One-armed Spiral Instability in a Low- T /| W | Postbounce Supernova Core

A three-dimensional, Newtonian hydrodynamic technique is used to follow the postbounce phase of a stellar core collapse event. For realistic initial data, we have employed post-core-bounce snapshots of the iron core of a 20 M ☉ star. The models exhibit strong differential rotation but have centrally condensed density stratifications. We demonstrate for the first time that such postbounce cores are subject to a so-called low- T /| W | nonaxisymmetric instability and, in particular, can become dynamically unstable to an ( m = 1)-dominated spiral mode at T /| W | ~ 0.08. We calculate the gravitational wave (GW) emission by the instability and find that the emitted waves may be detectable by current and future GW observatories from anywhere in the Milky Way.