Accretion of AGN Stars under Influence of Disk Geometry II: The Adiabatic Regime and Runaway Collapse Induced by Self-gravity

Abstract

Accretion onto massive stars embedded in Active Galactic Nuclei (AGN) disks around supermassive black holes (SMBHs) is regulated to the stellar Eddington rate in the fast-diffusion or radiatively-efficient limit, $c/τ> c_s$, where $τ$ is the optical depth of the accretion flow and $c_s$ the sound speed. However, when the ambient density is sufficiently high, the opposite slow-diffusion limit applies. In this regime, accretion proceeds quasi-adiabatically and forms a hydrostatic circumstellar envelope (CSE) that stalls further mass inflow in the absence of self-gravity. We perform 3D hydrodynamic simulations in the adiabatic limit to investigate the structure and evolution of such envelopes. For low thermal mass ratios, $q_{\rm th} \equiv M_\star/M_{\rm th}$ where $M_{\rm th}=c_s^3/(GΩ)$ is the thermal mass, the CSE boundary smoothly matches the ambient disk entropy and density without forming a shock. In contrast, when $q_{\rm th} \gg 1$, a strong shock develops at the envelope boundary, substantially increasing the entropy of the envelope and thereby regulating its structure and mass, $M_{\rm env}$. In marginally self-gravitating disks with Toomre parameter $Q \sim 1$, we find that at sufficiently large $q_{\rm th}$ the envelope mass satisfies $M_{\rm env}/M_\star \gtrsim 1$. This condition is equivalent to stating that the post-shock material entering the envelope possesses lower radiation entropy than the characteristic stellar value, which triggers dynamical runaway growth on a dynamical timescale once envelope self-gravity is included in our simulations. In realistic AGN disk environments with SMBH mass $\sim 10^8M_\odot$, runaway may occur close to the minimum self-gravitating radii and produce supermassive stars of $\sim 10^5M_\odot$.