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

¹ Argelander-Institut für Astronomie, Universität Bonn Auf dem Hügel 71 53121 Bonn, Germany
² Institute of Science and Technology Austria (ISTA) Am Campus 1 3400 Klosterneuburg, Austria
³ Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn, Germany
⁴ Department of Astronomy, The Oskar Klein Centre, Stockholm University AlbaNova 10691 Stockholm, Sweden
⁵ Department of Physics & Engineering Physics, Morgan State University 1700 East Cold Spring Lane Baltimore MD 21251, USA
⁶ Centro de Astrobiología (CSIC-INTA) Ctra. Torrejón a Ajalvir km 4 28850 Torrejón de Ardoz, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 October 2025
Accepted: 17 December 2025

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; T_eff < 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 L_max 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 L_max ≈ 10^5.6 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 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 > 10^5.6 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.