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

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 second moments of the MP and star mass distributions, μ2 ≡ Np⟨M⟩/N⋆⟨M⟩, satisfies μ2 ≫ 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 102 ≲ μ2 ≲ 108. MPs thus shorten the relaxation timescale by 101-107 relative to two-body relaxation by stars alone. We show that this increases by 101-103 the rate of large-periapse interactions with the MBH, where loss cone refilling by stellar two-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 star-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 can increase by orders of magnitude the ejection rate of hypervelocity stars. In contrast, the rate of 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 three-body exchange capture of 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.