N-body models of globular clusters: metallicities, half-light radii and mass-to-light ratios

Size differences of ≈20 per cent between red (metal-rich) and blue (metal-poor) subpopulations of globular clusters have been observed, generating an ongoing debate as to whether these originate from projection effects or the difference in metallicity. We present direct N-body simulations of metal-rich and metal-poor stellar populations evolved to study the effects of metallicity on cluster evolution. The models start with N = 100 000 stars and include primordial binaries. We also take metallicity-dependent stellar evolution and an external tidal field into account. We find no significant difference for the half-mass radii of those models, indicating that the clusters are structurally similar. However, utilizing observational tools to fit half-light (or effective) radii confirms that metallicity effects related to stellar evolution combined with dynamical effects such as mass segregation produce an apparent size difference of 17 per cent on average. The metallicity effect on the overall cluster luminosity also leads to higher mass-to-light ratios for metal-rich clusters.