Evolution of supermassive stars as a pathway to black hole formation

TL;DR

This paper investigates the structure and evolution of rotationally-stabilized supermassive stars, revealing their non-polytropic nature and proposing a pathway for forming massive black hole seeds through core-collapse and accretion in quasistars.

Contribution

It provides a new model of supermassive stars with convective cores and stable envelopes, estimating initial black hole seed masses of 10^4-10^5 solar masses, different from previous assumptions.

Findings

Supermassive stars have convective cores with stable envelopes, not n=3 polytropes.

Core-collapse in these stars can produce black hole seeds of 10^4-10^5 solar masses.

Black hole seeds can grow via accretion in quasistars, enabling early supermassive black hole formation.

Abstract

Supermassive stars, with masses greater than a million solar masses, are possible progenitors of supermassive black holes in galactic nuclei. Because of their short nuclear burning timescales, such objects can be formed only when matter is able to accumulate at a rate exceeding ~ 1 solar mass/yr. Here we revisit the structure and evolution of rotationally-stabilized supermassive stars, taking into account their continuous accumulation of mass and their thermal relaxation. We show that the outer layers of supermassive stars are not thermally relaxed during much of the star's main sequence lifetime. As a result, they do not resemble n=3 polytropes, as assumed in previous literature, but rather consist of convective (polytropic) cores surrounded by convectively stable envelopes that contain most of the mass. We compute the structures of these envelopes, in which the specific entropy is proportional to the enclosed mass M(R) to the 2/3-power. By matching the envelope solutions to convective cores, we calculate the core mass as a function of time. We estimate the initial black hole masses formed as a result of core-collapse, and their subsequent growth via accretion from the bloated envelopes ("quasistars") that result. The seed black holes formed in this way could have typical masses in the range ~ 10^4-10^5 solar masses, considerably larger than the remnants thought to be left by the demise of Population III stars. Supermassive black holes therefore could have been seeded during an epoch of rapid infall considerably later than the era of Pop III star formation.