Stellar collisions in young star clusters

We consider the effect of mass segregation on the observable integrated properties of star clusters. The measurable properties depend on a combination of the dynamical age of the cluster and the physical age of the stars in the cluster. To investigate all possible combinations of these two quantities, we propose an analytical model for the mass function of segregated star clusters that agrees with the results of Nbody simulations, in which any combination can be specified. For a realistic degree of mass segregation and a fixed density profile, we find with increasing age an increase in the measured core radii and a central surface brightness which decreases in all filters more rapidly than what is expected from stellar evolution alone. Within a Gyr the measured core radius increases by a factor of two and the central surface density in all filters of a segregated cluster will be overestimated by a similar factor if mass segregation is not taken into account during the conversion from light to mass. We find that the V − I colour of mass segregated clusters decreases with radius by about 0.1-0.2 mag, which could be observable. From recent observations of partially resolved extra-galactic clusters, a decreasing core radius with increasing wavelength was observed, which was attributed to mass segregation. However, our models predict that mass segregation has weaker dependence of the core radius on the wavelength, and therefore it is doubtful that the observed dependence can be attributed to the effects of mass segregation. These observations can not be reproduced by our models. In addition, we provide physical arguments based on the evolution of individual stars that one should not expect strong dependence of core radius as a function of the wavelength. We find that the differences between measured radii in different filters are always smaller than 5%. 10 Mass segregation in star clusters