THE YOUNG MAGELLANIC CLUSTER NGC-2004

The results of a new CCD photometric investigation of stars in the field of the LMC cluster NGC 2004 are reported and discussed. B, V photometry is given for more than 1500 objects, the large majority to be ascribed to the cluster. A new color-magnitude diagram is presented. It shows a well-defined blue sequence (BS) of H-burning stars, running from V almost-equal-to 21 mag up to V almost-equal-to 13 mag, together with a group of cluster He-burning red supergiants with V almost-equal-to 13 mag and a clump of field red giants at about V almost-equal-to 19 mag. Theoretical evolutionary computations are reported covering the range of massive stars evolving in the cluster. These evolutionary tracks can match the observed CM diagram provided that a low efficiency of the surface convection (a low value of the mixing length) is adopted. Comparison of the theoretical luminosity function with the upper portion of the observed BS distribution suggests a cluster age of about 8 million years, in agreement with previous evaluations. The completeness of the sample and the contribution of field stars have been estimated. The CM distribution of cluster stars has been compared with the results of synthetic clusters based on the adopted evolutionary scenario. We find that the observed distribution of cluster stars in the different evolutionary phases appears in good agreement with prediction based on theoretical evolutionary lifetimes. Moreover, the distribution of H-burning stars agrees with a Salpeter distribution in the whole range 2 less-than-or-equal-to M/M. less-than-or-equal-to 20. After a short review of the current theoretical scenario concerning massive stars, the CM diagram of field stars is discussed, reaching the conclusion that the observed clump of red giants is the natural consequence of the peculiar large lifetimes in the central He burning phase of stars with M congruent-to 3M..