The structure and evolution of quasi-stars

TL;DR

This paper models the evolution of quasi-stars, massive objects with black holes and gas envelopes, showing the black hole can grow to over 10% of the system's mass before hydrostatic equilibrium fails, within a few million years.

Contribution

It provides detailed stellar evolution models of quasi-stars, revealing their mass growth limits and the conditions affecting their hydrostatic stability.

Findings

Black holes in quasi-stars reach over 10% of total mass.

Hydrostatic equilibrium breaks down after a few million years.

Solutions are sensitive to the inner boundary radius, with no solutions below 0.3 of the Bondi radius.

Abstract

The existence of bright quasars at high redshifts implies that supermassive black holes were able to form in the early Universe. Though a number of mechanisms to achieve this have been proposed, none yet stands out. A recent suggestion is the formation of quasi-stars, initially stellar-mass black holes accreting from hydrostatic giant-like envelopes of gas, formed from the monolithic collapse of pre-galactic gas clouds. In this work, we modify the Cambridge STARS stellar evolution package to construct detailed models of the evolution of these objects. We find that, in all of our models, the black hole inside the envelope is able to reach slightly more than one-tenth of the total mass of the system before hydrostatic equilibrium breaks down. This breakdown occurs after a few million years of evolution. We show that the mechanism which causes the hydrostatic evolution to end is present in polytropic models. We also show that the solutions are highly sensitive to the size of the inner boundary radius and that no physical solutions exist if the inner boundary is chosen to be less than about 0.3 of the Bondi radius.