Accretion of AGN Stars under Influence of Disk Geometry

TL;DR

This study uses 3D radiation hydrodynamic simulations to explore how the geometry and temperature of AGN disks influence the accretion behavior of massive stars, revealing conditions for isotropic and anisotropic accretion and outflow dynamics.

Contribution

It introduces a realistic stratified disk geometry into accretion modeling, showing how disk temperature and scale height affect accretion flow anisotropy and outflow mechanisms.

Findings

Accretion remains isotropic in hot, thick disks.

Cold disks induce anisotropic, super-thermal accretion flows.

Super-Eddington outflows occur in polar regions, with sustained midplane accretion.

Abstract

Massive stars can form within or be captured by AGN disks, influencing both the thermal structure and metallicity of the disk environment. In a previous work, we investigated isotropic accretion onto massive stars from a gas-rich, high-entropy background. Here, we consider a more realistic scenario by incorporating the stratified geometry of the background disk in our 3D radiation hydrodynamic simulatons. We find that accretion remains relatively isotropic when the disk is hot enough and the scale height is thicker than the accretion flow's nominal supersonic critical radius $R{crit}$ (sub-thermal). However, when the disk becomes cold, the accretion flow becomes significantly anisotropic (super-thermal). Escaping stellar and accretion luminosity can drive super-Eddington outflows in the polar region, while rapid accretion is sustained along the midplane. Eventually, the effective cross-section is constrained by the Hill radius and the disk scale height rather than the critical radius when the disk is cold enough. For our setup (stellar mass $\sim 50 M\odot$ and background density $\rho\sim 10^{-10}$ g/cm$^3$) the accretion rates is capped below $\sim 0.02M\odot$/year and the effective accretion parameter $\alpha\sim 10^{-1}$ over disk temperature range $3 - 7 \times 10^4$ K. Spiral arms facilitate inward mass flux by driving outward angular momentum transport. Gap-opening effects may further reduce the long-term accretion rate, albeit to confirm which requires global simulations evolved over much longer viscous timescales.