Massive Perturber–driven Interactions between Stars and a Massive Black Hole

Abstract

We study the role of massive perturbers (MPs) in deflecting stars and binaries to almost radial ("loss-cone") orbits, where they pass near the central massive black hole (MBH), interact with it at periapse, and are ultimately destroyed. MPs dominate dynamical relaxation when the ratio of the 2nd moments of the MP and star mass distributions, mu_2=(N_p*<M_p^2>)/(N_s*<M_s^2>), satisfies mu_2>>1. We compile the MP mass function from published observations, and show that MPs in the nucleus of the Galaxy (mainly giant molecular clouds), and plausibly in late type galaxies generally, have 10^2<mu_2<10^8. MPs thus shorten the relaxation timescale by 10^1-10^7 relative to 2-body relaxation by stars alone. We show this increases by 10-1000 the rate of \emph{large}-periapse interactions with the MBH, where loss-cone refilling by stellar 2-body relaxation is inefficient. We extend the Fokker-Planck loss-cone formalism to approximately account for relaxation by rare encounters with MPs. We show that binary stars--MBH exchanges driven by MPs can explain the origin of the young main sequence B stars that are observed very near the Galactic MBH, and increase by orders of magnitude the ejection rate of hyper-velocity stars. In contrast, the rate of \emph{small}-periapse interactions of single stars with the MBH, such as tidal disruption, is only increased by a factor of a few. We suggest that MP-driven relaxation plays an important role in the 3-body exchange capture of single stars on very tight orbits around the MBH. These captured stars may later be disrupted by the MBH via tidal orbital decay or direct scattering into the loss cone; captured compact objects may inspiral into the MBH by the emission of gravitational waves from zero-eccentricity orbits.