{"arxiv":1,"department":[{"_id":"YlGo"}],"publication":"Astronomy & Astrophysics","_id":"21450","ddc":["520"],"volume":707,"oa":1,"date_created":"2026-03-15T23:01:35Z","abstract":[{"lang":"eng","text":"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."}],"quality_controlled":"1","status":"public","article_type":"original","external_id":{"arxiv":["2510.12594"]},"has_accepted_license":"1","month":"03","acknowledgement":"We thank our anonymous referee for carefully reading the manuscript and providing a constructive report with helpful feedback. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program #1233. The specific observations analyzed can be accessed via DOI: 10.17909/3c1d-6182. Moreover, this research is based in part on observations made with the NASA/ESA Hubble Space Telescope obtained from the\r\nSpace Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with programs #13664, GO-10915, and DD-11307. This research was supported in part by grant NSF PHY-2309135 to the Kavli Institute for Theoretical Physics (KITP). LRP acknowledges support by grants PID2019-105552RB-C41 and PID2022-137779OB-C41 funded\r\nby MCIN/AEI/10.13039/501100011033 by “ERDF A way of making Europe”. LRP acknowledges support from grant PID2022-140483NB-C22 funded by MCIN/AEI/10.13039/501100011033.","OA_type":"diamond","scopus_import":"1","article_number":"A116","file_date_updated":"2026-03-16T09:05:06Z","citation":{"apa":"Schootemeijer, A., Götberg, Y. L. L., Langer, N., Bortolini, G., Hirschauer, A. S., &#38; Patrick, L. (2026). A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href=\"https://doi.org/10.1051/0004-6361/202557675\">https://doi.org/10.1051/0004-6361/202557675</a>","ama":"Schootemeijer A, Götberg YLL, Langer N, Bortolini G, Hirschauer AS, Patrick L. A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18. <i>Astronomy &#38; Astrophysics</i>. 2026;707. doi:<a href=\"https://doi.org/10.1051/0004-6361/202557675\">10.1051/0004-6361/202557675</a>","ieee":"A. Schootemeijer, Y. L. L. Götberg, N. Langer, G. Bortolini, A. S. Hirschauer, and L. Patrick, “A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18,” <i>Astronomy &#38; Astrophysics</i>, vol. 707. EDP Sciences, 2026.","short":"A. Schootemeijer, Y.L.L. Götberg, N. Langer, G. Bortolini, A.S. Hirschauer, L. Patrick, Astronomy &#38; Astrophysics 707 (2026).","mla":"Schootemeijer, Abel, et al. “A Constant Upper Luminosity Limit of Cool Supergiant Stars down to the Extremely Low Metallicity of I Zw 18.” <i>Astronomy &#38; Astrophysics</i>, vol. 707, A116, EDP Sciences, 2026, doi:<a href=\"https://doi.org/10.1051/0004-6361/202557675\">10.1051/0004-6361/202557675</a>.","ista":"Schootemeijer A, Götberg YLL, Langer N, Bortolini G, Hirschauer AS, Patrick L. 2026. A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18. Astronomy &#38; Astrophysics. 707, A116.","chicago":"Schootemeijer, Abel, Ylva Louise Linsdotter Götberg, Norbert Langer, Giacomo Bortolini, Alec S. Hirschauer, and Lee Patrick. “A Constant Upper Luminosity Limit of Cool Supergiant Stars down to the Extremely Low Metallicity of I Zw 18.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences, 2026. <a href=\"https://doi.org/10.1051/0004-6361/202557675\">https://doi.org/10.1051/0004-6361/202557675</a>."},"date_updated":"2026-03-16T09:07:55Z","type":"journal_article","publication_status":"published","file":[{"file_size":2102107,"content_type":"application/pdf","creator":"dernst","access_level":"open_access","file_id":"21455","date_updated":"2026-03-16T09:05:06Z","success":1,"date_created":"2026-03-16T09:05:06Z","checksum":"02a0cd932340207c96fdd3059490ad29","file_name":"2026_AstronomyAstrophysics_Schootemeijer.pdf","relation":"main_file"}],"author":[{"first_name":"Abel","full_name":"Schootemeijer, Abel","last_name":"Schootemeijer"},{"id":"d0648d0c-0f64-11ee-a2e0-dd0faa2e4f7d","last_name":"Götberg","first_name":"Ylva Louise Linsdotter","orcid":"0000-0002-6960-6911","full_name":"Götberg, Ylva Louise Linsdotter"},{"first_name":"Norbert","full_name":"Langer, Norbert","last_name":"Langer"},{"full_name":"Bortolini, Giacomo","first_name":"Giacomo","last_name":"Bortolini"},{"last_name":"Hirschauer","first_name":"Alec S.","full_name":"Hirschauer, Alec S."},{"first_name":"Lee","full_name":"Patrick, Lee","last_name":"Patrick"}],"language":[{"iso":"eng"}],"OA_place":"publisher","doi":"10.1051/0004-6361/202557675","publisher":"EDP Sciences","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","year":"2026","intvolume":"       707","day":"01","date_published":"2026-03-01T00:00:00Z","oa_version":"Published Version","DOAJ_listed":"1","publication_identifier":{"issn":["0004-6361"],"eissn":["1432-0746"]},"PlanS_conform":"1","article_processing_charge":"No","title":"A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18","tmp":{"short":"CC BY (4.0)","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)","image":"/images/cc_by.png"}}