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

TL;DR

This study investigates how metallicity influences stellar winds in massive stars by analyzing 20 late-O and B supergiants in the SMC, revealing a clear metallicity dependence of wind properties and implications for stellar evolution.

Contribution

It provides a new metallicity-dependent recipe for wind momentum in massive stars and compares wind properties across different metallicities using consistent modeling techniques.

Findings

Wind momentum depends significantly on metallicity, mainly due to mass-loss rates.

No evidence of bi-stability jump effects between 25 and 21 kK.

Mass-loss rates are lower than previous estimates, affecting stellar evolution models.

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 $5.4 \leq \log{L_{\rm bol}/L_{\odot}} \leq6.1$ and $14 \leq T_{\rm eff}/{\rm kK} \leq 32$. 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.