Linear Growth of Spiral SASI Modes in Core‐Collapse Supernovae

Two-dimensional axisymmetric simulations have shown that the post-bounceaccretion shock in core collapse supernovae is subject to the SphericalAccretion Shock Instability, or SASI. Recent three-dimensional simulations haverevealed the existence of a non-axisymmetric mode of the SASI as well, wherethe postshock flow displays a spiral pattern. Here we investigate the growth ofthese spiral modes using two-dimensional simulations of the post-bounceaccretion flow in the equatorial plane of a core-collapse supernova. Byperturbing a steady-state model we are able to excite both one, two andthree-armed spiral modes that grow exponentially with time, demonstrating thatthese are linearly unstable modes closely related to the original axisymmetricsloshing modes. By tracking the distribution of angular momentum, we show thatthese modes are able to efficiently separate the angular momentum of theaccretion flow (which maintains a net angular momentum of zero), leading to asignificant spin-up of the underlying accreting proto-neutron star.Comment: To be published in The Astrophysical Journa