In the context of the multiple stellar population scenario in globular clusters ( GC ) , helium ( He ) has been proposed as a 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 particular , second generation stars belonging to the bluer part of the HB , are thought to be more He rich ( \Delta Y=0.03 or more ) but also more Na-rich/O-poor than those located in the redder part that should have Y equal to the cosmological value .
Up to now 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 \pm 0.01 , fully compatible with the primordial He content of the Universe , and were all Na-poor/O-rich .
Here we study hot blue horizontal branch ( BHB ) stars in the GC NGC 6121 ( M4 ) to measure their He plus O/Na content .
Our goal is to complete the partial results obtained for NGC 6752 , focusing our attention on targets located on the bluer part of the HB of M4 .
We observed 6 BHB stars using the UVES @ VLT2 spectroscopic facility .
Spectra of S/N \sim 150 were obtained and the very weak He line at 5875 Å measured for all our targets .
We compared this feature with synthetic spectra to obtain He abundances .
In addition O , Na , and Fe abundances were estimated .
Stars turned out to be all Na-rich and O-poor and to have a homogeneous He content with a mean value of Y=0.29 \pm 0.01 ( random ) \pm 0.01 ( systematic ) , which is enhanced by \Delta Y \sim 0.04 with respect to the most recent measurements of the primordial He content of the Universe ( Y \sim 0.24 \div 0.25 ) .
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 \div 0.03 with respect to the RHB stars .
The whole sample of stars has a metallicity of [ Fe/H ] =-1.06 \pm 0.02 ( internal error ) , in agreement with other studies available in the literature .
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 .