Modeling Supermassive Primordial Stars with MESA

TL;DR

This study models the evolution of supermassive primordial stars across various accretion rates using MESA, revealing their final masses and collapse behaviors, which depend on the accretion rate and influence early quasar formation.

Contribution

It provides a detailed grid of supermassive star evolution with a wider range of accretion rates than previous studies, highlighting their final masses and collapse mechanisms.

Findings

Final stellar masses range from 3,500 to 370,000 solar masses.

Stars with higher accretion rates evolve as red hypergiants and collapse via relativistic instability.

Stars with lower accretion rates evolve as blue supergiants and collapse after nuclear burning.

Abstract

Supermassive stars forming at $z \sim$ 15 - 20 are one of the leading contenders for the origin of the first quasars, over 200 of which have now been discovered at $z >$ 6. These stars likely form in pristine, atomically cooled haloes immersed in strong Lyman-Werner UV backgrounds or in highly supersonic baryon streaming flows. Atomic cooling triggers catastrophic baryon collapse capable of building up stars at rates of up to $\sim$1 M$_{\odot}$ yr$^{-1}$. Here we examine the evolution of supermassive stars with a much larger and finer grid of accretion rates than in previous studies with the \texttt{MESA} stellar evolution code. We find that their final masses range from 3.5 $\times$ 10$^3$ M$_{\odot}$ - 3.7 $\times$ 10$^5$ M$_{\odot}$ at accretion rates of 0.001 M$_{\odot}$ yr$^{-1}$ - 1 M$_{\odot}$ yr$^{-1}$, respectively. We also find that supermassive star evolution diverges at accretion rates of 0.01 M$_{\odot}$ yr$^{-1}$ - 0.02 M$_{\odot}$ yr$^{-1}$, above which they evolve as cool red hypergiants along the Hayashi track and collapse via the general relativistic instability during central hydrogen burning, and below which they evolve as hot blue supergiants and collapse at the end of their nuclear burning lifetimes after exiting the main sequence.