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

Abel Schootemeijer; Ylva Götberg; Norbert Langer; Giacomo Bortolini; Alec S. Hirschauer; Lee Patrick

doi:10.1051/0004-6361/202557675

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 $Z$ . Therefore, evolutionary models of massive stars at lower $Z$ are able to retain more of their H-rich layers and evolve into brighter cool supergiants (cool SGs; $T_{eff} < 7$ kK). Surprisingly, in galaxies in the range $0.2 ≲ Z / Z_{⊙} ≲ 1.5$ previous studies did not find a $Z$ -dependence of the upper luminosity limit $L_{max}$ of cool SGs. Here, we study four extra galaxies with HST and JWST. Observations of the extremely low- $Z$ dwarf galaxy I Zw 18 from JWST allow us to go down to $Z / Z_{⊙} \approx 1/40$ . For cool SGs in all studied galaxies including I Zw 18, we find a constant $L_{max} \approx 1 0^{5.6}$ L $_{⊙}$ , similar to literature results for $0.2 ≲ Z / Z_{⊙} ≲ 1.5$ . 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 $L > 1 0^{5.6}$ L $_{⊙}$ burn He as hot He-rich stars down to extremely low $Z$ . We argue that $Z$ -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 $M ≳ 30$ M $_{⊙}$ to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low $Z$ emit sufficiently hard ionizing radiation to produce lines of He II and C IV. In this scenario, late-phase $Z$ -independent mass loss produces hot He-rich stars. Due to the well-understood $Z$ -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.

Originally 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

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

Abstract

References in corpus (84)