Metal-poor single Wolf-Rayet stars: The interplay of optically thick winds and rotation

¹ Universität Heidelberg, Zentrum für Astronomie (ZAH), Institut für Theoretische Astrophysik, Albert Ueberle Str. 2, 69120 Heidelberg, Germany
² Universität Heidelberg, Interdiszipliäres Zentrum für Wissenschaftliches Rechnen, D-69120 Heidelberg, Germany
³ Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
⁴ Universität Heidelberg, Zentrum für Astronomie (ZAH), ARI, Monchhöfstr. 12–14, 69120 Heidelberg, Germany
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ INFN – Padova, Via Marzolo 8, I-35131 Padova, Italy
⁷ Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG Northern Ireland, UK

^⋆ Corresponding authors: lumen.boco@uni-heidelberg.de; mapelli@uni-heidelberg.de

Received: 30 June 2025
Accepted: 5 September 2025

Abstract

The Small Magellanic Cloud (SMC) hosts 12 known Wolf-Rayet (WR) stars, seven of which are apparently single. Their formation is a challenge for current stellar evolution models because line-driven winds are generally assumed to be quenched at a metallicity of Z ≤ 0.004. Here, we present a set of MESA models of single stars with zero-age main sequence masses of 20 − 80 M_⊙ considering different initial rotation speeds (Ω = 0 − 0.7 Ω_c), metallicities (Z = 0.002 − 0.0045), and wind mass-loss models (optically thin and thick winds). We show that if we account for optically thick winds, fast rotating (Ω ∼ 0.6 Ω_c) single metal-poor O-type stars (with M ≳ 20 M_⊙) shed their envelope and become WR stars even at the low metallicity of the SMC. The luminosity, effective temperature, evolutionary timescale, surface abundance, and rotational velocity of our simulated WR stars are compatible to the WRs observed in the SMC. We speculate that this scenario can also alleviate the excess of giant stars across the Humphreys-Davidson limit. Our results have key implications for black hole masses, (pair instability) supernova explosions, and other observable signatures.