Driving classical Wolf-Rayet winds: A {\Gamma}- and Z-dependent mass-loss

TL;DR

This study develops hydrodynamically consistent models for Wolf-Rayet star winds, revealing their dependence on luminosity, metallicity, and Eddington ratio, and challenges previous theories on mass-loss limits.

Contribution

Introduces the first hydrodynamic models for WR star winds considering Gamma and metallicity, showing their launch mechanism and variable mass-loss rates.

Findings

WR winds are launched at the iron opacity peak.

Mass-loss rates strongly depend on Eddington-Gamma.

Below SMC metallicity, WR mass-loss rates decrease rapidly.

Abstract

Classical Wolf-Rayet (WR) stars are at a crucial evolutionary stage for constraining the fates of massive stars. The feedback of these hot, hydrogen-depleted stars dominates their surrounding by tremendous injections of ionizing radiation and kinetic energy. The strength of a WR wind decides the eventual mass of its remnant, likely a massive black hole. However, despite their major influence and importance for gravitational wave detection statistics, WR winds are particularly poorly understood. In this paper, we introduce the first set of hydrodynamically consistent stellar atmosphere models for classical WR stars of both the carbon (C) and nitrogen (N) sequence, i.e. WC and WN stars, as a function of stellar luminosity-to-mass ratio (or Eddington Gamma), and metallicity. We demonstrate the inapplicability of the CAK wind theory for classical WR stars and confirm earlier findings that their winds are launched at the (hot) iron (Fe) opacity peak. For $\log Z/Z_\odot > -2$, Fe is also the main accelerator throughout the wind. Contrasting previous claims of a sharp lower mass-loss limit for WR stars, we obtain a smooth transition to optically thin winds. Furthermore, we find a strong dependence of the mass-loss rates on Eddington-$\Gamma$, both at solar and sub-solar metallicity. Increases in WC carbon and oxygen abundances turn out to slightly reduce the predicted mass-loss rates. Calculations at subsolar metallicities indicate that below the metallicity of the SMC, WR mass-loss rates decrease much faster than previously assumed, potentially allowing for high black hole masses even in the local universe.