Chemical inhomogeneities amongst first population stars in globular clusters

Evidence for He variations

¹ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: carmela.lardo@epfl.ch
² Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
³ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA

Received: 10 March 2018
Accepted: 24 May 2018

Abstract

Spreads in light element abundances among stars (also known as 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 (called the “chromosome map”), to maximise the separation between the different multiple populations. For each individual cluster its chromosome map separates the first population (with metal abundance patterns typical of field halo stars) from the second population (which 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, and that these spreads appear only in some clusters, challenges the scenarios put forward so far to explain the multiple population phenomenon.