paper

A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18

arXiv:2510.12594 · doi:10.1051/0004-6361/202557675

Abstract

Stellar wind mass loss is often assumed to depend on their metallicity . Therefore, evolutionary models of massive stars at lower are able to retain more of their H-rich layers and evolve into brighter cool supergiants (cool SGs; kK). Surprisingly, in galaxies in the range previous studies did not find a -dependence of the upper luminosity limit of cool SGs. Here, we study four extra galaxies with HST and JWST. Observations of the extremely low- dwarf galaxy I Zw 18 from JWST allow us to go down to . For cool SGs in all studied galaxies including I Zw 18, we find a constant L, similar to literature results for . In I Zw 18 and the other studied galaxies, the presence of Wolf-Rayet stars has been claimed. Although we cannot rule out that some of them become intermediate-temperature objects, this paints a picture in which evolved stars with L burn He as hot He-rich stars down to extremely low . We argue that -independent late-phase mass loss would be the most likely mechanism responsible. Regardless of the exact stripping mechanism, for the Early Universe our results imply a limitation on black hole masses and a contribution of stars born with M to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low emit sufficiently hard ionizing radiation to produce lines of He II and C IV. In this scenario, late-phase -independent mass loss produces hot He-rich stars. Due to the well-understood -dependence of radiation-driven winds of hot stars, a window of opportunity would open below 0.2Z, where self-stripped He-rich stars can exist without dense WR winds that absorb hard ionizing radiation.

arXiv commentOriginally submitted to arXiv on 14.10.2025 after submitting the paper to A&A. This version is the version that is published in A&A

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