The helium content of globular clusters: NGC6121 (M4)
  He has been proposed as a key element to interpret the observed multiple MS,
SGB, and RGB, as well as the complex horizontal branch (HB) morphology. Stars
belonging to the bluer part of the HB, are thought to be more He rich (\Delta
Y=0.03 or more) and more Na-rich/O-poor than those located in the redder part.
This hypothesis was only partially confirmed in NGC 6752, where stars of the
redder zero-age HB showed a He content of Y=0.25+-0.01, fully compatible with
the primordial He content of the Universe, and were all Na-poor/O-rich. Here we
study hot blue HB (BHB) stars in the GC NGC 6121 (M4) to measure their He plus
O/Na content. We observed 6 BHB stars using the UVES@VLT2 spectroscopic
facility. In addition to He, O, Na, and Fe abundances were estimated. Stars
turned out to be all Na-rich and O-poor and to have a homogeneous enhanced He
content with a mean value of Y=0.29+-0.01(random)+-0.01(systematic). The high
He content of blue HB stars in M4 is also confirmed by the fact that they are
brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are
He-enhanced by \Delta Y=0.02-0.03 with respect to the RHB stars. The whole
sample of stars has a metallicity of [Fe/H]=-1.06+-0.02 (internal error). This
is a rare direct measurement of the (primordial) He abundance for stars
belonging to the Na-rich/O-poor population of GC stars in a temperature regime
where the He content is not altered by sedimentation or extreme mixing as
suggested for the hottest, late helium flash HB stars. Our results support
theoretical predictions that the Na-rich/O-poor population is also more He-rich
than the Na-poor/O-rich generation and that a leading contender for the 2^{nd}
parameter is the He abundance.
