On the Rotation of Supermassive Stars

TL;DR

This paper models the evolution of supermassive Population III stars, revealing they must accrete angular momentum at about 1% of Keplerian levels, making them slow rotators with minimal deformation, which constrains their formation process.

Contribution

It introduces a stellar evolution model including accretion and rotation, establishing a tight angular momentum constraint for supermassive star formation.

Findings

Supermassive stars must accrete about 1% of Keplerian angular momentum.

Such stars are slow rotators with surface velocities less than 20% of critical velocity.

Rotation-induced deformation of these stars is negligible.

Abstract

Supermassive stars born from pristine gas in atomically-cooled haloes are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star including simultaneously accretion and rotation. We find that, for supermassive stars to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the $\Omega\Gamma$-limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that supermassive stars are slow rotators, with a surface velocity less than 10-20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible.