Herschel survey of brown dwarf disks in Rho Ophiuchi
  Recent observations of the Rho Ophiuchi cluster with the Herschel Space
Observatory allow us to probe the spectral energy distribution (SED) of the
brown dwarf population in the far-IR, where the disk emission peaks. We
performed aperture photometry at 70, 100, and 160 micron, and constructed SEDs
for all previously known brown dwarfs detected. These were complemented with
ancillary photometry at shorter wavelengths. We compared the observed SEDs to a
grid of synthetic disks produced with the radiative transfer code MCFOST, and
used the relative figure of merit estimated from the Bayesian inference of each
disk parameter to analyse the structural properties. We detected 12 Class II
brown dwarfs with Herschel, which corresponds to one-third of all currently
known brown dwarf members of Rho Ophiuchi. We do not detect any of the known
Class III brown dwarfs. Comparison to models reveals that the disks are best
described by an inner radius between 0.01 and 0.07 AU, and a flared disk
geometry with a flaring index between 1.05 and 1.2. Furthermore, we can exclude
values of the disk scale-height lower than 10 AU (measured at a fiducial radius
of 100 AU). We combined the Herschel data with recent ALMA observations of the
brown dwarf GY92 204 (ISO-Oph 102), and by comparing its SED to the same grid
of disk models, we derived an inner disk radius of 0.035 AU, a scale height of
15 AU with a flaring index of beta~1.15, an exponent for dust settling of -1.5,
and a disk mass of 0.001 MSun. This corresponds to a disk-to-central object
mass ratio of ~1%. The structural parameters constrained by the extended SED
coverage (inner radius and flaring index) show a narrow distribution for the 11
young brown dwarfs detected in Rho Ophiuchi, suggesting that these objects
share the same disk evolution and, perhaps, formation.
