A.A. Kaas 1, S. Bontemps 1, M. Huldtgren 1, G. Olofsson 1, L. Nordh 1, P. André 2, & al.
1 Stockholm Observatory, Sweden
2 CEA/SAp, C.E. Saclay, France
We present the first results of a mid-infrared survey of the young and
active star formation region surrounding the Serpens Cloud Core. The
mapping was made with ISOCAM in the two broad band filters LW2 (5 - 8 mu)
and LW3 (12 - 17 mu) with the purpose of obtaining a complete census of
the young stellar population. The Serpens Cloud Core contains a compact
cluster embedded in a clumpy high-extinction cloud (
)
located
only
5 degrees from the galactic plane. These factors all contribute to the
problem of distinguishing young stellar objects (YSOs) from reddened
background stars. Previous YSO population studies in the NIR suffer from
sample incompleteness if based on NIR excess alone, due to the problem of
extinction. In addition, they cover a small region only. With ISOCAM, however,
we can safely identify and characterize the YSOs in terms of intrinsic IR
excess due to circumstellar dust.
In total 0.22 square degrees ware mapped and fluxes obtained for 550 and 210
point sources in LW2 and LW3, respectively. These filters provide an excellent
discriminator between sources with and without IR excess, and the colour
index [14.3/6.7] is used to obtain a robust sample of YSOs, by which we
here mean Class I and Class II sources only. Among the 191 sources detected
in both filters we thus identify 81 as YSOs, of which 28 where known from
before. A lower limit of 400 YSOs per 
,
assuming a distance of 260
pc,
is found for the Cloud Core. We expect some unidentified Class III sources
to lurk among the remaining 110 sources without IR excess.
For the most active areas we obtained deep K images and placed the sources in a K-LW2/K-LW3 colour-colour diagramme which allows us to distinguish between Class I and Class II sources for the first time in Serpens. Compared to other star formation regions the number of Class I sources is large relative to the number of Class II sources. This could be an indication that the Serpens Cloud is younger. In addition, several Class I sources are redder than any Class I source in e.g. the Rho Ophiuchi cloud.
We present and discuss luminosity functions for the Class I and Class II source samples, addressing the problem of luminosity evolution from the accretion phase of Class I sources to the stage of gravitational contraction for Class II sources.