identify 19 objects with
effective temperatures at or below 3200 K, 8 of which are newly identified
very-low-mass probable members of rho Oph. Among these eight, six objects have
Teff <= 3000 K, confirming their likely substellar nature. These six new brown
dwarfs comprise one fifth of the known substellar population in \rho Oph. We
estimate that the number of missing substellar objects in our survey area is
~15, down to 0.003 - 0.03 MSun and for Av = 0 - 15. The upper limit on the
low-mass star to brown dwarf ratio in rho Oph is 5.1 +- 1.4, while the disk
fractions are ~40% and ~60% for stars and BDs, respectively. Both results are
in line with those for other nearby star forming regions.
