Helium in first and second-generation stars in Globular Clusters from
  spectroscopy of red giants
  (abridged) Recent spectroscopic and photometric observations show the
existence of various generations of stars in GCs, differing in the abundances
of products of H-burning at high temperatures (the main final product being
He). It is important to study the connections between stars properties and He
content. We consider here the about 1400 stars on the Red Giant Branch (RGB)
observed with FLAMES@VLT in 19 Galactic GCs, part of out Na-O anticorrelation
projet. Stars with different He are expected to have different temperatures
(i.e. colours), slightly different [Fe/H], and different luminosity levels of
the RGB bump. All these differences are small, but our study has the necessary
precision, good statistics, and homogeneity to detect them. We also computed
suitable sets of stellar models (BaSTI) for various assumptions about the
initial helium content. Differences in observable quantities that can be
attributed to variations in He content are generally detectable between stars
of the Primordial (P, first-generation) and Extreme (E, second-generation)
populations, but not between the Primordial and Intermediate ones (I). The only
exception (differences are significant also between P and I populations) is
NGC2808, where three populations are clearly separated also on the Main
Sequence and the Horizontal Branch. The average enhancement in the He mass
fraction Y between P and E stars is about 0.05-0.11, depending on the
assumptions. The differences in Y, for NGC2808 alone, are about 0.11-0.14
between P and I stars, and about 0.15-0.19 between P and E stars, again
depending on the assumptions. The RGB bump luminosity of first and
second-generation stars has different levels; the implied Y difference is more
difficult to quantify, but is in agreement with the other determinations.
