Mass loss from late-type WN stars and its Z-dependence: very massive stars approaching the Eddington limit

TL;DR

This paper presents self-consistent models of late-type WN star winds, revealing their strong Z-dependent mass loss mechanisms, especially at low metallicity, with implications for early universe chemical enrichment and gamma-ray burst progenitors.

Contribution

It introduces the first self-consistent atmosphere/wind models for late-type WN stars, highlighting their mass loss dependence on metallicity and Eddington factor, and proposes a new parametrized mass loss recipe.

Findings

WNL stars are very massive stars near the Eddington limit with optically thick winds.

Mass loss strongly depends on metallicity, Eddington factor, and temperature.

Low-metallicity stars can still experience significant WR-type mass loss, aided by primary nitrogen enrichment.

Abstract

The mass loss from Wolf-Rayet (WR) stars is of fundamental importance for the final fate of massive stars and their chemical yields. Its Z-dependence is discussed in relation to the formation of long-duration Gamma Ray Bursts (GRBs) and the yields from early stellar generations. However, the mechanism of formation of WR-type stellar winds is still under debate. We present the first fully self-consistent atmosphere/wind models for late-type WN stars. We investigate the mechanisms leading to their strong mass loss, and examine the dependence on stellar parameters, in particular on the metallicity Z. We identify WNL stars as very massive stars close to the Eddington limit, potentially still in the phase of central H-burning. Due to their high L/M ratios, these stars develop optically thick, radiatively driven winds. These winds show qualitatively different properties than the thin winds of OB stars. The resultant mass loss depends strongly on Z, but also on the Eddington factor, and the stellar temperature. We combine our results in a parametrized mass loss recipe for WNL stars. According to our present model computations, stars close to the Eddington limit tend to form strong WR-type winds, even at very low Z. Our models thus predict an efficient mass loss mechanism for low metallicity stars. For extremely metal-poor stars, we find that the self-enrichment with primary nitrogen can drive WR-type mass loss. These first WN stars might play an important role in the enrichment of the early ISM with freshly produced nitrogen.