The Peculiar Radial Distribution of Multiple Populations in the massive globular cluster M80

TL;DR

This study reveals that in the globular cluster M80, first-generation stars are more centrally concentrated than second-generation stars, likely due to differences in He content affecting stellar mass and distribution.

Contribution

The paper demonstrates that observed radial distributions of multiple populations can be explained by mass differences from He enrichment, not initial spatial arrangements.

Findings

First-generation stars are more centrally concentrated than second-generation stars.

A mass difference of 0.05-0.10 M_sun explains the radial distribution.

He enrichment of second-generation stars influences their spatial distribution.

Abstract

We present a detailed analysis of the radial distribution of light-element multiple populations (LE-MPs) in the massive and dense globular cluster M80 based on the combination of UV and optical Hubble Space Telescope data. Surprisingly, we find that first generation stars (FG) are significantly more centrally concentrated than extreme second generation ones (SG) out to $\sim 2.5 r_h$ from the cluster center. To understand the origin of such a peculiar behavior, we used a set of $N$-body simulations following the long-term dynamical evolution of LE-MPs. We find that, given the advanced dynamical state of the cluster, the observed difference does not depend on the primordial relative distributions of FG and SG stars. On the contrary, a difference of $\sim 0.05-0.10 M_{\odot}$ between the average masses of the two sub-populations is needed to account for the observed radial distributions. We argue that such a mass difference might be the result of the higher He abundance of SG stars (of the order of $\Delta Y\sim 0.05-0.06$) with respect to FG. Interestingly, we find that a similar He variation is necessary to reproduce the horizontal branch morphology of M80. These results demonstrate that differences in mass among LE-MPs, due to different He content, should be properly taken into account for a correct interpretation of their radial distribution, at least in dynamically evolved systems.