The Link between Hot and Cool Outflows

TL;DR

This paper demonstrates how understanding hot and cool stellar outflows, combined with metallicity and temperature-dependent mass loss, is essential for predicting black hole masses below the pair-instability supernova gap, especially for low-metallicity blue supergiants.

Contribution

It introduces a stellar evolution model showing low-Z blue supergiants can retain large envelopes and produce massive black holes below the PISN gap by maintaining temperatures above 8000 K.

Findings

Low-Z BSGs can avoid strong winds if kept above 8000 K.

Such stars can retain large envelopes before collapse.

This explains the formation of black holes like the one in GW190521.

Abstract

The link between hot and cool stellar outflows is shown to be critical for correctly predicting the masses of the most massive black holes (BHs) below the so-called pair-instability supernova (PISN) mass gap. Gravitational Wave (GW) event 190521 allegedly hosted an "impossibly" heavy BH of 85 Solar Masses. Here we show how our increased knowledge of both metallicity Z and temperature dependent mass loss is critical for our evolutionary scenario of a low-Z blue supergiant (BSG) progenitor of an initially approx 100 Solar Masses star to work. We show using MESA stellar evolution modelling experiments that as long as we can keep such stars above 8000 K such low-Z BSGs can avoid strong winds, and keep a very large envelope mass intact before core collapse. This naturally leads to the Cosmic Time dependent maximum BH function below the PISN gap.