Inside the supernova: A powerful convective engine

Abstract

We present an extensive study of the inception of supernova explosions by following the evolution of the cores of two massive stars (15 M ⊙ and 25 M ⊙ ) in multidimension. Our calculations begin at the onset of core collapse and stop several hundred milliseconds after the bounce, at which time successful explosions of the appropriate magnitude have been obtained. Similar to the classical delayed explosion mechanism of Wilson (1985), the explosion is powered by the heating of the envelope due to neutrinos emitted by the protoneutron star as it radiates the gravitational energy liberated by the collapse. However, as was shown by Herant, Benz & Colgate (1992), this heating generates strong convection outside the neutrinosphere, which we demonstrate to be critical to the explosion. By breaking a purely stratified hydrostatic equilibrium, convection moves the nascent supernova away from a delicate radiative equilibrium between neutrino emission and absorption. Thus, unlike what has been observed in one-dimensional calculations, explosions are rendered quite insensitive to the details of the physical input parameters such as neutrino cross-sections or nuclear

The Development of Explosions in Axisymmetric Ab Initio Core-Collapse Supernova Simulations of 12–25 Stars

“…Second, material advecting toward the gain surface is continually heated by neutrinos, gaining entropy as it moves inwards. As has been pointed out by many authors (e.g., Herant et al 1992Herant et al , 1994Janka & Müller 1993;Burrows et al 1995;Janka & Müller 1995;Thompson 2000;Janka 2001;Fryer & Warren 2004;Thompson et al 2005;Bruenn et al 2006;Buras et al 2006b;Murphy & Burrows 2008;Scheck et al 2008;Müller et al 2012a), convective flows enhance the conditions necessary for the revival of the shock. They increase the effective neutrino heating efficiency because rising fluid elements have lower neutrino emissivities after adiabatic expansion, reducing losses due to neutrino cooling, and the non-radial motions increase the dwell time of fluid elements in the gain region, thereby increasing the time they are subject to neutrino heating.…”

“…A number of early multidimensional hydrodynamics simulations demonstrated that neutrino-driven convection does indeed develop in the heating layer and enhances the revival of the shock (Herant et al 1992(Herant et al , 1994Janka & Müller 1993Burrows et al 1995). These authors and others pointed out that convectively unstable entropy gradients occur in the heating layer for several reasons.…”

“…Two decades ago, the first two-dimensional (2D) simulations (Herant et al 1992(Herant et al , 1994 transformed the field. The fundamental feedback between mass accretion, against which the shock must work, and the neutrino luminosities powering the explosion was finally unchained in these models and others (e.g., Burrows et al 1995;Janka & Müller 1996), allowing continued accretion and explosion to coexist, something impossible in 1D models.…”

Simulations of stellar core collapse, bounce, and postbounce evolution with Boltzmann neutrino transport, and implications for the core collapse supernova mechanism

“…The shock is distorted by this convective activity. In the Herant et al [4] simulations, large-scale convection developed beneath the shock, leading to increased neutrino energy deposition, the accumulation of mass and energy in the gain region, and a thermodynamic engine they claimed ensured explosion, although Herant et al stressed the need for more sophisticated multidimensional, multigroup transport in future models. [They used two-dimensional "gray" (neutrino-energy-integrated, as opposed to multigroup) flux-limited diffusion in neutrino-thick regions and a neutrino lightbulb approximation in neutrinothin regions.…”

“…The heating is mediated primarily by the absorption of electron neutrinos and antineutrinos on the dissociation-liberated nucleons behind the shock. This past decade has also seen the emergence of multidimensional supernova models, which have investigated the role convection, rotation, and magnetic fields may play in the explosion [4][5][6][7][8][9][10], in some cases invoking new explosion paradigms.…”