The properties of supermassive stars in galaxy merger-driven direct collapse

I. Models without rotation

Département d’Astronomie, Université de Genève, Chemin Pegasi 51, CH-1290 Versoix, Switzerland

^⋆ Corresponding author.

Received: 1 March 2025
Accepted: 23 June 2025

Abstract

Context. The formation of the most massive quasars observed at high redshifts requires extreme accretion rates (> 1 M_⊙ yr⁻¹). Inflows of 10−1000 M_⊙ yr⁻¹ are found in hydrodynamical simulations of galaxy mergers, leading to the formation of supermassive discs (SMDs) with high metallicities (> Z_⊙). Supermassive stars (SMSs) born in these SMDs could be the progenitors of the most extreme quasars.

Aims. Here, we study the properties of non-rotating SMSs forming in high-metallicity SMDs.

Methods. Using the stellar evolution code GENEC, we carried out a numerical computation of the hydrostatic structures of non-rotating SMSs with metallicities of Z = 1 − 10 Z_⊙ by following their evolution under constant accretion at rates of 10−1000 M_⊙ yr⁻¹. We determined the final mass of the SMSs, set by general-relativistic (GR) instability, by applying the relativistic equation of adiabatic pulsations to the hydrostatic structures.

Results. We find that non-rotating SMSs with metallicities of Z = 1 − 10 Z_⊙ and accreting at rates of 10−1000 M_⊙ yr⁻¹ evolve as red supergiant protostars until their final collapse. All the models reach the GR instability during H-burning. The final mass is ∼10⁶ M_⊙ and this result is found to be nearly independent of the metallicity and the accretion rate.