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Astron. Astrophys. 330, 990-998 (1998)

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1. Introduction

The knowledge of the distribution of young stars within molecular clouds is of fundamental importance since it provides insights into the nature of star-forming mechanisms. Young Stellar Objects (YSOs) are associated with varying amounts of gas and dust and it is expected that the youngest objects will be invisible at optical wavelengths due to obscuration by opaque circumstellar dust. Therefore observations at infrared wavelengths provide one of the best methods for identifying the young stellar population within molecular clouds. In the past, infrared studies of star-forming regions were limited by the poor sensitivity of the instruments as well as their low spatial resolution. Infrared array technology has advanced considerably in the last few years, and is now at a stage allowing to survey large regions of star formation (see De Poy et al. 1990, Lada et al. 1991, Zinnecker et al. 1993). An increase in spatial resolution and sensitivity almost always provides new insights into old problems.

Two distinct areas need to be addressed by such data: i) the determination of which stars in any given field are members of the embedded young stellar population, i.e. separating the young Pre-Main Sequence (PMS) stars from the population of 'normal' background/foreground main sequence and giant stars and, ii) to determine the nature (i.e. evolutionary state, age, luminosity, etc) of the derived young embedded PMS population. This second point has recently been addressed by Lada & Adams (1992) who studied the location of known classes of YSOs (i.e. classical T Tauri Stars - CTTS, Weak-line T Tauri Stars - WTTS, Herbig Ae/Be - HAEBE stars, and IR protostars: the class I sources of Lada & Wilking 1984) in near-IR colour-colour diagrams. They concluded that these relatively well-known evolutionary classes occupy different regions of the near-IR colour-colour diagram and that given the JHK photometry, the evolutionary state of PMS stars can be inferred relatively unambiguously. Star clusters are important laboratories for studying the initial luminosity function because they consist of statistically significant groups of stars who share the common heritage of forming from the same parental cloud, and they are not old enough to have lost a significant number of members due to stellar evolution or dynamical effects such as evaporation or violent relaxation (Lada & Wilking 1984, Lada et al. 1991). Moreover in these very young clusters ([FORMULA] yr), low-mass stars are brighter than at any other time in their PMS evolution.

At a distance of 310pc (De Lara et al. 1991), this region has received attention since Strom et al. (1976) reported a small red nebulosity, called the Serpens Object or the Serpens Reflection Nebula. Based on the more than fifty low-mass stars identified in the core by a near-infrared survey, the Serpens molecular cloud is one of the most spectacular examples of a protostellar nursery, harboring a stellar density exceeding 450 stars/pc-3 (Eiroa & Casali 1992). A Recent submillimeter continuum survey has uncovered half a dozen mm/submillimeter peaks, four of which lack an infrared counterpart (Casali et al. 1993). A low resolution CO and H2CO survey revealed a dense core in the dark cloud complex (Loren et al. 1979). More recently, Hurt and Barsony (1996) found several sources sharing the characteristics of Class 0 protostars, the short-lived (a few [FORMULA] yr) earliest protostellar stage (André, Ward-Thompson & Barsony 1993, Barsony 1994).

In this paper, we present new deep optical and near-infrared observations of the Serpens cloud core using an array detector. Our results increase the number of sources detected in the central part of the cloud and allow to make a more complete census of its membership, determine the nature of its embedded members, and construct the infrared luminosity function of the cluster. To investigate the nature of the underlying mass function, we calculate models which predict the evolution of the luminosity function of a cluster of PMS stars. We compare these models with the Serpens K luminosity function to place constraints on the star formation history of this cluster and on the nature of the underlying mass function.

The optical and near IR observations are presented in Sect.  2, and the results in Sect.  3. The interpretation of these results in terms of general properties of the cluster is presented and discussed in Sect.  4and our conclusions are summarized in Sect.  5.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998