The Final Fates of Accreting Supermassive Stars

TL;DR

This study models the evolution and collapse of rapidly accreting supermassive stars, revealing their structural stability and collapse conditions, which are crucial for understanding the formation of supermassive black holes in the early universe.

Contribution

It provides detailed stellar evolution calculations including nuclear burning and GR effects, showing how accretion rate influences the star's stability and collapse mass.

Findings

Stars with higher accretion rates are more stable against GR instability.

Collapse occurs at higher masses for higher accretion rates.

Black holes of similar mass are expected to form after collapse.

Abstract

The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\gtrsim 10^5~M_\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\dot M \lesssim 0.1~M_\odot~{\rm yr}^{-1}$. With $\dot{M} \simeq 0.3 - 1~M_\odot~{\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage at $M \simeq 2 - 3.5~\times 10^5~M_\odot$. In an extreme case with $10~M_\odot~{\rm yr}^{-1}$, the star does not collapse until the mass reaches $\simeq 8.0\times 10^5~M_\odot$, where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.