GLOBULAR-CLUSTERS IN THE MAGELLANIC CLOUDS .1. BV-CCD-PHOTOMETRY FOR 11 CLUSTERS

We present here BV CCD data for 11 intermediate-age LMC clusters, namely NGC 1756, 1831, 1868, 1987, 2107, 2108, 2162, 2173, 2190, 2209 and 2249. Although statistical sampling, field contamination and crowding problems have made the analysis and discussion very hard to accomplish, the observational data essentially confirm the existence of the predicted red giant branch phase transition. In particular, from the colour-magnitude diagrams of the 11 LMC clusters down to V similar to 22, we can conclude the following. (1) In the (V-TO, V-Cl,V-m) plane, the models yield a very good overall description of the data. A similar agreement can also be found in the [V-TO, Delta(V-TO - V-Cl,V-m)] plane, where the ordinate is, moreover, distance- and reddening-independent. (2) With the current sample, it is still impossible to choose decisively between 'classical' and 'overshooting' models. Both sets yield a good fit to the data in luminosity, classical models being apparently better if the observational results are taken at face value. (3) Regardless of the adopted distance modulus and reddening, the separation in colour between the main-sequence band (H-burning) and the Red Clump (He-burning) is smaller than predicted by any theoretical tracks, either classical or with overshooting. In particular, the main sequence is too red by about 0.05-0.10 mag and the Red Clump is more extended than expected. (4) The existence of the so-called red giant branch phase transition seems to be confirmed. In particular, the behaviour of the luminosities of Red Clump stars and the red giant branch development are qualitatively consistent with the theoretical predictions. Finally, we have identified a small subset of clusters (NGC 2209, 2190 and 2162) for future deeper study (maybe with the Hubble Space Telescope), which are most suitable for a further detailed investigation on this subject.