Evolution of Stellar Orbits Around Merging Massive Black-Hole Binary

TL;DR

This study investigates how stars' orbital axes around merging black-hole binaries tend to align with the remnant black hole's spin, influenced by relativistic effects and initial conditions, revealing potential signatures of black hole merger history.

Contribution

It demonstrates that stellar orbital axes generally align with the black hole spin after merger, incorporating relativistic effects and analyzing the impact of mass ratios and initial orientations.

Findings

Orbital axes tend to align with the remnant BH spin after merger.

Alignment is robust for nearly coplanar initial orientations.

Complex evolution occurs for extreme mass ratios, but the trend persists.

Abstract

We study the long-term orbital evolution of stars around a merging massive or supermassive black-hole (BH) binary, taking into account the general relativistic effect induced by the BH spin. When the BH spin is significant compared to and misaligned with the binary orbital angular momentum, the orbital axis ($\hat{\mathbf{l}}$) of the circumbinary star can undergo significant evolution during the binary orbital decay driven by gravitational radiation. Including the spin effect of the primary (more massive) BH, we find that starting from nearly coplanar orbital orientations, the orbital axes $\hat{\mathbf{l}}$ of circumbinary stars preferentially evolve towards the spin direction after the merger of the BH binary, regardless of the initial BH spin orientation. Such alignment phenomenon, i.e., small final misalignment angle between $\hat{\mathbf{l}}$ and the spin axis of the remanent BH $\hat{\mathbf{S}}$, can be understood analytically using the principle of adiabatic invariance. For the BH binaries with extremely mass ratio ($m_2/m_1\lesssim0.01$), $\hat{\mathbf{l}}$ may experience more complicated evolution as adiabatic invariance breaks down, but the trend of alignment still works reasonably well when the initial binary spin-orbit angle is relatively small. Our result suggests that the correlation between the orientations of stellar orbits and the spin axis of the central BH could provide a potential signature of the merger history of the massive BH.