The Helium content of Globular Clusters: light element abundance
  correlations and HB morphology. I. NGC6752
  Helium has been proposed as the key element to interpret the observed
multiple main sequences (MS), subgiant branches (SGB) and red giant branches
(RGB), as well as the complex horizontal branch (HB) morphology in Globular
Clusters (GC). However, up to now, He was never directly measured in suitable
GC stars (8500<Teff<11500 K) with the purpose of verify this hypothesis. We
studied 7 hot blue horizontal branch (BHB) stars (Teff<11500 K) in the GC NGC
6752 with the purpose to measure their Helium content. In addition
Fe,Cr,Si,Ti,O,Na, and Ba abundances were measured. We could measure He
abundance only for stars warmer than Teff=8500 K. All our targets with
measurable He are zero age HB (ZAHB) objects and turned out to have a
homogeneous He content with a mean value of Y=0.245+-0.012, compatible with the
most recent measurements of the primordial He content of the Universe (Y~0.25).
The whole sample of stars have a metallicity of [Fe/H]=-1.56+-0.03 and
[alpha/Fe]=+0.21+-0.03. Our HB targets show the same Na-O anticorrelation
identified among the TO-SGB-RGB stars. This is the first direct measurement of
the He abundance for a significative sample of GC stars in a temperature regime
where the He content is not altered by sedimentation processing or extreme
mixing as suggested for the hottest, late helium flasher HB stars.
