Nuclear spiral arms are small-scale transient spiral structures found in the centers of galaxies. Similarly to their galactic-scale counterparts, nuclear spiral arms exert torques on nearby objects. Within the central few hundred parsecs of a supermassive black hole (MBH), such torques affect the orbits of stars and binaries, causing orbital diffusion in energy and angular momentum. Such diffusion can drive stars and binaries to close approaches with the MBH, disrupting single stars in tidal disruption events (TDEs), or disrupting binaries, leaving a star on a tight orbit around the MBH, and an unbound star escaping the galaxy, i.e., a hypervelocity star (HVS). Here, we consider diffusion by nuclear spiral arms in the central few hundred parsecs of galactic nuclei, specifying to the Milky Way Galactic Center. We determine nuclear spiral arm-driven diffusion rates using test-particle integrations, and we compute disruption rates. Our TDE rates are up to $20\%$ higher compared to relaxation by single stars. For binaries, the enhancement is up to a factor of $\sim 100$, and our rates are roughly consistent with the observed numbers of HVSs and S-stars. Our scenario is complementary to relaxation driven by massive perturbers. In addition, our rates depend on the inclination of the binary with respect to the Galactic plane. Therefore, our scenario of diffusion by nuclear spiral arms provides an explanation for the observed anisotropic distribution of HVSs. Nuclear spiral arms may also be important for accelerating the coalescence of binary MBHs, and for supplying nuclear star clusters with stars and gas.
A. Hamers and H. Perets
Fri, 21 Apr 17
Comments: Submitted to AAS Journals. 20 pages, 16 figures