On the eccentricity evolution of massive black hole binaries in stellar backgrounds. (5th February 2020)

• Record Type: Journal Article
• Authors: Bonetti, Matteo; Rasskazov, Alexander; Sesana, Alberto; Dotti, Massimo; Haardt, Francesco; Leigh, Nathan W C; Arca Sedda, Manuel; Fragione, Giacomo; Rossi, Elena
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 493:Number 1(2020)
• Page Start: L114
• Page End: L119
• Publication Date: 2020-02-05
• Subjects: black hole physics -- gravitational waves -- stars: kinematics and dynamics -- galaxies: nuclei; Astronomy -- Research -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnrasl/slaa018
• Languages: English
• ISSNs: 1745-3933

Abstract

ABSTRACT: We study the dynamical evolution of eccentric massive black hole binaries (MBHBs) interacting with unbound stars by means of an extensive set of three-body scattering experiments. Compared to previous studies, we extend the investigation down to a MBHB mass ratio of q = m 2 / m 1 = 10 −4, where m 1 and m 2 are the masses of the primary and secondary hole, respectively. Contrary to a simple extrapolation from higher mass ratios, we find that for q ≲ 10 −3 the eccentricity growth rate becomes negative, i.e. the binary circularizes as it shrinks. This behaviour is due to the subset of interacting stars captured in metastable counter-rotating orbits; those stars tend to extract angular momentum from the binary, promoting eccentricity growth for q > 10 −3, but tend to inject angular momentum into the binary driving it towards circularization for q < 10 −3 . The physical origin of this behaviour requires a detailed study of the orbits of this subset of stars and is currently under investigation. Our findings might have important consequences for intermediate massive black holes (IMBHs) inspiralling on to MBHs (e.g. a putative $10^3\, \rm M_{\odot }$ black hole inspiralling on to Sgr A*).