On the eccentricity evolution of massive black hole binaries in stellar backgrounds

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 $q\lesssim 10^{-3}$ the eccentricity growth rate becomes negative, i.e., the binary {\it circularises} 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 circularisation 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 MBHs (IMBHs) inspiralling onto MBHs (e.g. a putative $10^3{\mathrm{M}}_{\odot }$ black hole inspiralling onto SgrA${}^{\ast }$).

6 pages, 5 figures; accepted for publication in MNRAS

References in corpus (6)

• Laser Interferometer Space Antenna
• Interaction of massive black hole binaries with their stellar environment: I. Ejection of hypervelocity stars
• Implementing Few-Body Algorithmic Regularization with Post-Newtonian Terms
• On the coexistence of stellar-mass and intermediate-mass black holes in globular clusters
• Self-consistent simulations of Nuclear Cluster formation through Globular Cluster orbital decay and merging
• Analytic computation of the secular effects of encounters on a binary: features arising from second-order perturbation theory

Cited by in corpus (3)

• Eccentricity evolution of massive black hole binaries from formation to coalescence
• The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
• Dynamics of supermassive black hole triples in the ROMULUS25 cosmological simulation