The Hubble Space Telescope UV legacy survey of galactic globular clusters – XVI. The helium abundance of multiple populations

Abstract

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