An ISOCAM deep census of low-mass stars in ρ Ophiuchi -: Mass function in a young embedded cluster

We present the initial results of an extensive imaging survey of the rho Ophiuchi molecular cloud with the mid-infrared camera ISOCAM at 6.7 mu m and 14.3 mu m. This two-wavelength mid-IR survey is invaluable to recognize most of the young stars with IR excesses embedded in the cloud, and therefore provides a more complete census of the young stars in rho Ophiuchi. Most of these ISOCAM-selected sources are also near-IR sources. It is then possible to estimate with a good accuracy their stellar luminosities and to build a new luminosity function of the rho Oph embedded cluster. Modelling of this luminosity function based on time-dependent mass-luminosity relationships from recent PMS tracks shows that the mass function in rho Oph flattens at a characteristic, transition mass of 0.5 - 1.0 M-.. If the transition is close to 0.5 M-. like for the IMF favored by Kroupa et al. (1993), we find that the rho Oph embedded cluster should be younger than 0.5 Myr. If the cluster is somewhat older (similar to 1.0 Myr), the rho Oph luminosity function is then well reproduced with a mass function which flattens at M* similar to 1.0 M-. which is the value favored by, e.g., Scale (1998).