Optically thick winds of very massive stars suppress intermediate-mass black hole formation

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Universität Heidelberg, Zentrum für Astronomie (ZAH), Institut für Theoretische Astrophysik, Albert Ueberle Str. 2, 69120 Heidelberg, Germany
³ Universität Heidelberg, Interdiszipliäres Zentrum für Wissenschaftliches Rechnen, 69120 Heidelberg, Germany
⁴ Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ INFN, Sezione di Padova, Via Marzolo 8, 35131 Padova, Italy
⁶ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, 67100 L’Aquila, Italy
⁷ INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi, Italy
⁸ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

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Received: 24 October 2025
Accepted: 19 December 2025

Abstract

Intermediate-mass black holes (IMBHs) are the link between stellar-mass and supermassive black holes. Gravitational waves have started to unveil a population of IMBHs in the ~100–300 M_⊙ range. Here, we investigate the formation of IMBHs from non-rotating very massive stars (VMSs, > 100 M_⊙). We calculate new VMS models that account for the transition from optically thin to optically thick winds, and study how this enhanced mass loss affects IMBH formation and the black hole mass function at intermediate and high metallicity (Z = 10⁻⁴−0.02). We show that optically thick winds suppress the formation of IMBHs from direct VMS collapse at metallicities Z > 0.001, one order of magnitude lower than predicted by previous models. Our models indicate that the stellar progenitors of GW231123 must have had a metallicity Z < 0.002, if the primary black hole formed via direct VMS collapse.