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

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4. Summary and Conclusions

The analysis of our CCD BVRI photometry in the southern part of the star-forming region NGC 2264 has lead to the identification of 24 previously unknown likely members of the association, on the basis of photometric color excesses. We have collected a critical compilation of the existing membership data for NGC 2264, building a sample which we show to be statistically complete for [FORMULA]. Through star positions on the evolutionary tracks of D'Antona & Mazzitelli (1998), we have estimated their individual masses and ages, which we have then used to build an Initial Mass Function for the studied part of the region as well as to determine the star-formation history as a function of time in the region.

Our main result is that the IMF seems to differ from the field one (i.e. the Scalo 1986 IMF), due to the presence of a "bump" in the IMF at high masses

This features could point toward the presence of two somewhat distinct star-formation mechanisms being at work in NGC 2264, one for low-mass and one for high-mass stars (see e.g. Larson, 1986; Silk 1985; Lee & Chun, 1986). Following the discussion of Larson (1986) one may envision a scenario in which massive stars (such as the B1.5V star W178 which is at the center of the field studied here, or the O7V star S Mon) form at an early stage, and, due to the heating and/or strong ionization of the surrounding matter, prevent to some degree the formation of less massive stars, thus explaining the observed excess of high-mass stars in the IMF. The likehood of such an history of star formation will be greatly enhanced if the suggested difference in stellar age between samples of different mass will be confirmed with greater confidence.

We note that our conclusions are robust in the face of the existing uncertainty in the distance to NGC 2264. If we were to adopt the Pérez et al. (1987) distance of 950 pc, in place of the 760 pc (Sung et al. 1997) which was assumed throughout the present paper, the absolute age and mass estimates would change, thus shifting the mass at which the IMF shows a break, but the general conclusion about the presence of a bimodal IMF would not change.

A more complete survey of the region, both covering a wider area of the SFR and reaching down to fainter magnitudes in order to achieving completeness at lower masses, will be needed in the future to assess whether the bimodal structures of the IMF and the difference in characteristic ages of the high- and low-mass populations persist when a more complete census of the region is taken. Both deep optical and X-ray observations will be useful in this context.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999