ISOCAM observations of the rho 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 sur vey 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 s urveyed for point sources at 6.7 mum and 14.3 mum. A total of 425 sources a re detected in similar to0.7 deg(2), including 16 Class I, 123 Class II, an d 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 r ecognized 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 populatio n 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, reaso ns to believe that several tens of Class III YSOs remain to be identified b elow L-* similar to 0.2 L-circle dot. The mid-IR luminosities of most (simi lar to 65%) Class II objects are consistent with emission from purely passi ve circumstellar disks. The stellar luminosity function of the complete sam ple 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-* simil ar 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 p lausible star formation histories, allows us to derive the mass distributio n of the Class II YSOs which arguably reflects the initial mass function (I MF) of the embedded cluster. After correction for the presence of unresolve d binary systems, we estimate that the IMF in rho Ophiuchi is well describe d by a two-component power law with a low-mass index of -0.35 +/- 0.25, a h igh-mass index of 1.7 (to be compared with the Salpeter value of -1.35), an d 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-ci rcle dot.