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Issue A&A
Volume 372, Number 1, June II 2001
Page(s) 173 - 194
Section Formation, structure and evolution of stars
DOI 10.1051/0004-6361:20010474



A&A 372, 173-194 (2001)
DOI: 10.1051/0004-6361:20010474

ISOCAM observations of the $\mathsf{\rho}$ Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster

S. Bontemps1, 2, P. André3, A. A. Kaas4, 2, L. Nordh2, G. Olofsson2, M. Huldtgren2, A. Abergel5, J. Blommaert6, F. Boulanger5, M. Burgdorf6, C. J. Cesarsky3, D. Cesarsky5, E. Copet7, J. Davies8, E. Falgarone9, G. Lagache5, T. Montmerle3, M. Pérault9, P. Persi10, T. Prusti6, J. L. Puget5 and F. Sibille11

1  Observatoire de Bordeaux, BP 89, 33270 Floirac, France
2  Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
3  Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
4  ESA/ESTEC, Astrophysics Division, The Netherlands
5  IAS, Université Paris XI, 91405 Orsay, France
6  ISO/SOC, Astrophysics Division of ESA, Villafranca, Spain
7  DESPA, Obs. Paris-Meudon, 5 Pl. J. Janssen, 92195 Meudon, France
8  JAC, 660 N. A'Ohoku Place, University Park, Hilo, HI 96720, USA
9  ENS Radioastronomie, 24 rue Lhomond, 75231 Paris, France
10  IAS, CNR, Area di Ricerca Tor Vergata, 00133 Roma, Italy
11  Observatoire de Lyon, 69230 Saint Genis Laval, France

(Received 9 May 2000 / Accepted 07 March 2001 )

Abstract
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 $\rho$ Ophiuchimolecular cloud L1688, as well as the two secondary clouds L1689N and L1689S, have been completely surveyed for point sources at 6.7 $\mu$m and 14.3 $\mu$m. A total of 425 sources are detected in ~0.7 deg2, 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 $F_\nu$ ~ 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 $L_\star \sim 0.2 L_\odot$. The mid-IR luminosities of most (~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 $L_\star \sim 0.03 L_\odot$. It is basically flat (in logarithmic units) below $L_\star \sim 2 L_\odot$, exhibits a possible local maximum at $L_\star \sim 1.5 L_\odot$, 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 $\rho$ Ophiuchiis well described by a two-component power law with a low-mass index of $-0.35\pm0.25$, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at $M_{flat}=0.55\pm0.25 M_\odot$. This IMF is flat with no evidence for a low-mass cutoff down to at least ~ $0.06 M_\odot$.


Key words: stars: formation -- stars: low-mass, brown dwarfs -- stars: luminosity function, mass function -- stars: pre-main sequence -- ISM: individual objects: $\rho$ Ophiuchi cloud

Offprint request: S. Bontemps, bontemps@observ.u-bordeaux.fr

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