Helium in first and second-generation stars in globular clusters from spectroscopy of red giants

The stars in a cluster (GC) have always been considered coeval and of the same metallicity., this assumption has been challenged on the basis of spectroscopic and observations, which show various generations of stars in, differing in the abundances of products of-burning at high temperatures. The main final product of this burning is He. It is then important study the connections between stars properties and He content. consider here the nearly 1400 stars on the red giant branch (RGB) observed FLAMES@VLT in 19 Galactic Globular Clusters (GCs) in the course of our on the Na-O anticorrelation. Stars with different He are expected to different temperatures (i.e., different colours), slightly different [Fe/H], and different luminosity levels of the RGB bump. All these are small, but our study has the necessary precision, good, and homogeneity to detect them. Besides considering the observed and the temperatures and metallicities determined in our survey, we suitable sets of stellar models – fully consistent with those present the BaSTI archive – for various assumptions about the initial He. We find that differences in observable quantities that can be attributed variations in He content are generally detectable between stars of the (P, first-generation) and extreme (E, second-generation) populations, not between the primordial and intermediate ones (I). The only exception, differences are also significant between P and I populations, is the NGC 2808, where three populations are clearly separated also on the main and possibly on the horizontal branch. The average enhancement in the mass fraction Y between P and E stars is about 0.05–0.11, depending on the. The differences in Y, for NGC 2808 alone, are about 0.11–0.14 P and I stars, and about 0.15–0.19 between P and E stars, again on the assumptions. When we consider the RGB bump luminosity of first second-generation stars we find different levels; the implied Y is more difficult to quantify, but agrees with the other.