Formation of black hole stars via star--black hole collisions
Abstract
In dense stellar environments such as globular clusters and active galactic nucleus (AGN) disks, stellar-mass black holes (sBHs) may frequently collide with massive stars. We investigate this process using semi-analytic models, three-dimensional hydrodynamical simulations, and one-dimensional stellar evolution calculations, focusing on collisions between sBHs and a 100 M⊙ main-sequence star. We find that gas drag retains the BH within the stellar envelope unless the impact velocity exceeds ∼2√(G(M⋆+M•)/R⋆). The post-collision outcome depends primarily on the BH-to-star mass ratio. For M•≳30 M⊙, the retained envelope is either quasi-spherical or disc-like, but remains dynamically unstable because of shock heating. In contrast, for M•≲10 M⊙, the collision forms a ``black hole star'' (BH*): a quasi-hydrostatic, extended stellar envelope surrounding the embedded BH. These results agree with our analytic prediction that BH* formation necessarily requires M•≲0.2 M⋆. Follow-up \texttt{MESA} calculations further show that, for these low-mass BHs, the shock-heated remnant thermally relaxes without triggering runaway expansion. We discuss several astrophysical implications of BH*s, including their evolution, the possibility of gravitational-wave events from BH binaries assembled within a stellar envelope, and repeated star--sBH collisions as a pathway for rapid BH growth in dense stellar systems. This mechanism may contribute to the formation of massive BHs in high-redshift nuclear star clusters and may be relevant to the origin of the ``little red dots'' discovered by JWST.
Related
- Formation of Merging Black Hole Binaries Inside Massive AGN Stars
- When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes
- Evolution of Massive Main-Sequence Stars in Binary Population Synthesis. II. Consequences for Black Hole High-Mass X-ray Binaries and Binary Black Holes
- Reviving primordial black hole formation in slow first-order phase transitions
- Luminosity Signatures of Dark Sector Particles from Black Hole Evaporation in Neutron Stars
BibTeX
@misc{shi2026formation,
title = {{Formation of black hole stars via star--black hole collisions}},
author = {Yanlong Shi and Qingru Hu and Zhenghao Xu and Douglas N. C. Lin and Norman Murray},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.27596},
eprint = {2608.27596},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.27596},
}
RIS
TY - UNPB
TI - Formation of black hole stars via star--black hole collisions
AU - Yanlong Shi
AU - Qingru Hu
AU - Zhenghao Xu
AU - Douglas N. C. Lin
AU - Norman Murray
PY - 2026
DA - 2026/08/27
PB - arXiv
DO - 10.48550/arxiv.2608.27596
UR - https://arxiv.org/abs/2608.27596
AB - In dense stellar environments such as globular clusters and active galactic nucleus (AGN) disks, stellar-mass black holes (sBHs) may frequently collide with massive stars. We investigate this process using semi-analytic models, three-dimensional hydrodynamical simulations, and one-dimensional stellar evolution calculations, focusing on collisions between sBHs and a 100 M_⊙ main-sequence star. We find that gas drag retains the BH within the stellar envelope unless the impact velocity exceeds ∼2√(G(M_⋆+M_•)/R_⋆). The post-collision outcome depends primarily on the BH-to-star mass ratio. For M_•≳30 M_⊙, the retained envelope is either quasi-spherical or disc-like, but remains dynamically unstable because of shock heating. In contrast, for M_•≲10 M_⊙, the collision forms a ``black hole star'' (BH*): a quasi-hydrostatic, extended stellar envelope surrounding the embedded BH. These results agree with our analytic prediction that BH* formation necessarily requires M_•≲0.2 M_⋆. Follow-up \texttt{MESA} calculations further show that, for these low-mass BHs, the shock-heated remnant thermally relaxes without triggering runaway expansion. We discuss several astrophysical implications of BH*s, including their evolution, the possibility of gravitational-wave events from BH binaries assembled within a stellar envelope, and repeated star--sBH collisions as a pathway for rapid BH growth in dense stellar systems. This mechanism may contribute to the formation of massive BHs in high-redshift nuclear star clusters and may be relevant to the origin of the ``little red dots'' discovered by JWST.
ER -