Global hot-star wind models for stars from Magellanic Clouds

TL;DR

This paper presents hydrodynamic models predicting stellar wind mass-loss rates for O stars in the Magellanic Clouds, accounting for lower metallicity and providing formulas for these rates based on stellar parameters.

Contribution

It introduces comprehensive global wind models for Magellanic Cloud stars, including a metallicity-dependent mass-loss rate formula, improving understanding of stellar winds in low-metallicity environments.

Findings

Mass-loss rates are lower than Galactic stars due to metallicity effects.

Predicted mass-loss rates scale with metallicity as Z^{0.59}.

Models align with some observational diagnostics, especially UV line profiles.

Abstract

We provide mass-loss rate predictions for O stars from Large and Small Magellanic Clouds. We calculate global (unified, hydrodynamic) model atmospheres of main sequence, giant, and supergiant stars for chemical composition corresponding to Magellanic Clouds. The models solve radiative transfer equation in comoving frame, kinetic equilibrium equations (also known as NLTE equations), and hydrodynamical equations from (quasi-)hydrostatic atmosphere to expanding stellar wind. The models allow us to predict wind density, velocity, and temperature (consequently also the terminal wind velocity and the mass-loss rate) just from basic global stellar parameters. As a result of their lower metallicity, the line radiative driving is weaker leading to lower wind mass-loss rates with respect to the Galactic stars. We provide a formula that fits the mass-loss rate predicted by our models as a function of stellar luminosity and metallicity. On average, the mass-loss rate scales with metallicity as $ \dot M\sim Z^{0.59}$. The predicted mass-loss rates are lower than mass-loss rates derived from H$\alpha$ diagnostics and can be reconciled with observational results assuming clumping factor $C_\text{c}=9$. On the other hand, the predicted mass-loss rates either agree or are slightly higher than the mass-loss rates derived from ultraviolet wind line profiles. The calculated \ion{P}{v} ionization fractions also agree with values derived from observations for LMC stars with $T_\text{eff}\leq40\,000\,$K. Taken together, our theoretical predictions provide reasonable models with consistent mass-loss rate determination, which can be used for quantitative study of stars from Magellanic Clouds.