The saturation of SASI by parasitic instabilities

TL;DR

This paper investigates how parasitic instabilities like Kelvin-Helmholtz and Rayleigh-Taylor can cause nonlinear saturation of SASI in core-collapse supernovae, affecting shock oscillation amplitudes and explosion asymmetries.

Contribution

It introduces a new mechanism for SASI saturation via parasitic instabilities, supported by analytical estimates and numerical simulations, linking physical parameters to SASI amplitude reduction.

Findings

Parasitic instabilities grow on SASI modes at large amplitudes.

Acoustic feedback decreases when parasitic instabilities distort structures.

Saturation explains the decrease in SASI power with varying physical conditions.

Abstract

The Standing Accretion Shock Instability (SASI) is commonly believed to be responsible for large amplitude dipolar oscillations of the stalled shock during core collapse, potentially leading to an asymmetric supernovae explosion. The degree of asymmetry depends on the amplitude of SASI, which nonlinear saturation mechanism has never been elucidated. We investigate the role of parasitic instabilities as a possible cause of nonlinear SASI saturation. As the shock oscillations create both vorticity and entropy gradients, we show that both Kelvin-Helmholtz and Rayleigh-Taylor types of instabilities are able to grow on a SASI mode if its amplitude is large enough. We obtain simple estimates of their growth rates, taking into account the effects of advection and entropy stratification. In the context of the advective-acoustic cycle, we use numerical simulations to demonstrate how the acoustic feedback can be decreased if a parasitic instability distorts the advected structure. The amplitude of the shock deformation is estimated analytically in this scenario. When applied to the set up of Fernandez & Thompson (2009a), this saturation mechanism is able to explain the dramatic decrease of the SASI power when both the nuclear dissociation energy and the cooling rate are varied. Our results open new perspectives for anticipating the effect, on the SASI amplitude, of the physical ingredients involved in the modeling of the collapsing star.