ISOCAM observations of the ρ Ophiuchi cloud:: Luminosity and mass functions of the pre-main sequence embedded cluster

We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main rho Ophiuchi molecular cloud L1688, as well as the two secondary clouds L1689N and L1689S, have been completely surveyed for point sources at 6.7 mum and 14.3 mum. A total of 425 sources are detected in similar to0.7 deg(2), including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to F-v similar to 10-15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. There are, however, reasons to believe that several tens of Class III YSOs remain to be identified below L-* similar to 0.2 L-circle dot. The mid-IR luminosities of most (similar to 65%) Class II objects are consistent with emission from purely passive circumstellar disks. The stellar luminosity function of the complete sample of Class II YSOs is derived with good accuracy down to L-* similar to 0.03 L-circle dot. It is basically at (in logarithmic units) below L-* similar to 0.2 L-circle dot, exhibits a possible local maximum at L-* similar to 1.5 L-circle dot, and sharply falls off at higher luminosities. A modeling of the luminosity function, using available pre-main sequence tracks and plausible star formation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the initial mass function (IMF) of the embedded cluster. After correction for the presence of unresolved binary systems, we estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low-mass index of -0.35 +/- 0.25, a high-mass index of 1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M-flat = 0.55 +/- 0.25 M-circle dot. This IMF is at with no evidence for a low-mass cutoff down to at least similar to0.06 M-circle dot.