THE STELLAR POPULATION IN THE RHO OPHIUCHI CLUSTER

We report a near-infrared survey of 200 arcmin2 in the cores of the rh o Ophiuchi interstellar clouds, leading to the detection of 91 sources . We develop a new method for estimating the luminosities of these sou rces based on fitting of isochrones from theoretical models. Applicati on of this method allows us to determine a luminosity function and acc ompanying initial mass function for the cluster of young stars forming in these clouds. The IMF extends to well below the bottom of the main sequence (0.08 M.) with no obvious feature at the transition between stellar and substellar masses. The power-law exponent of the IMF at lo w stellar and substellar masses is roughly - 1.3 to - 1.1, with formal errors of 0.2. These values are consistent with extrapolation of many estimates of the local IMF to substellar masses. At least seven objec ts appear to be substellar, even allowing for the largest plausible er rors in the observations, the analysis, and the theoretical evolutiona ry models for these sources. It therefore seems likely that brown dwar fs form in significant numbers and that the process of cloud fragmenta tion is not biased against substellar masses. These results indicate t hat the widespread difficulty in finding large numbers of brown dwarfs in older stellar populations arise from the rapid cooling and possibl y from atmospheric absorptions that may depress the near infrared spec tra outputs of these objects.