The helium content of globular clusters: light element abundance correlations and HB morphology

Abstract

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.

Evolution of long-lived globular cluster stars

“…We assume an initial helium mass fraction Y ini of 0.248 (i.e., slightly higher than the primordial helium abundance 0.2463 ± 0.0003 predicted by standard BBN by Coc et al 2013 Carretta et al 2007Carretta et al , 2009aVillanova et al 2009). …”

The metallicity of the open cluster Tombaugh 2

“…For Li, Ba, and Eu, whose abundances were obtained by the spectral synthesis method. σ EW was obtained using the χ 2 method, as described in Villanova et al (2009). In Table 4 we report in the second column the average σ EW for each element.…”

G2C2 – IV. A novel approach to study the radial distributions of multiple populations in Galactic globular clusters

“…Helium abundance variations are linked to the observed light element enhancements for the multiple generations in globular clusters (e.g. Carretta et al 2006;D'Antona & Ventura 2007;Salaris et al 2008;Villanova et al 2009;Marino et al 2011;Villanova et al 2012;Monelli et al 2013;Salaris et al 2013;Milone et al 2014;Gratton et al 2014;Mucciarelli et al 2014;Gratton et al 2015;Milone 2015) and may be taken to trace abundance variations in CNONa, even if these by themselves may not affect the morphology of the HB. Carretta et al (2010b) show that the maximum temperature of the HB correlates with the range of Na/O abundance in red giants.…”