The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XVI. The helium abundance of multiple populations

TL;DR

This study uses HST UV data to measure helium differences between multiple stellar populations in 57 globular clusters, revealing helium variation as a key factor influencing horizontal branch morphology.

Contribution

First comprehensive measurement of helium differences between 1G and 2G stars across a large sample of globular clusters using multi-wavelength photometry and synthetic spectra.

Findings

2G stars are helium-enhanced compared to 1G in all clusters.

Maximum helium variation correlates with cluster mass and HB color extension.

Helium variation is a major parameter affecting HB morphology.

Abstract

Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first- (1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host sub-stellar populations with different chemical composition. We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G stars are consistent with being enhanced in helium with respect to 1G. The maximum helium variation ranges from less than 0.01 to more than 0.10 in helium mass fraction and correlates with both the cluster mass and the color extension of the horizontal branch (HB). These findings demonstrate that the internal helium variation is one of the main (second) parameters governing the HB morphology.