Quasistars: Accreting black holes inside massive envelopes

TL;DR

This paper investigates the structure and evolution of quasistars, which are massive envelopes with accreting black holes, showing how they can rapidly grow black hole seeds to thousands of solar masses within a few million years.

Contribution

It provides analytical and numerical models of quasistars, revealing their temperature evolution, limiting conditions, and their role in forming massive black hole seeds quickly.

Findings

Black hole growth can exceed Eddington limit for the black hole but not for the envelope.

Photospheric temperature decreases with black hole mass, hitting a limit around 4000 K for metal-free quasistars.

Quasistars can produce black hole seeds of 1000-10000 solar masses in less than a few million years.

Abstract

We study the structure and evolution of "quasistars", accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate onto the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass. This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasistars. We derive analytically the scaling of the photospheric temperature with the black hole mass and envelope mass, and show that it decreases with time as the black hole mass increases. Once the photospheric temperature becomes lower than 10,000 K, the photospheric opacity drops precipitously and the photospheric temperature hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hydrostatic solution for the convective envelope exists. For metal-free (Population III) opacities this limiting temperature is approximately 4000 K. After a quasistar reaches this limiting temperature, the envelope is rapidly dispersed by radiation pressure. We find that black hole seeds with masses between 1000 and 10000 solar masses could form via this mechanism in less than a few Myr.