The Growth of the Central Black Holes in Quasi-stars

Abstract

Observations by JWST have confirmed the presence of supermassive black holes (BHs) at redshifts $z\gtrsim 10$, lending support to scenarios in which BHs experience rapid growth through intense gas accretion. Here we investigate the growth of a BH embedded at the center of a quasi-star, a theoretically predicted object formed via direct collapse. In a quasi-star, the central BH accretes at a highly super-Eddington rate, while the excess energy is transported outward by convection and radiated at approximately the Eddington luminosity of the entire star. We employ the open-source stellar evolution code \texttt{MESA} to construct quasi-star models and follow the time-dependent growth of the central BH under different prescriptions for the accretion rate at the inner boundary $R_i$, and further considering the effect of winds. For the case $R_i=N{R}_{\mathrm{B}}$, where $N$ is a constant and $R_{\mathrm{B}}$ is the Bondi radius corresponding to the mass of the BH and the gas infalling onto it, our models terminate when the BH mass reaches a critical value $M_{\mathrm{crit}}(N)=c_{s,i}^3/(12\sqrt{N^3{G}^3\ddot{I}€\ddot{I}{}_i})$ (where $c_{s,i}$ and $\ddot{I}{}_i$ are the sound speed and density at $R_i$, respectively), a limit we also derive analytically. Models that feature an inner convective region matched to an outer adiabatic envelope exhibit BH growth up to approximately $M_{\mathrm{BH}}/M_{\star }\simeq 0.33$, largely independent of the stellar mass $M_{\star }$ itself. This ratio is approximately preserved even in the presence of mass loss, as several properties of the model are independent of the quasi-star's total mass.

17 pages, 12 figures