On the role of the WNH phase in the evolution of very massive stars: Enabling the LBV instability with feedback

TL;DR

This paper introduces the WNH classification for luminous Wolf-Rayet stars with hydrogen, clarifies their evolutionary status, and links their feedback-driven mass loss to the LBV instability, enhancing understanding of massive star evolution.

Contribution

It defines WNH stars as a distinct class, clarifies their evolutionary phase as pre-LBVs, and explains how feedback from winds influences their progression and instability.

Findings

WNH stars have higher masses than H-free WR stars.

WNH stars' H mass fractions exceed those of LBVs.

Feedback from WNH winds drives LBV instability.

Abstract

(abridged) We propose the new designation ``WNH'' for luminous Wolf-Rayet (WR) stars of the nitrogen sequence with H in their spectra. These are commonly called WNL stars (e.g., WN7h), but this new shorthand avoids confusion because there are late-type WN stars without H and early-type WN stars with H. Clearly differentiating WNH stars from H-free WN stars is critical when discussing progenitors of Type Ib/c SNe and GRBs -- the massive WNH stars are not likely SN progenitors, and are distinct from core-He burning WR stars. We show that the masses of WNH stars are systematically higher than H-poor WR stars (both WN and WC), with little overlap, while H mass fractions of luminous WNH stars exceed those of LBVs. We favor the interpretation that the most luminous WNH stars are still core-H burning, pre-LBVs. While on the main sequence, a star's mass is reduced due to winds while its luminosity slowly rises, so the star increases its Eddington factor and mass-loss rate, pushing it even closer to the Eddington limit. With this feedback, observed masses and mass-loss rates of WNH stars are a natural outcome near the end of core-H burning. Feedback from the strong WNH wind itself then enables the instability seen later as an LBV. Steadily climbing mass-loss rates also explain continuity in spectral traits from O3 V to O3 If* to WNH.