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Astron. Astrophys. 345, 521-530 (1999)

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1. Introduction

Star-forming regions (SFRs) have always been the ideal laboratory for the study of the mechanisms of stellar formation, as well as for the study of the structure and time evolution of newly formed stellar systems from the early pre-main sequence (PMS) phase down to the young main sequence (MS). In spite of the several advances which have been made possible by the availability of new spectral domains (and notably of the soft X-ray band), and of the large attention which they have received in the recent literature, several key questions regarding the mechanisms of star formation are still unanswered, both from an observational as well as from a theoretical point of view.

In particular, the key issue of whether star formation is essentially an universal, self-regulating process (thus leading to an universal initial mass function - IMF) or whether its outcome depends strongly on the local conditions and/or on the triggering event which started the collapse of the original cloud (which would result in different IMFs for different regions) is still unsolved. The derivation of the IMF, as well as of the star-formation rate in different SFRs with different observed conditions supplies an important constraint to whether star formation is an universal process or not.

At a distance of about 760 pc (Sung et al. 19971, and at an estimated age varying between [FORMULA] years (Walker 1956; Mermilliod 1981) and [FORMULA] years (Sagar et al. 1986), the star forming region NGC 2264 allows us to study the earlier stages of stellar evolution.

The disadvantage of its larger distance with respect to other well studied regions with similar characteristics (e.g. the Taurus-Auriga or Orion clouds) is compensated by two facts, which greatly ease the interpretation of the observational data: a small interstellar visual absorption ([FORMULA] mag.; Pérez et al. 1987) for the cluster members and the presence, in the immediate proximity of the region, of an optically thick reflection nebula (see e.g. Herbig 1954) that effectively obscures background stars.

Fig. 1 shows a photographic image (taken from the STScI Digital Sky Survey) of the central part of NGC 2264, in which the presence of the cloud is clearly revealed by the distinct decrease in the surface density of stars toward the area around and between the two brightest stars of the association: S Mon (north) and W178 (HD47887, south). It can also be seen that, contrary to most stellar associations that show a single clustering of stars, NGC 2264 has a rather complicated spatial structure: we can identify two physically distinct agglomerations, one to the North and one to the South (see Sagar et al., 1988).

[FIGURE] Fig. 1. An image of NGC 2264 from the STScI Digital Sky Survey. The square (20 arcmin on a side) indicates the extent of the CCD frames which are discussed in the present paper. Note the distinct decrease in star density toward the center of the region due to the presence of the molecular cloud. North is up, east to left.

Since the classical study of Walker (1956) suggested that a large fraction of the stars in the area are in the pre-main sequence stage, several workers have tried to characterize the stellar population of the region, using different techniques. Proper motion studies (Vasilevskis et al. 1965), X-ray observations (Simon et al. 1985, Patten et al. 1994, Flaccomio et al. in prep.), [FORMULA] surveys (Herbig 1954; Ogura 1984; Marcy 1980; Sung et al. 1997), IRAS infrared imaging (Margulis et al. 1989) and photometric studies (Adams et al. 1983; Rydgren 1979; Lada et al. 1993) have progressively enriched the knowledge of the young stellar population of NGC 2264.

The identification of the optical counterparts of a deep X-ray survey of NGC 2264 using the ROSAT HRI instrument recently conducted by Flaccomio et al. (in prep.) shows that the existing studies of the region do not provide an unbiased list of members down to sufficiently faint magnitudes. Therefore, CCD observations of part of the region in the Johnson-Cousin BVRI bands were conducted, covering the 20 arcmin square shown in Fig. 1. The area, which is roughly centered on the bright B1.5 V star W178, includes a good part of the southern star-forming core as well as the characteristic cone nebula.

The observations and their reductions are discussed in Sect. 2 of the present paper, while the results obtained are presented and discussed in Sect. 3, and summarized in Sect. 4.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999