CCD photometry of galactic open clusters - II. NGC 2243

Abstract

Summary New CCD photometry of 607 stars in the old open cluster NGC 2243 is presented. The resulting colour-magnitude diagram establishes the presence of a fraction of binary stars up to 30 per cent of the total population. Three additional features in the stellar distribution of the diagram are confirmed to be significant: a gap just below the top of the main sequence, six blue straggler candidates and an incipient red horizontal branch (curiously split into two almost parallel substructures). The detailed comparison of the observational colour-magnitude diagram with the synthetic ones derived from various models of stellar evolution has allowed us to determine the main parameters of NGC 2243 and to infer interesting indications for the models themselves. The cluster metallicity is Z = 0.003−0.006, the age is in the range 3−5 Gyr, the reddening E(B-V) = 0.06 − 0.08 and the distance modulus (m − M)0 = 12.7 − 12.8. The dependence of all these quantities on the adopted stellar evolution models is discussed, emphasizing in particular that the large difference in the derived ages depends on whether or not overshooting from convective cores is taken into account in the stellar interior. It is also worth noting that the gap is not predicted by overshooting models and that even the standard models do not reproduce it exactly.