NGC 2849 and NGC 6134: two more BOCCE open clusters

Abstract

We present CCD photometry of two southern open clusters. As part of the Bologna Open Cluster Chemical Evolution project we obtained BVI and UBVI imaging for NGC 2849 and NGC 6134, respectively. By means of the synthetic colour-magnitude diagram method and using various evolutionary sets of stellar evolution tracks with various metallicities, we determined at the same time age, distance and reddening. We also determined an approximate metallicity for NGC 2849, for which the information is not available from sounder methods like high-resolution spectroscopy. NGC 2849 turned to be 0.85-1.0 Gyr old with a solar metallicity. The foreground reddening is E(B - V) = 0.28 - 0.32, and the true distance modulus (m - M) 0 = 13.8-13.9. For NGC 6134 we did not obtain fully consistent answers from the V, B - V and V, V - I photometry, an unexpected problem, since both the metallicity and the reddening are known (from high-resolution spectroscopy and the U - B, B - V two colours diagram, respectively). This may either indicate a difficulty of current models (evolutionary tracks and/or models of atmosphere) to accurately reproduce colours, or be related to differences in the metal mixture assumed by the models and those of the clusters. Assuming the spectroscopic abundance and the colour excess [E(B - V) = 0.35] from the U - B, B - V plot, we derived a best age between 0.82 and 0.95 Gyr and a distance modulus 10.5. In agreement with previous studies, the NGC 6134 colour-magnitude diagram shows also a clear main sequence gap at V ˜ 15 and B-V ˜ 0.9-1.0 that is unexplained by canonical stellar evolution models.