Chemical Inhomogeneities Amongst First Population Stars in Globular Clusters: Evidence for He variations

TL;DR

This study investigates chemical inhomogeneities in first population stars of globular clusters, revealing variations in helium and nitrogen abundances that challenge existing models of multiple stellar populations.

Contribution

It provides evidence for initial helium and nitrogen abundance spreads in first population stars, highlighting cluster-to-cluster differences in chemical inhomogeneities.

Findings

Extended distribution of first population stars in chromosome maps suggests chemical inhomogeneity.

Helium and nitrogen abundance spreads are linked to the observed distributions.

Some clusters show no such inhomogeneities, indicating diverse formation histories.

Abstract

Spreads in light element abundances among stars (a.k.a. multiple populations) are observed in nearly all globular clusters. One way to map such chemical variations using high-precision photometry is to employ a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters (the so called "chromosome map"), to maximise the separation between the different multiple populations. For each individual cluster its chromosome map separates the so-called first population -with metal abundance patterns typical of field halo stars- from the second population, that displays distinctive abundance variations among a specific group of light-elements. Surprisingly, the distribution of first population stars in chromosome maps of several -but not all- clusters has been found to be more extended than expected from purely observational errors, suggesting a chemically inhomogeneous origin. We consider here three clusters with similar metallicity ([Fe/H]~-1.3) and different chromosome maps, namely NGC 288, M 3 and NGC 2808, and argue that the first population extended distribution (as observed in two of these clusters) is due to spreads of the initial helium abundance and possibly a small range of nitrogen abundances as well. The presence of a range of initial He and N abundances amongst stars traditionally thought to have homogeneous composition, plus the fact that these spreads appear only in some clusters, challenge the scenarios put forward so far to explain the multiple population phenomenon.