The photometric evolution of star clusters and the preferential loss of low-mass bodies - with an application to globular clusters

To obtain an accurate description of broad-band photometric star cluster evolution, certain effects should be accounted for, such as the preferential loss of low-mass stars and the retain of stellar remnants. Moreover, the IMF and metallicity affect photometry as well. Due to their advanced evolutionary state, globular clusters (GCs) are interesting test cases for cluster models describing these effects. In this paper we describe cluster models in which the above effects are included. The photometric evolution of clusters is predicted, and the results are applied to Galactic GCs. The presented cluster models represent an analytical description of the evolution of the underlying stellar mass function due to stellar evolution and dynamical cluster dissolution. Stellar remnants are included by using initial-remnant mass relations, while cluster photometry is computed from the Padova 1999 isochrones. Our study shows that the preferential loss of low-mass stars, the retain of stellar remnants, as well as IMF and metallicity variations strongly influence the magnitude, colour and mass-to-light ratio evolution of clusters. The different effects can be clearly separated with our models. The models are applied to the Galactic GC population, of which the magnitude, colour and mass-to-light ratio ranges are well reproduced. We also show that the mass-to-light ratios of clusters of similar ages and metallicities cannot be assumed to be constant for all cluster luminosities. Instead, mass-to-light ratio increases with cluster luminosity and mass. These models underline the importance of detailed cluster models when considering cluster photometry. With the analytic framework provided in this paper, observed cluster properties can be interpreted in a more complete perspective. (Abridged)