# The Evolution of Supermassive Population III Stars

## TL;DR

This study models the evolution of supermassive Population III stars with various accretion rates, revealing conditions under which they can grow to supermassive sizes without significant ionising feedback, supporting their role as quasar progenitors.

## Contribution

The paper introduces new stellar evolution models for supermassive Population III stars across a wide range of accretion rates, including relativistic effects, to better understand their growth and collapse.

## Key findings

- Stars with accretion rates >0.001 M_sun/yr evolve as cool supergiants.
- Stars with accretion rates ≤0.001 M_sun/yr become hot and blue.
- Ionising feedback remains weak at high accretion rates, favoring direct collapse.

## Abstract

Supermassive primordial stars forming in atomically-cooled halos at $z \sim15-20$ are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star evolves under accretion rates of $0.1 - 1$ $M_\odot$ yr$^{-1}$ until the general relativistic instability triggers its collapse to a black hole at masses of $\sim10^5$ $M_\odot$. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionising radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates $0.001 - 10$ $M_\odot$ yr$^{-1}$, computed with the GENEVA stellar evolution code including general relativistic corrections to the internal structure. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than $0.001$ $M_\odot$ yr$^{-1}$ the stars evolve as red, cool supergiants with surface temperatures below $10^4$ K towards masses $>10^5$ $M_\odot$, and become blue and hot, with surface temperatures above $10^5$ K, only for rates $\lesssim0.001$ $M_\odot$ yr$^{-1}$. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionising feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars.