Evolution of stellar orbits around merging massive black hole binary

We study the long-term orbital evolution of stars around a merging massive or supermassive black hole binary (BHB), taking into account the general relativistic effect induced by the black hole (BH) spin. When the BH spin is significant compared to and misaligned with the binary orbital angular momentum, the orbital axis ((l) over cap) 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 (l) over cap of circumbinary stars preferentially evolve towards the spin direction after the merger of the BHB, regardless of the initial BH spin orientation. Such alignment phenomenon, i.e. small final misalignment angle between (l) over cap and the spin axis of the remnant BH (S) over cap, can be understood analytically using the principle of adiabatic invariance. For the BHBs with extremely mass ratio (m(2)/m(1) less than or similar to 0.01), (l) over cap 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.

Automated summary

This study investigates the long-term orbital evolution of stars around merging black hole binary systems, taking into account the effect of black hole spin. The results show that, regardless of the initial black hole spin orientation, the orbital axes of stars tend to align with the black hole spin direction after the merger. This alignment phenomenon could serve as a potential signature of the merger history of massive black holes.