Non-radial instabilities and progenitor asphericities in core-collapse supernovae

Abstract

Since core-collapse supernova simulations still struggle to produce robust neutrino-driven explosions in 3D, it has been proposed that asphericities caused by convection in the progenitor might facilitate shock revival by boosting the activity of non-radial hydrodynamic instabilities in the post-shock region. We investigate this scenario in depth using 42 relativistic 2D simulations with multi-group neutrino transport to examine the effects of velocity and density perturbations in the progenitor for different perturbation geometries that obey fundamental physical constraints (like the anelastic condition). As a framework for analysing our results, we introduce semi-empirical scaling laws relating neutrino heating, average turbulent velocities in the gain region, and the shock deformation in the saturation limit of non-radial instabilities. The squared turbulent Mach number, Ma 2 , reflects the violence of aspherical motions in the gain layer, and explosive runaway occurs for Ma 2 0.3, corresponding to a reduction of the critical neutrino luminosity by ∼25% compared to 1D. In the light of this theory, progenitor asphericities aid shock revival mainly by creating anisotropic mass flux onto the shock: Differential infall efficiently converts velocity perturbations in the progenitor into density perturbations δρ/ρ at the shock of the order of the initial convective Mach number Ma prog . The anisotropic mass flux and ram pressure deform the shock and thereby amplify post-shock turbulence. Large-scale (ℓ = 2, ℓ = 1) modes prove most conducive to shock revival, whereas small-scale perturbations require unrealistically high convective Mach numbers. Initial density perturbations in the progenitor are only of order Ma 2 prog and therefore play a subdominant role.

Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism

“…One recalls that the original delayed mechanism of Wilson required for explosion only a modest enhancement of ∼25% in the neutrino luminosity. 5 From the work of Thompson et al (2000) and Tubbs (1979), we find that the crossover energy between upscattering and downscattering is nearer 6k B T (not 3k B T , as in Müller and Janka 2015), where k B is Boltzmann's constant and T is the temperature, around densities of ∼10 11 g cm −3 to ∼10 13 g cm −3 and so our approximate redistribution rate for ν μ s is proportional to κ scat (ε μ − 6k B T )/m n c 2 , where κ scat is the ν μ scattering opacity and m n is the neutron mass.…”

“…However, when spherical progenitor models do not readily lead to explosion, the initial perturbation spectrum in the progenitor's convective silicon and oxygen zones could certainly affect the timescales for the generation of turbulence behind the stalled shock and be a factor in the onset of explosion (Couch and Ott 2013;Müller and Janka 2015;Abdikamalov et al 2016;Müller et al 2017). Specifically, the magnitude, character, and spectra of seed perturbations will affect how quickly turbulence reaches the non-linear regime and, perhaps, whether turbulence grows to non-linearity at all during the finite time the accreta are in the unstable gain region.…”

“…Therefore, the seed perturbations that arise during oxygen and silicon burning prior to collapse might be key inputs into core-collapse supernova theory, and Couch and Ott (2013), Müller and Janka (2015), , Müller (2016), and Müller et al (2017) have begun to explore this. However, mixing-length theory, though inadequate as a comprehensive theory, still provides a measure of the magnitude of velocity perturbations at the onset of collapse , and they are only a few hundred to ∼500 km s −1 , with Mach numbers bounded by ∼0.08 (Woosley and Heger 2007).…”

“…Furthermore, it is not clear that all these currently published under-powered 3D explosions will actually succeed as an explosion after traversing the entire star and after the mass cut between ejecta and residual core is determined hydrodynamically. An exception is the recent work of Müller et al (2017), who perform 3D multi-group simulations using their simpler FMT method (Müller and Janka 2015) for an 18-M progenitor model evolved in 3D to collapse. Such a 3D progenitor naturally manifests seed perturbations that Müller et al (2017) show are instrumental in leading to an explosion with a respectable energy.…”

The two promising scenarios to explode core collapse supernovae

“…Schreier & Soker (2016) suggest that such an accretion belt might launch jets. In a series of paper Gilkis & Soker (Gilkis & Soker 2014 argue that the pre-collapse turbulence regions that exist in the core, and more so for the turbulence that was assumed and used by Couch & Ott (2013), Couch & Ott (2015), and Mueller & Janka (2015), might lead to the formation of an intermittent accretion belt around the neutron star that is formed at the center of the collapsing core.…”