Dynamical formation of high-eccentricity compact binaries through BH--BH*/TZO collisions
Abstract
The rapidly accumulating discoveries of binary stellar-mass black-hole (sBH) coalescences, detected by LIGO, have opened a new window into the formation and evolution of compact binaries. In particular, residual orbital eccentricity may provide a distinctive signature of their formation channels. Here, we investigate a scenario in which high-eccentricity compact binaries form through the sequential capture of multiple sBHs by massive main-sequence stars, using a combination of hydrodynamical and semianalytic few-body simulations. We find that sBHs with M•≲ 0.2 M⋆ can be captured by massive stars and settle into a quasi-hydrostatic black-hole star (BH*) through gas dynamical friction. A subsequent encounter with a second sBH can then produce a compact binary embedded within the stellar envelope. Our hydrodynamical simulations show that through captures with small impact parameter, some binaries are born with high eccentricity (e≳ 0.5), with its orbital frequency already entering the LISA band. Our semianalytic models further demonstrate that gas dynamical friction can pump the eccentricity to e10 Hz>0.9 before gravitational-wave emission eventually circularizes the binary during the final stage of coalescence. Once formed, the binary can merge quickly in ∼ 10 hours. This channel may operate in dense stellar environments, such as star clusters and active galactic nucleus (AGN) disks. The same mechanism can also be applied to Thorne-Żytkow objects. A high-eccentricity binary in the LIGO band could therefore provide a distinctive signature of this formation scenario.
Related
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BibTeX
@misc{hu2026dynamical,
title = {{Dynamical formation of high-eccentricity compact binaries through BH--BH*/TZO collisions}},
author = {Qingru Hu and Yanlong Shi and Douglas N. C. Lin and Norman Murray},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.27595},
eprint = {2608.27595},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.27595},
}
RIS
TY - UNPB
TI - Dynamical formation of high-eccentricity compact binaries through BH--BH*/TZO collisions
AU - Qingru Hu
AU - Yanlong Shi
AU - Douglas N. C. Lin
AU - Norman Murray
PY - 2026
DA - 2026/08/27
PB - arXiv
DO - 10.48550/arxiv.2608.27595
UR - https://arxiv.org/abs/2608.27595
AB - The rapidly accumulating discoveries of binary stellar-mass black-hole (sBH) coalescences, detected by LIGO, have opened a new window into the formation and evolution of compact binaries. In particular, residual orbital eccentricity may provide a distinctive signature of their formation channels. Here, we investigate a scenario in which high-eccentricity compact binaries form through the sequential capture of multiple sBHs by massive main-sequence stars, using a combination of hydrodynamical and semianalytic few-body simulations. We find that sBHs with M_•≲ 0.2 M_⋆ can be captured by massive stars and settle into a quasi-hydrostatic black-hole star (BH*) through gas dynamical friction. A subsequent encounter with a second sBH can then produce a compact binary embedded within the stellar envelope. Our hydrodynamical simulations show that through captures with small impact parameter, some binaries are born with high eccentricity (e≳ 0.5), with its orbital frequency already entering the LISA band. Our semianalytic models further demonstrate that gas dynamical friction can pump the eccentricity to e_10 Hz>0.9 before gravitational-wave emission eventually circularizes the binary during the final stage of coalescence. Once formed, the binary can merge quickly in ∼ 10 hours. This channel may operate in dense stellar environments, such as star clusters and active galactic nucleus (AGN) disks. The same mechanism can also be applied to Thorne-Żytkow objects. A high-eccentricity binary in the LIGO band could therefore provide a distinctive signature of this formation scenario.
ER -