The properties of supermassive stars in galaxy merger driven direct collapse I: models without rotation

The formation of the most massive quasars observed at high redshifts requires extreme accretion rates ($>1$ M${}_{\odot }$ yr${}^{-1}$). Inflows of $10-1000$ M${}_{\odot }$ yr${}^{-1}$ are found in hydrodynamical simulations of galaxy mergers, leading to the formation of supermassive discs (SMDs) with high metallicities ($>$ Z${}_{\odot }$). Supermassive stars (SMSs) born in these SMDs could be the progenitors of the most extreme quasars. Here, we study the properties of non-rotating SMSs forming in high metallicity SMDs. Using the stellar evolution code GENEC, we compute numerically the hydrostatic structures of non-rotating SMSs with metallicities $Z=1-10$ Z${}_{\odot }$ by following their evolution under constant accretion at rates $10-1000$ M${}_{\odot }$ yr${}^{-1}$. We determine the final mass of the SMSs, set by the general-relativistic (GR) instability, by applying the relativistic equation of adiabatic pulsations to the hydrostatic structures. We find that non-rotating SMSs with metallicities $Z=1-10$ Z${}_{\odot }$ accreting at rates $10-1000$ M${}_{\odot }$ 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 $\sim {10}^6$ M${}_{\odot }$, nearly independently of the metallicity and the accretion rate.