X-Shooting ULLYSES: Massive stars at low metallicity

XIV. Properties of SMC late-O and B supergiants reveal the metallicity dependence of winds in the Magellanic Clouds

¹ Astrophysics Research Cluster, School of Mathematical and Physical Sciences, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
² School of Chemical, Materials and Biological Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁵ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁶ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁷ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str 24/25, 14476 Potsdam, Germany
⁹ Lennard-Jones Laboratories, Keele University ST5 5BG, UK
¹⁰ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany

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

Received: 27 October 2025
Accepted: 28 January 2026

Abstract

Context. Hot massive stars lose mass through radiation-driven winds, producing significant chemical, radiative, and mechanical feedback in the surrounding environment. The properties of these winds play a crucial role in determining the star’s evolutionary path. Considering the physics of radiation-driven winds, the wind properties should depend on the metal content of the stellar atmosphere. Therefore, studying the wind properties of massive stars in different metallicities (Z) provides a sanity check on prescriptions that are widely used in evolutionary calculations.

Aims. We first aim to obtain the stellar and wind properties of a sample of late-O and B supergiants in the Small Magellanic Cloud (SMC). Using these properties, we aim to quantify the dependence of wind properties on metallicity by comparing them with those of a Large Magellanic Cloud (LMC) counterpart study, which has a similar sample and data, and employed the same modelling techniques used in this study.

Methods. Spectroscopic modelling of UV and optical data from ULLYSES and XShootU was performed using the radiative transfer code CMFGEN. We also employed an updated Bayesian inference method similar to BONNSAI to explore the evolutionary history of our sample.

Results. We derived the stellar and wind properties of 20 late-O and B supergiants. We derived the following metallicity-dependent recipe for wind momentum: log D_mom = (1.64 − 0.75 log Z/Z_⊙) log (L_bol/10⁶ L_⊙) + 1.38 log Z/Z_⊙ + 29.17, which is applicable for 5.4 ≤ log L_bol/L_⊙ ≤ 6.1 and 14 ≤ T_eff/kK ≤ 32.

Conclusions. We find a significant dependence of the wind momentum on the metallicity, which is largely due to the mass-loss rates. We do not find any evidence of a discontinuity in either the mass-loss rate or the ratio of the terminal wind velocity to the escape velocity, v_∞/v_esc, between 25 and 21 kK, which could be attributed to the bi-stability jump, although when taking into account the effect of luminosity in the transformed mass-loss rate, the behaviour appears to be different. Stellar parameters are consistent across different methods and radiative transfer codes, whereas mass-loss rates differ significantly with our values being generally lower. We find a discrepancy between the evolutionary and spectroscopic masses in 40% of our sample, with the evolutionary mass usually being systematically higher. The mass-loss rates of blue supergiants are far too low to strip the stellar envelope and the subsequent formation of classical Wolf-Rayet (WR) stars, leading to the conclusion that luminous blue variable eruptions or binary interactions are necessary to explain characteristics of the WR population in the SMC.