Relaxation near Supermassive Black Holes Driven by Nuclear Spiral Arms: Anisotropic Hypervelocity Stars, S-stars, and Tidal Disruption Events

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.

The former companion of hyper-velocity star S5-HVS1

“…We note that molecular clouds, and spiral arms near the GC (a few to ten pc) can enhance the rate of relaxation and hence loss-cone filling compared to stellar perturbers. The full loss-cone case is consistent with the number of young HVSs and S-stars in the GC (if the S-stars are mainly from the Hills mechanism, Perets et al 2007;Hamers & Perets 2017). In the following, we draw the distribution of pericenter distance rp by considering two extreme cases of empty and full loss cones.…”

Rates of Stellar Tidal Disruption

“…We note, however, that various dynamical processes can shorten relaxation times outside the influence radius (Perets et al 2007;Hamers and Perets 2017). At these regions MS stars are typically already in the full loss cone regime and are not significantly affected by such processes.…”

Nearby High-speed Stars in Gaia DR2

“…hypervelocity trajectories in this small region (e.g., Hills 1988;Yu & Tremaine 2003;Bromley et al 2006;Kenyon et al 2014;Hamers & Perets 2017); observations support this assessment (e.g., Brown et al 2006a;Kollmeier & Gould 2007;Kollmeier et al 2009Kollmeier et al , 2010Brown et al 2014). On the basis of the relatively short lifetimes of giants compared to these main sequence stars, the likelihood of observing an evolved star in this region is even lower.…”