Mapping the Outcomes of Stellar Evolution in the Disks of Active Galactic Nuclei

TL;DR

This paper explores how stars in AGN disks evolve under different conditions, revealing that accretion rates significantly influence their life cycles, including the potential for runaway growth, supernovae, and gamma-ray bursts.

Contribution

It provides a detailed analysis of stellar evolution outcomes in AGN disks across various SMBH masses and disk models, highlighting the impact of accretion rates on star fate.

Findings

Stars in outer disks evolve like interstellar medium stars.

Inner disk stars can become massive or 'immortal' due to accretion.

Runaway accretion can prevent steady-state star evolution.

Abstract

The disks of Active Galactic Nuclei (AGNs) are expected to be populated by numerous stars, either formed in the outer regions of the disk via gravitational instability, or captured from the nearby nuclear star cluster. Regardless of their formation mechanism, these stars experience altered evolutionary paths, mostly shaped by the accretion of dense disk material. In this study, through the comparison of different timescales, we chart the evolutionary outcomes of these AGN stars as a function of disk radius and across a range of supermassive black hole (SMBH) masses, spanning from $10^6$ to $10^9 \rm M_\odot$, for two popular AGN disk models. We find that, in the outer regions of the disk, stars evolve similarly to those in the interstellar medium, but in the inner and denser regions accretion quickly turns low-mass stars into massive stars, and their fate depends on just how quickly they accrete. If accretion occurs at a faster rate than nuclear burning, they can reach a quasi-steady `immortal' state. If stars accrete faster than they can thermally adjust, runaway accretion occurs, potentially preventing a quasi-steady state and altering the disk structure. During the AGN lifetime, in the regions of the disk that produce massive stars, supernovae (SNe) and Gamma-Ray Bursts (GRBs) may occur within the disk over a wide range of optical depths and ambient densities. Subsequently, in the final phase of the AGN, as the disk becomes depleted, formerly immortal stars will be unable to replenish their fuel, leading to additional SNe and GRBs.