Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters III: Analysis of 30 Clusters

TL;DR

This study employs a Bayesian approach to analyze 30 Galactic Globular Clusters, revealing helium differences between stellar populations and their relation to cluster mass, while addressing model-data discrepancies.

Contribution

Introduces a Bayesian method to simultaneously estimate multiple parameters of stellar populations in globular clusters, providing new insights into their helium variations and formation scenarios.

Findings

Helium differences range from ~0.04 to 0.11 among populations.

Proportion of first population increases with cluster mass.

Helium content correlates with cluster mass and formation models.

Abstract

We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival ACS Treasury observations of 30 Galactic Globular Clusters to characterize two distinct stellar populations. A sophisticated Bayesian technique is employed to simultaneously sample the joint posterior distribution of age, distance, and extinction for each cluster, as well as unique helium values for two populations within each cluster and the relative proportion of those populations. We find the helium differences among the two populations in the clusters fall in the range of ~0.04 to 0.11. Because adequate models varying in CNO are not presently available, we view these spreads as upper limits and present them with statistical rather than observational uncertainties. Evidence supports previous studies suggesting an increase in helium content concurrent with increasing mass of the cluster and also find that the proportion of the first population of stars increases with mass as well. Our results are examined in the context of proposed globular cluster formation scenarios. Additionally, we leverage our Bayesian technique to shed light on inconsistencies between the theoretical models and the observed data.