# Relaxation near supermassive black holes driven by nuclear spiral arms: anisotropic hypervelocity stars, S-stars and tidal disruption events

## TL;DR

This paper investigates how nuclear spiral arms in galactic centers influence star and binary star dynamics near supermassive black holes, leading to increased tidal disruption events, hypervelocity stars, and possibly affecting black hole mergers.

## Contribution

It introduces a model for diffusion caused by nuclear spiral arms, quantifies their impact on star disruption rates, and explains the anisotropic distribution of hypervelocity stars.

## Key findings

- TDE rates increase by up to 20% due to spiral arms.
- Binary disruption rates are enhanced by up to a factor of 100.
- The scenario explains the observed anisotropic distribution of HVSs.

## Abstract

Nuclear spiral arms are small-scale transient spiral structures found in the centers of galaxies. Similarly to their galactic-scale counterparts, nuclear spiral arms can perturb the orbits of stars. In the case of the Galactic Center (GC), these perturbations can affect the orbits of stars and binaries in a region extending to several hundred parsecs around the supermassive black hole (MBH), causing diffusion in orbital energy and angular momentum. This diffusion process 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 tightly bound to the MBH, and an unbound star escaping the galaxy, i.e., a hypervelocity star (HVS). Here, we consider diffusion by nuclear spiral arms in galactic nuclei, specifying to the Milky Way GC. We determine nuclear spiral arm-driven diffusion rates using test-particle integrations, and 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 ~100, and our rates are comparable to 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 provides a novel potential source 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.