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Register a new userTurbulent magnetic field amplification from spiral SASI modes in core-collapse supernovae
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We describe the initial implementation of magnetohydrodynamics (MHD) in our astrophysical simulation code genasis. Then, we present MHD simulations exploring the capacity of the stationary accretion shock instability (SASI) to generate magnetic fields by adding a weak magnetic field to an initially spherically symmetric fluid configuration that models a stalled shock in the post-bounce supernova environment. Upon perturbation and nonlinear SASI development, shear flows associated with the spiral SASI mode contributes to a widespread and turbulent field amplification mechanism. While the SASI may contribute to neutron star magnetization, these simulations do not show qualitatively new features in the global evolution of the shock as a result of SASI-induced magnetic field amplification.
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The stationary accretion shock instability (SASI) plays a central role in modern simulations of the explosion phase of core-collapse supernovae (CCSNe). It may be key to realizing neutrino powered explosions, and possibly links birth properties of pulsars (e.g., kick, spin, and magnetic field) to supernova dynamics. Using high-resolution magnetohydrodynamic simulations, we study the development of turbulence, and subsequent amplification of magnetic fields in a simplified model of the post-bounce core-collapse supernova environment. Turbulence develops from secondary instabilities induced by the SASI. Our simulations suggest that the development of turbulence plays an important role for the subsequent evolution of the SASI. The turbulence also acts to amplify weak magnetic fields via a small-scale dynamo.
We summarize our current understanding of gravitational wave emission from core-collapse supernovae. We review the established results from multi-dimensional simulations and, wherever possible, provide back-of-the-envelope calculations to highlight the underlying physical principles. The gravitational waves are predominantly emitted by protoneutron star oscillations. In slowly rotating cases, which represent the most common type of the supernovae, the oscillations are excited by multi-dimensional hydrodynamic instabilities, while in rare rapidly rotating cases, the protoneutron star is born with an oblate deformation due to the centrifugal force. The gravitational wave signal may be marginally visible with current detectors for a source within our galaxy, while future third-generation instruments will enable more robust and detailed observations. The rapidly rotating models that develop non-axisymmetric instabilities may be visible up to a megaparsec distance with the third-generation detectors. Finally, we discuss strategies for multi-messenger observations of supernovae.
We compare the yields of Ti44 and Ni56 produced from post-processing the thermodynamic trajectories from three different core-collapse models -- a Cassiopeia A progenitor, a double shock hypernova progenitor, and a rotating 2D explosion -- with the yields from exponential and power-law trajectories. The peak temperatures and densities achieved in these core-collapse models span several of the distinct nucleosynthesis regions we identify, resulting in different trends in the Ti44 and Ni56 yields for different mass elements. The Ti44 and Ni56 mass fraction profiles from the exponential and power-law profiles generally explain the tendencies of the post-processed yields, depending on which regions are traversed by the model. We find integrated yields of Ti44 and Ni56 from the exponential and power-law trajectories are generally within a factor 2 or less of the post-process yields. We also analyze the influence of specific nuclear reactions on the Ti44 and Ni56 abundance evolution. Reactions that affect all yields globally are the 3a, p(e-,nu)n and n(e+,nubar)p. The rest of the reactions are ranked according to their degree of impact on the synthesis of Ti44. The primary ones include Ti44(a,p)V47, Ca40(a,g)Ti44, V45(p,g)Cr46, Ca40(a,p)Sc43, F17(a,p)Ne20, Na21(a,p)Mg24, Sc41(p,g)Ti42, Sc43(p,g)Ti44, Ti44(p,g)V45, and Ni57(p,g)Cu58, along with numerous weak reactions. Our analysis suggests that not all Ti44 need be produced in an a-rich freeze-out in core-collapse events, and that reaction rate equilibria in combination with timescale effects for the expansion profile may account for the paucity of Ti44 observed in supernovae remnants.
Knowledge of the progenitors of core-collapse supernovae is a fundamental component in understanding the explosions. The recent progress in finding such stars is reviewed. The minimum initial mass that can produce a supernova has converged to 8 +/- 1 solar masses, from direct detections of red supergiant progenitors of II-P SNe and the most massive white dwarf progenitors, although this value is model dependent. It appears that most type Ibc supernovae arise from moderate mass interacting binaries. The highly energetic, broad-lined Ic supernovae are likely produced by massive, Wolf-Rayet progenitors. There is some evidence to suggest that the majority of massive stars above ~20 solar masses may collapse quietly to black-holes and that the explosions remain undetected. The recent discovery of a class of ultra-bright type II supernovae and the direct detection of some progenitor stars bearing luminous blue variable characteristics suggests some very massive stars do produce highly energetic explosions. The physical mechanism is open to debate and these SNe pose a challenge to stellar evolutionary theory.
Nuclear shell burning in the final stages of the lives of massive stars is accompanied by strong turbulent convection. The resulting fluctuations aid supernova explosion by amplifying the non-radial flow in the post-shock region. In this work, we investigate the physical mechanism behind this amplification using a linear perturbation theory. We model the shock wave as a one-dimensional planar discontinuity and consider its interaction with vorticity and entropy perturbations in the upstream flow. We find that, as the perturbations cross the shock, their total turbulent kinetic energy is amplified by a factor of $sim!2$, while the average linear size of turbulent eddies decreases by about the same factor. These values are not sensitive to the parameters of the upstream turbulence and the nuclear dissociation efficiency at the shock. Finally, we discuss the implication of our results for the supernova explosion mechanism. We show that the upstream perturbations can decrease the critical neutrino luminosity for producing explosion by several percent.
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