Spin-Orbit Alignment in Merging Binary Black Holes Following Collisions with Massive Stars

TL;DR

This paper investigates how stellar collisions and debris accretion in dense star clusters influence the spin--orbit alignment of merging binary black holes, potentially explaining observed positive effective spins.

Contribution

It demonstrates through hydrodynamic simulations that stellar collisions can reorient accretion disks, leading to small positive effective spins in merging BBHs, challenging previous isotropic spin assumptions.

Findings

Stellar debris forms accretion disks around black holes after collisions.

Tidal torques can reorient these disks near pericenter passages.

Resulting BBHs tend to have small, positive effective spins.

Abstract

Merging binary black holes (BBHs) formed dynamically in dense star clusters are expected to have uncorrelated spin--orbit orientations since they are assembled through many random interactions. However, measured effective spins in BBHs detected by LIGO/Virgo/KAGRA hint at additional physical processes that may introduce anisotropy. Here we address this question by exploring the impact of stellar collisions, and accretion of collision debris, on the spin--orbit alignment in merging BBHs formed in dense star clusters. Through hydrodynamic simulations, we study the regime where the disruption of a massive star by a BBH causes the stellar debris to form individual accretion disks bound to each black hole. We show that these disks, which are randomly oriented relative to the binary orbital plane after the initial disruption of the star, can be reoriented by strong tidal torques in the binary near pericenter passages. Following accretion by the BHs on longer timescales, BBHs with small but preferentially positive effective spin parameters ($\chi_{\rm eff} \lesssim 0.2$) are formed. Our results indicate that BBH collisions in young massive star clusters could contribute to the observed trend toward small positive $\chi_{\rm eff}$, and we suggest that the standard assumption often made that dynamically assembled BBHs should have isotropically distributed BH spins is not always justified.