The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XII. The RGB Bumps of multiple stellar populations

摘要

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters isproviding a major breakthrough in our knowledge of Globular Clusters (GCs) andtheir stellar populations. Among the main results, we discovered that all thestudied GCs host two main discrete groups consisting of first generation (1G)and second generation (2G) stars. We exploit the multiwavelength photometryfrom this project to investigate, for the first time, the Red Giant Branch Bump(RGBB) of the two generations in a large sample of GCs. We identified, withhigh statistical significance, the RGBB of 1G and 2G stars in 26 GCs and foundthat their magnitude separation as a function of the filter wavelength followscomparable trends. The comparison of observations to synthetic spectra revealsthat the RGBB luminosity depends on the stellar chemical composition and thatthe 2G RGBB is consistent with stars enhanced in He and N and depleted in C andO with respect to 1G stars. For metal-poor GCs the 1G and 2G RGBB relativeluminosity in optical bands mostly depends on helium content, Y. We used theRGBB observations in F606W and F814W bands to infer the relative heliumabundance of 1G and 2G stars in 18 GCs, finding an average helium enhancement$\Delta$Y=0.011$\pm$0.002 of 2G stars with respect to 1G stars. This is thefirst determination of the average difference in helium abundance of multiplepopulations in a large number of clusters and provides a lower limit to themaximum internal variation of helium in GCs.