---
res:
  bibo_abstract:
  - Stellar wind mass loss of massive stars is often assumed to depend on their metallicity
    Z. Therefore, evolutionary models predict that massive stars in lower-Z environments
    are able to retain more of their hydrogen-rich layers and evolve into brighter
    cool supergiants (cool SGs; Teff < 7 kK). Surprisingly, in galaxies in the metallicity
    range 0.2 ≲ Z/Z⊙ ≲ 1.5, previous studies have not found a metallicity dependence
    on the upper luminosity limit Lmax of cool SGs. Here, we add four galaxies to
    the sample studied for this purpose with data from the Hubble Space Telescope
    and the James Webb Space Telescope (JWST). Observations of the extremely metal-poor
    dwarf galaxy I Zw 18 from JWST allow us to extend the studied metallicity range
    down to Z/Z⊙ ≈ 1/40. For cool SGs in all studied galaxies, including I Zw 18,
    we find a constant value of Lmax ≈ 105.6 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 previously inferred. Although we cannot rule out that some of them
    become intermediate-temperature objects, this paints a picture in which evolved
    stars with L > 105.6 L⊙ burn helium as hot, helium-rich stars down to extremely
    low metallicity. We argue that metallicity-independent late-phase mass loss would
    be the most likely mechanism responsible for this. Regardless of the exact stripping
    mechanism (winds or, for example, binary interaction), 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 metallicity emit sufficiently hard ionizing
    radiation to produce He II and C IV lines. In this scenario, late-phase metallicity-independent
    mass loss produces hot, helium-rich stars. Due to the well-understood metallicity
    dependence on the radiation-driven winds of hot stars, a window of opportunity
    would open below 0.2 Z⊙, where self-stripped helium-rich stars can exist without
    dense Wolf-Rayet winds that absorb hard ionizing radiation.@eng
  bibo_authorlist:
  - foaf_Person:
      foaf_givenName: Abel
      foaf_name: Schootemeijer, Abel
      foaf_surname: Schootemeijer
  - foaf_Person:
      foaf_givenName: Ylva Louise Linsdotter
      foaf_name: Götberg, Ylva Louise Linsdotter
      foaf_surname: Götberg
      foaf_workInfoHomepage: http://www.librecat.org/personId=d0648d0c-0f64-11ee-a2e0-dd0faa2e4f7d
    orcid: 0000-0002-6960-6911
  - foaf_Person:
      foaf_givenName: Norbert
      foaf_name: Langer, Norbert
      foaf_surname: Langer
  - foaf_Person:
      foaf_givenName: Giacomo
      foaf_name: Bortolini, Giacomo
      foaf_surname: Bortolini
  - foaf_Person:
      foaf_givenName: Alec S.
      foaf_name: Hirschauer, Alec S.
      foaf_surname: Hirschauer
  - foaf_Person:
      foaf_givenName: Lee
      foaf_name: Patrick, Lee
      foaf_surname: Patrick
  bibo_doi: 10.1051/0004-6361/202557675
  bibo_volume: 707
  dct_date: 2026^xs_gYear
  dct_isPartOf:
  - http://id.crossref.org/issn/0004-6361
  - http://id.crossref.org/issn/1432-0746
  dct_language: eng
  dct_publisher: EDP Sciences@
  dct_title: A constant upper luminosity limit of cool supergiant stars down to the
    extremely low metallicity of I Zw 18@
...