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

TL;DR

This study uses N-body simulations to investigate how metallicity influences the apparent size and mass-to-light ratios of globular clusters, finding that metallicity effects can produce observable size differences and affect luminosity.

Contribution

The paper demonstrates through simulations that metallicity-related stellar evolution and dynamical effects can explain observed size differences in globular clusters.

Findings

Metallicity causes a 17% apparent size difference in clusters.

Metal-rich clusters have higher mass-to-light ratios.

Structural sizes are similar regardless of metallicity.

Abstract

Size differences of approx. 20% between red (metal-rich) and blue (metal-poor) sub-populations of globular clusters have been observed, generating an ongoing debate as to weather 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 = 100000 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% on average. The metallicity effect on the overall cluster luminosity also leads to higher mass-to-light ratios for metal-rich clusters.