%0 Journal Article
%T One-arm Spiral Instability in Hypermassive Neutron Stars Formed by Dynamical-Capture Binary Neutron Star Mergers
%A Vasileios Paschalidis
%A William E. East
%A Frans Pretorius
%A Stuart L. Shapiro
%J Physics
%D 2015
%I arXiv
%X Using general-relativistic hydrodynamical simulations, we show that merging binary neutron stars can form hypermassive neutrons stars that undergo the one-arm spiral instability. We study the particular case of a dynamical capture merger where the stars have a small spin, as may arise in globular clusters, and focus on an equal-mass scenario where the spins are aligned with the orbital angular momentum. We find that this instability develops when post-merger fluid vortices lead to the generation of a toroidal remnant - a configuration whose maximum density occurs in a ring around the center-of-mass - with high vorticity along its rotation axis. The instability quickly saturates on a timescale of $\sim 10$ ms, with the $m=1$ azimuthal density multipole mode dominating over higher modes. The instability also leaves a characteristic imprint on the post-merger gravitational wave signal that could be detectable if the instability persists in long-lived remnants.
%U http://arxiv.org/abs/1510.03432v2