The helium content of globular clusters: light element abundance correlations and HB morphology I. NGC 6752

Context: In the context of the multiple stellar population scenario in globular clusters (GC), helium (He) 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. However, up to now, He has never been directly measured in GC stars (8500 < T_eff < 11 500 K) to verify this hypothesis. Aims: We studied hot blue horizontal branch (BHB) stars in the GC NGC 6752 to measure their He content. Our goal is to verify the feasibility of He measurement from high resolution spectra in stars cooler than 11 500 K, where chemical abundances are not altered by sedimentation or levitation. Methods: We observed 7 BHB stars using the UVES@VLT2 spectroscopic facility. Spectra of S/N~200 were obtained and the very weak He line at 5875 Å measured. We compared this feature with synthetic spectra to obtain He abundances. In addition, iron peak (Fe, Cr), α (Si, Ti), light (O, Na), and s-element (Ba) abundances were measured. Results: We could measure He abundance only for stars warmer than T_eff = 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 has a metallicity of [Fe/H] = -1.56±0.03 and [ α/Fe] = +0.21±0.03, in agreement with other studies available in the literature. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars. Conclusions: 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 or extreme mixing as suggested for the hottest, late helium flasher HB stars.