Helium-Star Models with Optically Thick Winds: Implications for the Internal Structures and Mass-Loss Rates of Wolf-Rayet Stars

TL;DR

This paper develops helium star models with optically thick winds to better understand the internal structures and mass-loss rates of Wolf-Rayet stars, aligning theoretical predictions with observed properties.

Contribution

It introduces a new modeling approach connecting hydrostatic helium cores to trans-sonic winds, providing improved estimates of mass-loss rates and internal structures of WR stars.

Findings

Mass-loss rates match observed WR star values.

Sonic points occur at specific temperatures below the Fe-opacity peak.

Predicted photospheric temperatures are warmer than observed.

Abstract

We construct helium (He) star models with optically thick winds and compare them with the properties of Galactic Wolf-Rayet (WR) stars. Hydrostatic He-core solutions are connected smoothly to trans-sonic wind solutions that satisfy the regularity conditions at the sonic point. Velocity structures in the supersonic parts are assumed by a simple beta-type law. By constructing a center-to-surface structure, a mass-loss rate can be obtained as an eigenvalue of the equations. Sonic points appear at temperatures ~ 1.8e5 - 2.8e5 K below the Fe-group opacity peak, where the radiation force becomes comparable to the local gravity. Photospheres are located at radii 3-10 times larger than sonic points. The obtained mass-loss rates are comparable to those of WR stars. Our mass-loss rate - luminosity relation agrees well with the relation recently obtained by Graefener et al. (2017). Photospheric temperatures of WR stars tend to be cooler than our predictions. We discuss the effects of stellar evolution, detailed radiation transfer, and wind clumping, which are ignored in this paper.