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
Abstract
Considering the physics of radiation-driven winds of massive stars, the wind properties should depend on the metal content of the stellar atmosphere. Therefore, studying the winds of massive stars in different metallicities provides a sanity check on prescriptions that are widely used in evolutionary calculations. We obtained the stellar and wind properties of a sample of 20 late-O and B supergiants in the Small Magellanic Cloud (SMC) from a quantitative combined UV and optical spectroscopic analysis using CMFGEN. By comparing these properties with those of a Large Magellanic Cloud counterpart study, which has a similar sample and data, and employed the same modelling techniques used in this study, We derived a metallicity-dependent recipe for wind momentum, which is applicable for 5.4 ≤ L bol/L ≤6.1 and 14 ≤ T eff/ kK ≤ 32. 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 between 25 and 21~kK, which could be attributed to the bi-stability jump. 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 the characteristics of the WR population in the SMC.