Multiple stellar populations at less evolved stages

IV. Evidence of helium enrichments in four Magellanic globular clusters

¹ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, China
² CSST Science Center for the Guangdong-Hongkong-Macau Greater Bay Area, Sun Yat-sen University, Zhuhai 519082, China
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
⁴ School of Mathematics and Physics, The University of Queensland, St. Lucia, QLD 4072, Australia

^★ Corresponding author; lichengy5@mail.sysu.edu.cn

Received: 11 November 2024
Accepted: 14 January 2025

Abstract

Most globular clusters in the local group show multiple stellar populations, a pattern defined by variations of several light elements, with some also displaying dispersions in heavy elements. Since helium is the most immediate product of hydrogen burning, almost all models suggest that second-generation stars should show different levels of helium enrichment than first-generation stars. Therefore, investigating helium enrichment in stellar populations of globular clusters can constrain different theoretical models. Using the deep photometry carried out by the Hubble Space Telescope, we have analyzed the morphologies of the main sequences of four Large Magellanic Cloud globular clusters, Hodge 11, NGC 1841, NGC 2210, and NGC 2257. We aim to constrain the helium distributions among their main sequence stars. We found a clear broadening of main sequences in all four clusters. After excluding the effects of photometric errors, differential reddening, unresolved binaries, and metallicity dispersions, this broadening would suggest a substantial helium enrichment in their star populations, ranging from δY = 0.08 to δY ≥ 0.12, depending on the presumed helium distributions. Helium-enriched stars are comparable in number to normal stars within these clusters, and may even dominate, comprising approximately ∼40% to over 80% of the total stellar population. However, given the great distance of the Large Magellanic Cloud, it is difficult to rule out the significant impact of crowding, so our results may only represent an upper limit.