Stellar Evolution in AGN Disks

TL;DR

This paper investigates how stars embedded in AGN disks evolve under extreme conditions, revealing they can rapidly become very massive, influence disk metallicity, and contribute to gravitational wave sources.

Contribution

It introduces a detailed model of stellar evolution within AGN disks, highlighting the effects of accretion, winds, and chemical mixing on star mass and evolution.

Findings

AGN stars can grow rapidly to over 100 solar masses.

These stars undergo core-collapse, forming compact remnants.

AGN stars influence disk metallicity and gravitational wave sources.

Abstract

Active Galactic Nuclei are powered by geometrically-thin accretion disks surrounding a central supermassive black hole. Here we explore the evolution of stars embedded in these extreme astrophysical environments (AGN stars). Because AGN disks are much hotter and denser than the interstellar medium, AGN stars are subject to very different boundary conditions than normal stars. They are also strongly affected by both mass accretion, which can runaway given the vast mass of the disk, and mass loss due to super-Eddington winds. Moreover, chemical mixing plays a critical role in the evolution of these stars by allowing fresh hydrogen accreted from the disk to mix into their cores. We find that, depending on the local AGN density and sound speed and the duration of the AGN phase, AGN stars can rapidly become very massive (M > 100 M$_\odot$). These stars undergo core-collapse, leave behind compact remnants and contribute to polluting the disk with heavy elements. We show that the evolution of AGN stars can have a profound impact on the evolution of AGN metallicities, as well as the production of gravitational waves sources observed by LIGO-Virgo. We point to our galactic center as a region well-suited to test some of our predictions of this exotic stellar evolutionary channel.