The stellar population in the ρ Ophiuchi cluster

We report a near-infrared survey of 200 arcmin² in the cores of the ρ Ophiuchi interstellar clouds, leading to the detection of 91 sources. We develop a new method for estimating the luminosities of these sources based on fitting of isochrones from theoretical models. Application of this method allows us to determine a luminosity function and accompanying 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 low 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 objects appear to be substellar, even allowing for the largest plausible errors in the observations, the analysis, and the theoretical evolutionary models for these sources. It therefore seems likely that brown dwarfs form in significant numbers and that the process of cloud fragmentation is not biased against substellar masses. These results indicate that the widespread difficulty in finding large numbers of brown dwarfs in older stellar populations arise from the rapid cooling and possibly from atmospheric absorptions that may depress the near infrared spectra outputs of these objects.