The evolution of supermassive Population III stars

Abstract

Supermassive primordial stars forming in atomically-cooled halos at $z\n\\sim15-20$ are currently thought to be the progenitors of the earliest quasars\nin the Universe. In this picture, the star evolves under accretion rates of\n$0.1 - 1$ $M_\\odot$ yr$^{-1}$ until the general relativistic instability\ntriggers its collapse to a black hole at masses of $\\sim10^5$ $M_\\odot$.\nHowever, the ability of the accretion flow to sustain such high rates depends\ncrucially on the photospheric properties of the accreting star, because its\nionising radiation could reduce or even halt accretion. Here we present new\nmodels of supermassive Population III protostars accreting at rates $0.001 -\n10$ $M_\\odot$ yr$^{-1}$, computed with the GENEVA stellar evolution code\nincluding general relativistic corrections to the internal structure. We use\nthe polytropic stability criterion to estimate the mass at which the collapse\noccurs, which has been shown to give a lower limit of the actual mass at\ncollapse in recent hydrodynamic simulations. We find that at accretion rates\nhigher than $0.001$ $M_\\odot$ yr$^{-1}$ the stars evolve as red, cool\nsupergiants with surface temperatures below $10^4$ K towards masses $>10^5$\n$M_\\odot$, and become blue and hot, with surface temperatures above $10^5$ K,\nonly for rates $\\lesssim0.001$ $M_\\odot$ yr$^{-1}$. Compared to previous\nstudies, our results extend the range of masses and accretion rates at which\nthe ionising feedback remains weak, reinforcing the case for direct collapse as\nthe origin of the first quasars.\n