The Influence of Disk Composition on the Evolution of Stars in the Disks of Active Galactic Nuclei

TL;DR

This paper explores how the composition of accretion disks in active galactic nuclei influences the evolution, luminosity, and chemical enrichment processes of embedded stars, with implications for transient events and galaxy evolution.

Contribution

It presents a theoretical investigation into the effects of helium-rich disk environments on star evolution, highlighting changes in luminosity, mass loss, and star longevity.

Findings

Stars are more luminous at higher helium fractions.

Higher helium content leads to less massive, shorter-lived stars.

Disk composition impacts transient event rates and chemical enrichment.

Abstract

Disks of gas accreting onto supermassive black holes, powering active galactic nuclei (AGN), can capture stars from nuclear star clusters or form stars in situ via gravitational instability. The density and thermal conditions of these disks can result in rapid accretion onto embedded stars, dramatically altering their evolution in comparison to stars in the interstellar medium. Theoretical models predict that, when subjected to sufficiently rapid accretion, fresh gas replenishes hydrogen in the cores of these stars as quickly as it is burned into helium, reaching a quasi-steady state. Such massive, long-lived ("immortal") stars may be capable of dramatically enriching AGN disks with helium, and would increase the helium abundance in AGN broad-line regions relative to that in the corresponding narrow-line regions and hosts. We investigate how the helium abundance of AGN disks alters the evolution of stars embedded therein. We find, in agreement with analytical arguments, that stars at a given mass are more luminous at higher helium mass fractions, and so undergo more radiation-driven mass-loss. We further find that embedded stars tend to be less massive in disks with higher helium mass fractions, and that immortal stars are less common in such disks. Thus, disk composition can alter the rates of electromagnetic and gravitational wave transients as well as further chemical enrichment by embedded stars.