Göran Olofsson 1, L. Nordh 1, S. Bontemps 1, A. A. Kaas 1, M. Huldtgren 1, P. André 2, C. J. Cesarsky 2, T. Montmerle 2, A. Abergel 3, F. Boulanger 3, D. Cesarsky 3, G. Lagache 3, J. L. Puget 3, J. Blommaert 4, M. Burgdorf 4, T. Prusti 4, E. Falgarone 5, M. Perault 5, E. Copet 6, J. Davies 7, M. Casali 8, P. Persi 9, & F. Sibille 10
1 Stockholm Observatory, 2 CEA, Saclay, 3 IAS, Orsay, 4 ISO/SOC, Villafranca, 5 ENS, Paris, 6 DESPA, Meudon, 7 JAC, Hilo, 8 ROE, Edinburgh, 9 IAS, CNR, Frascati, 10Observatoire de Lyon
We present an overview of the extensive ISOCAM survey in two filters (LW2 and LW3 centred at 6.7 and 14.3 microns) of selected parts of the closest star formation regions. All together the survey covers 2.3 square degrees and the sensitivity limit for point sources varies (depending on confusing nebulosity etc.) but it is usually around 1-3 mJy in LW2 and a factor two worse in LW3. The ultimate goals of this programme are to extend the current knowledge of the initial mass function (IMF) for stars towards lower masses and, which is closely connected, to gain a better estimate on the star formation efficiency in different clouds. It was anticipated that the two selected filter wavelengths (expected to be little effected by interstellar reddening) should separate stars with circumstellar dust emission from reddened background stars. This turned out to be true and in fact, within the regions surveyed, we can at least double the number of stars identified as Young Stellar Objects (YSOs). The 'new' YSOs are in general fainter and/or more embedded than the previously known.
Having identified the YSOs, the next step is to derive the individual luminosity (and finally the mass) of each star. Some of the sources have rising spectra versus the far IR (class I) and for these we need the PHOT observations (which complemented the ISOCAM survey) but for the majority of the sources the bulk of the energy is emitted in the near infrared which means that the ISOCAM data have to be combined with ground-based observations at shorter wavelengths. At the J wavelength, the cloud obscuration in the densest regions tends to prevent detection but in H and K the sources are normally easily observed. Assuming an intrinsic colour (not very critical) the source can be dereddened and the luminosity calculated. In the next step - going from luminosity to mass - we need a theoretical model and an estimate of the age. For the low-mass stars - which are of prime importance in our investigation - the assumed age is not very crucial as they are resting in the deuterium burning phase. Even if we neglect duplicity, we arrive at an IMF which undoubtedly rises to the sensitivity limit of the survey. For the closest regions this limit is well below the brown dwarf limit and for these regions some 10-30% of the YSOs are brown dwarf candidates. We discuss a few of these putative brown dwarfs in some detail.