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Abstract Stars are likely embedded in the gas disks of active galactic nuclei (AGN). Theoretical models predict that in the inner regions of the disk, these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order 102–4, confined to the innerrcap∼ 3000rs∼ 0.03 pc. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star–star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star–compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates toward the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows from the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around 10M⊙. Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order-unity impact on disk structure and evolution, they must be included in realistic disk models.
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This content will become publicly available on February 21, 2026
Mapping the Outcomes of Stellar Evolution in the Disks of Active Galactic Nuclei
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 masses, spanning from 106to 109M⊙, 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.
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- Award ID(s):
- 2006839
- PAR ID:
- 10643491
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 981
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 16
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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