The Helium content of Globular Clusters: light element abundance correlations and HB morphology. I. NGC6752

TL;DR

This study directly measured helium content in hot blue horizontal branch stars of NGC 6752, finding a homogeneous primordial-like helium abundance, and linked it to other elemental abundances and cluster features.

Contribution

First direct measurement of helium abundance in a significant sample of globular cluster stars in the relevant temperature range, confirming primordial helium levels.

Findings

Helium abundance is homogeneous with a mean Y=0.245±0.012.

Stars show the Na-O anticorrelation typical of multiple populations.

Helium content aligns with primordial values, supporting its role in cluster morphology.

Abstract

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.