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 $M_\bullet\lesssim 0.2\,M_{\star}$ 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 ($e\gtrsim 0.5$), with its orbital frequency already entering the LISA band. Our semianalytic models further demonstrate that gas dynamical friction can pump the eccentricity to $e_{\rm 10\,Hz}>0.9$ before gravitational-wave emission eventually circularizes the binary during the final stage of coalescence. Once formed, the binary can merge quickly in $\sim 10$ 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.