Stellar Evolution with Radiative Feedback in AGN Disks

TL;DR

This paper models the evolution of stars within AGN disks, revealing how radiative feedback and wind-driven mass loss influence their growth, chemical yields, and potential to explain high metallicity in AGN environments.

Contribution

It introduces one-dimensional MESA simulations of embedded stars in AGN disks, incorporating radiative feedback and wind loss, to explore their unique evolutionary paths and chemical enrichment effects.

Findings

Embedded stars can reach large masses but are limited by feedback.

Radiative feedback prevents runaway stellar growth in AGN disks.

Metamorphic stars enrich disks with heavy elements, explaining high metallicity.

Abstract

Stars embedded in the inner pc region of an active galactic nucleus (AGN) experience extreme accretion conditions that significantly alter their evolution. We present one-dimensional MESA simulations of stars growing and decaying within AGN disks, implementing radiative-feedback-regulated accretion which limits stellar growth near the Eddington luminosity, as well as wind-driven mass loss. Unlike stand-alone stars in the field, these embedded stars follow unique evolutionary tracks with well-determined mass evolution and chemical yields. We distinguish two regimes: ``immortal" stars that indefinitely remain on the main sequence due to efficient hydrogen mixing; and ``metamorphic" stars that evolves off the main sequence, ultimately enriching the disk with heavy elements upon hydrogen and helium exhaustion in their cores. Results indicate that embedded stars in AGN disks can attain large masses, but gas retention and limited mixing likely render the ``immortal" track unsustainable. We show radiative feedback plays a critical role in preventing runaway growth, since it regulates the inflow to at most of order-unity the Eddington-limited mass-loss rate. Embedded metamorphic stars significantly enrich AGN disks with helium and $\alpha$-elements, potentially explaining the observed high metallicity in broad-line regions (BLR) without excessive helium enrichment. This study underscores the critical interplay between stellar feedback and accretion physics in shaping the stellar populations and chemical evolution within AGN disks.