# Massive stars in advanced evolutionary stages, and the progenitor of GW150914

## TL;DR

This paper investigates the evolution of massive stars, especially Wolf-Rayet stars, across different metallicities, to understand their role as progenitors of black hole mergers like GW150914, highlighting uncertainties in stellar modeling.

## Contribution

It provides empirical analysis of Wolf-Rayet populations at various metallicities and compares these with stellar evolution models, emphasizing the impact of metallicity and binary interactions on massive star evolution.

## Key findings

- Galactic stellar winds may prevent high-mass remnants at solar metallicity.
- Low-metallicity environments like the SMC support quasi-homogeneous evolution of WN stars.
- Close binary systems can produce high-mass remnants that merge within a Hubble time.

## Abstract

The recent discovery of a gravitational wave from the merging of two black holes of about 30 solar masses each challenges our incomplete understanding of massive stars and their evolution. Critical ingredients comprise mass-loss, rotation, magnetic fields, internal mixing, and mass transfer in close binary systems. The imperfect knowledge of these factors implies large uncertainties for models of stellar populations and their feedback. In this contribution we summarize our empirical studies of Wolf-Rayet populations at different metallicities by means of modern non-LTE stellar atmosphere models, and confront these results with the predictions of stellar evolution models. At the metallicity of our Galaxy, stellar winds are probably too strong to leave remnant masses as high as 30 solar masses, but given the still poor agreement between evolutionary tracks and observation even this conclusion is debatable. At the low metallicity of the Small Magellanic Cloud, all WN stars which are (at least now) single are consistent with evolving quasi-homogeneously. O and B-type stars, in contrast, seem to comply with standard evolutionary models without strong internal mixing. Close binaries which avoided early merging could evolve quasi-homogeneously and lead to close compact remnants of relatively high masses that merge within a Hubble time.