The Effects of Accretion Feedback on Stellar Evolution in AGN Disks

TL;DR

This study models how feedback from accreting stars in AGN disks influences their growth, structure, and evolution, revealing that feedback limits accretion rates and affects stellar properties and populations.

Contribution

It introduces a semi-analytical model incorporating accretion feedback effects into stellar evolution calculations within AGN disks, expanding understanding of stellar growth limits and properties.

Findings

Accretion feedback limits stellar accretion rates below 0.1 solar masses per year.

Feedback suppresses runaway accretion, allowing long-lived equilibrium states.

Including gap opening and feedback alters stellar masses, radii, and luminosities significantly.

Abstract

Stars embedded in the accretion disks of active galactic nuclei (AGN) can accrete rapidly from their surroundings, dramatically altering their structure and evolution. However, feedback from the release of gravitational potential energy and radiative enthalpy by accreting gas can limit accretion rates, as recently demonstrated in radiation hydrodynamics simulations. To determine the importance of these effects neglected in earlier stellar evolution calculations, we incorporate these feedback processes into a semi-analytical model of stellar structure and evolution and conduct a suite of calculations spanning a broad parameter space of AGN disk conditions drawn from $\alpha$-disk models with central black hole masses $M_\bullet/M_\odot \in [10^6, 10^9]$. We find that accretion feedback limits stellar accretion rates below $\sim 10^{-1}\,M_\odot\,\mathrm{yr}^{-1}$, reducing the sensitivity of stellar evolution on disk properties. This suppression eliminates runaway accretion in models where it would otherwise occur, broadening the parameter space over which stars can reach long-lived ``immortal'' equilibria between accretion and mass loss. When gap opening is also accounted for, accretion feedback significantly alters stellar properties: it can reduce accretion and mass-loss rates by over an order of magnitude, reducing the strength of accretion shocks and thereby increasing equilibrium stellar masses and radii. These higher masses correspond to higher intrinsic luminosities, suggesting that neglecting accretion feedback may lead to an underestimate of disk chemical enrichment rates. Additionally, accretion feedback is important for predicting the properties of stellar populations within AGN disks, and associated transient phenomena.