The evolution of accreting population III stars at 10⁻⁶–10³ M_⊙ yr⁻¹

¹ Geneva Observatory, Geneva University, 1290 Versoix, Switzerland
² Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
³ Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK
⁴ Institut fuer Astrophysik, Universität Zurich, Winterthurerstrasse 190, 805 Zurich, Switzerland

Received: 9 February 2024
Accepted: 16 July 2024

Abstract

Context. The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos.

Aims. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate.

Methods. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10⁻⁶–10³ M_⊙ yr⁻¹ and study how these rates determine final masses. The stars are evolved until either the end central Si burning or they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the star at one accretion rate.

Results. At rates below ∼2.5 × 10⁻⁵ M_⊙ yr⁻¹ the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M_⊙ is ∼5 × 10⁻⁵ M_⊙ yr⁻¹, well within the range of infall rates found in numerical simulations of halos that cool via H₂, ≲10⁻³ M_⊙ yr⁻¹. At rates of 5 × 10⁻⁵ M_⊙ yr⁻¹ to 4 × 10⁻² M_⊙ yr⁻¹, like those expected for halos cooling by both H₂ and Lyα, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures log(T_eff) = 3.8 and luminosities that can reach 10^10.5 L_⊙. At accretion rates of 100–1000 M_⊙ yr⁻¹ the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of ∼10⁶ M_⊙ without ever having become a star.

Conclusions. Our models corroborate previous studies of Pop III stellar evolution with and without hydrodynamics over separate, smaller ranges in accretion rate. They also reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse.