Multiple Stellar Populations at Less Evolved Stages. III. A Possible Helium Spread in NGC 2210

Abstract

Helium variations are common features of globular clusters (GCs) with multiple stellar populations. All the formation scenarios predict that secondary population stars are enhanced in helium, but the exact helium content depends on the polluters. Therefore, searching for helium variations in a star cluster is a straightforward method to understand whether it hosts multiple populations or not and to constrain the formation scenario. Although this topic has been well explored for Galactic GCs, GCs beyond the Milky Way are challenging to study because of their large distances. This work studies the helium distribution of GK-type main-sequence (MS) dwarfs in an old (∼12.5 Gyr) GC in the Large Magellanic Cloud, NGC 2210, using the deep photometry observed by the Hubble Space Telescope. We compare the observed morphology of the MS with that of synthetic populations with different helium distributions. We confirm that NGC 2210 dwarfs have a helium spread, with an internal dispersion of δ Y ∼ 0.06–0.07. The fraction of helium-enriched stars depends on the δ Y distribution. A continuous δ Y distribution would indicate that more than half of MS stars are helium enriched (∼55%). If the δ Y distribution is discrete (bimodal), a fraction of ∼30% enriched stars is able to explain the observed morphology of the MS. We also find that the He-enriched population stars are more centrally concentrated than He-normal stars.