Helium Stars: Towards an Understanding of Wolf-Rayet Evolution

TL;DR

This study models helium star evolution to understand Wolf-Rayet stars, showing how mass-loss rates influence their temperature, composition, and relation to supernova progenitors, challenging existing evolutionary scenarios.

Contribution

It introduces a grid of pure helium star models varying mass-loss rates, providing new insights into WR star temperature ranges, evolutionary paths, and supernova progenitor predictions.

Findings

Mass-loss rate variations reproduce observed WR star temperatures.

WN and WO stars originate from more massive stars, WC from lower masses.

WR stars are confirmed as progenitors of core-collapse supernovae.

Abstract

Wolf-Rayet (WR) stars are massive stars that have lost most or all of their hydrogen via powerful stellar winds. Recent observations have indicated that hydrogen-free WR stars have cooler temperatures than those predicted by current evolutionary models. To investigate how varying mass-loss rate affects WR evolution, we have created a grid of pure helium star models. We compare our results with Galactic and LMC WR observations and show that the temperature ranges of observed WR stars can be reproduced by varying the mass-loss rate, which effectively determines the size of the helium envelope around the core. We also find that WN and WO stars arise from more massive stars, whereas WC stars come from lower masses. This contradicts the standard Conti scenario by which WN and WC stars evolve in an age sequence. We also predict the magnitudes of our models at core-collapse and compare with observations of nearby progenitors of Type Ib/c supernovae. We confirm the findings of previous studies that suggest WR stars are the progenitors of core-collapse supernovae: the progenitors would remain unobserved except in the cases where the progenitor is a low-mass helium giant, as is the case of iPTF13bvn.