Quantifying mass segregation and new core radii for 54 milky way globular clusters

TL;DR

This study measures core radii and mass segregation in 54 Milky Way globular clusters using star count data, revealing how mass segregation correlates with cluster properties and providing new insights into their internal structure.

Contribution

It introduces a method to quantify mass segregation and core radii by fitting star counts with King-Michie models, highlighting the relationship between mass segregation and cluster characteristics.

Findings

Mass segregation varies across clusters and can be quantified by a power-law slope.

The slope correlates strongly with core relaxation time and cluster size.

Most clusters show increased central concentration of more massive stars.

Abstract

We present core radii for 54 Milky Way globular clusters determined by fitting King-Michie models to cumulative projected star count distributions. We find that fitting star counts rather than surface brightness profiles produces results that differ significantly due to the presence of mass segregation. The sample in each cluster is further broken down into various mass groups, each of which is fit independently, allowing us to determine how the concentration of each cluster varies with mass. The majority of the clusters in our sample show general agreement with the standard picture that more massive stars will be more centrally concentrated. We find that core radius vs. stellar mass can be fit with a two parameter power-law. The slope of this power-law is a value that describes the amount of mass segregation present in the cluster, and is measured independently of our distance from the cluster. This value correlates strongly with the core relaxation time and physical size of each cluster. Supplementary figures are also included showing the best fits and likelihood contours of fit parameters for all 54 clusters.