The photometric evolution of dissolving star clusters. II. Realistic models. Colours and M/L ratios

Abstract

Evolutionary synthesis models are the primary means of constructing spectrophotometric models of stellar populations, and deriving physical parameters from observations compared with these models. One of the basic assumptions of evolutionary synthesis models has been the time-independence of the stellar mass function, apart from the successive removal of high-mass stars by stellar evolution. However, dynamical simulations of star clusters in tidal fields have demonstrated that the mass function can be changed by the preferential removal of low-mass stars from clusters. Aims. We combine the results of dynamical simulations of star clusters in tidal fields with our evolutionary synthesis code GALEV. We extend the models to consider the total cluster disruption time as additional parameter. Methods. Following up on our earlier work, which was based on simplifying assumptions, we reanalyse the mass-function evolution found in N-body simulations of star clusters in tidal fields, parametrise it as a function of age and total disruption time of the cluster, and use this parametrisation to compute GALEV models as a function of age, metallicity, and total cluster disruption time. Results. We study the impact of cluster dissolution on colours (which generally become redder) and magnitudes (which become fainter) of star clusters, their mass-to-light ratios (which can deviate by a factor of ~2–4 from predictions of standard models without cluster dissolution), and quantify the effect of the altered integrated photometry on cluster age determination. In most cases, clusters appear to be older than they are, where the age difference can range from 20% to 200%. By comparing our model results with observed M/L ratios for old compact objects in the mass range 104.5–108 , we find a strong discrepancy for objects more massive than 107 , such that observed M/L ratios are higher than predicted by our models. This could be caused either by differences in the underlying stellar mass function or be an indication of the presence of dark matter in these objects. Less massive objects are well described by the models.