The young stellar population in the Serpens Cloud Core: An ISOCAM survey ^*,**,***

¹ Nordic Optical Telescope, Apdo 474, 38700 Santa Cruz de La Palma, Spain
² Stockholm Observatory, Roslagstullsbacken 21, 10691 Stockholm, Sweden
³ Observatoire de Bordeaux, BP 89, 33270 Floirac, France
⁴ Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁵ Research and Scientific Support Department of ESA, Postbus 229, 2200 AG Noordwijk, The Netherlands
⁶ Instituto Astrofisica Spaziale e Fisica Cosmica, CNR, Rome, Italy
⁷ Instituto de Astrofísica de Andalucia, Granada, Spain
⁸ IAS, Université Paris XI, 91405 Orsay, France
⁹ SIRTF Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125
¹⁰ Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ Joint Astronomy Centre, 660 N. A'Ohoku Place, University Park, Hilo, HI 96720, USA
¹² ENS Radioastronomie, 24 Rue Lhomond, 75231 Paris, France
¹³ Laboratoire d'Astrophysique de Grenoble, 38041 Grenoble Cedex, France
¹⁴ Observatoire de Lyon, 69230 Saint Genis Laval, France

Corresponding author: A. A. Kaas, akaas@not.iac.es

Received: 1 December 2003
Accepted: 18 March 2004

Abstract

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 μm) and LW3 (12-18 μm) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 μm band and 139 in the 14.3 μm band to a limiting sensitivity of ~2 mJy. We identified 53 Young Stellar Objects (YSOs) with mid-IR excess from the single colour index , and 8 additional YSOs from the diagram. Only 32 of these 61 sources were previously known to be YSO candidates. Only about 50% of the mid-IR excess sources show excesses in the near-IR  diagram. In the 48 square arcmin field covering the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as ρ  Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for ρ  Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a “microburst” of star formation has taken place very recently, probably within the last 10⁵ yrs.