Deep Near-Infrared Imaging of the rho Oph Cloud Core: Clues to the
  Origin of the Lowest-Mass Brown Dwarfs
  A search for young substellar objects in the rho Oph cloud core region has
been made using the deep-integration Combined Calibration Scan images of the
2MASS extended mission in J, H and Ks bands, and Spitzer IRAC images at 3.6,
4.5, 5.8 and 8.0 microns. The field of view of the combined observations was 1
deg x 9.3 arcmin, and the 5 sigma limiting magnitude at J was 20.5. Comparison
of the observed SEDs with the predictions of the COND and DUSTY models, for an
assumed age of 1 Myr, supports the identification of many of the sources with
brown dwarfs, and enables the estimation of effective temperature, Teff. The
cluster members are then readily distinguishable from background stars by their
locations on a plot of flux density versus Teff. The range of estimated Teff
extends down to ~ 750 K, suggesting the presence of objects of sub-Jupiter
mass. The results also suggest that the mass function for the rho Oph cloud
resembles that of the sigma Orionis cluster based on a recent study, with both
rising towards lower masses. The other main result from our study is the
apparent presence of a progressive blueward skew in the distribution of J-H and
H-Ks colors, such that the blue end of the range becomes increasingly bluer
with increasing magnitude. We suggest that this behavior might be understood in
terms of the 'ejected stellar embryo' hypothesis, whereby some of the
lowest-mass brown dwarfs could escape to locations close to the front edge of
the cloud, and thereby be seen with less extinction.
