Mass segregation in young star clusters: can it be detected from the integrated photometric properties?

TL;DR

This study models how mass segregation affects the observable properties of young star clusters, showing that it influences core radius, surface brightness, and color gradients, but current models cannot fully explain some recent observations.

Contribution

The paper introduces an analytical model for mass-segregated star clusters that aligns with N-body simulations and explores how mass segregation impacts observable properties over time.

Findings

Core radius increases with age in segregated clusters.

Central surface brightness decreases more rapidly than stellar evolution predicts.

Color gradients due to mass segregation are potentially observable.

Abstract

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 N-body 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 that 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 when not taking into account mass segregation in 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 half-light radius with increasing wavelength was observed, which was attributed to mass segregation. These observations can not be reproduced by our models. We find that the differences between measured radii in different filters are always smaller than 5%.