Supernova theory, numerical and analytic, has made remarkable progress in the past decade. This progress was made possible by more sophisticated simulation tools, especially for neutrino transport, improved microphysics, and deeper insights into the role of hydrodynamic instabilities. Violent, large-scale nonradial mass motions are generic in supernova cores. The neutrino-heating mechanism, aided by nonradial flows, drives explosions, albeit low-energy ones, of O-Ne-Mg-core and some Fe-core progenitors. The characteristics of the neutrino emission from newborn neutron stars were revised, new features of the gravitational-wave signals were discovered, our notion of supernova nucleosynthesis was shattered, and our understanding of pulsar kicks and explosion asymmetries was significantly improved. But simulations also suggest that neutrino-powered explosions might not explain the most energetic supernovae and hypernovae, which seem to demand magnetorotational driving. Now that modeling is being advanced from...

https://www.annualreviews.org/doi/pdf/10.1146/annurev-nucl-102711-094901

Explosion Mechanisms of Core-Collapse Supernovae

Core-collapse theory brings together many facets of high-energy and nuclear astrophysics and the numerical arts to present theorists with one of the most important, yet frustrating, astronomical questions: ``What is the mechanism of core-collapse supernova explosions?'' A review of all the physics and the 50-year history involved would soon bury the reader in minutiae that could easily obscure the essential elements of the phenomenon, as we understand it today. Moreover, much remains to be discovered and explained, and a complicated review of an unresolved subject in flux could grow stale fast. Therefore, this paper describes various important facts and perspectives that may have escaped the attention of those interested in this puzzle. Furthermore, an attempt to describe the modern theory's physical underpinnings and a brief summary of the current state of play are given. In the process, a few myths that have crept into modern discourse are identified. However, there is much more to do and humility in the face of this age-old challenge is clearly the most prudent stance as its eventual resolution is sought.

Colloquium : Perspectives on core-collapse supernova theory

An $8.8{M}_{\ensuremath{\bigodot}}$ electron-capture supernova was simulated in spherical symmetry consistently from collapse through explosion to essentially complete deleptonization of the forming neutron star. The evolution time ($\ensuremath{\sim}9\text{ }\text{ }\mathrm{s}$) is short because high-density effects suppress our neutrino opacities. After a short phase of accretion-enhanced luminosities ($\ensuremath{\sim}200\text{ }\text{ }\mathrm{ms}$), luminosity equipartition among all species becomes almost perfect and the spectra of ${\overline{\ensuremath{\nu}}}_{e}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu},\ensuremath{\tau}}$ very similar, ruling out the neutrino-driven wind as $r$-process site. We also discuss consequences for neutrino flavor oscillations.

Neutrino signal of electron-capture supernovae from core collapse to cooling

We present two-dimensional hydrodynamic simulations of stellar core collapse and develop the framework for a detailed analysis of the energetic aspects of neutrino-powered supernova explosions. Our results confirm that the neutrino-heating mechanism remains a viable explanation of the explosion of a wider mass range of supernova progenitors with iron cores, but the explosion sets in later and develops differently than thought so far. The calculations were performed with an energy-dependent treatment of the neutrino transport based on the ray-by-ray plus approximation, in which the neutrino number, energy, and momentum equations are closed with a variable Eddington factor obtained by iteratively solving a model Boltzmann equation. We focus here on the evolution of a 15 M ☉ progenitor and provide evidence that shock revival and an explosion are initiated at about 600 ms after core bounce, powered by neutrino energy deposition. This is significantly later than previously found for an 11.2 M ☉ star, for which we also present a continuation of the explosion model published by Buras et al. The onset of the blast is fostered in both cases by the standing accretion-shock instability. This instability exhibits highest growth rates for the dipole and quadrupole modes, which lead to large-amplitude bipolar shock oscillations and push the shock to larger radii, thus increasing the time accreted matter is exposed to neutrino heating in the gain layer. As a consequence, also convective overturn behind the shock is strengthened, which otherwise is suppressed or damped because of the small shock stagnation radius. When the explosion sets in, the shock reveals a pronounced global deformation with a dominant dipolar component. In both the 11.2 M ☉ and 15 M ☉ explosions long-lasting equatorial downflows supply the gain layer with fresh gas, of which a sizable fraction is heated by neutrinos and leads to the build-up of the explosion energy of the ejecta over possibly hundreds of milliseconds. A soft nuclear equation of state that causes a rapid contraction, and a smaller radius of the forming neutron star and thus a fast release of gravitational binding energy, seems to be more favorable for the development of an explosion. Rotation has the opposite effect because in the long run it leads to a more extended and cooler neutron star and thus lower neutrino luminosities and mean energies and overall less neutrino heating. Neutron star g-mode oscillations, although we see their presence, and the acoustic mechanism play no important role in our simulations. While numerical tests show that our code is also well able to follow large-amplitude core g-modes if they are instigated; the amplitude of such oscillations remains small in our supernova runs and the acoustic energy flux injected by the ringing neutron star and by the deceleration of supersonic downflows near the neutron star surface is small compared to the neutrino energy deposition.

http://iopscience.iop.org/article/10.1088/0004-637X/694/1/664/pdf

Delayed neutrino-driven supernova explosions aided by the standing accretion-shock instability

In this paper we present a new mechanism for core-collapse supernova explosions that relies on acoustic power generated in the inner core as the driver. In our simulation using an 11 M☉ progenitor, an advective-acoustic oscillation a la Foglizzo with a period of ~25-30 ms arises ~200 ms after bounce. Its growth saturates due to the generation of secondary shocks, and kinks in the resulting shock structure funnel and regulate subsequent accretion onto the inner core. However, this instability is not the primary agent of explosion. Rather, it is the acoustic power generated early on in the inner turbulent region stirred by the accretion plumes and, most importantly, but later on, by the excitation and sonic damping of core g-mode oscillations. An l = 1 mode with a period of ~3 ms grows at late times to be prominent around ~500 ms after bounce. The accreting proto-neutron star is a self-excited oscillator, "tuned" to the most easily excited core g-mode. The associated acoustic power seen in our 11 M☉ simulation is sufficient to drive the explosion >550 ms after bounce. The angular distribution of the emitted sound is fundamentally aspherical. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. The ultimate source of the acoustic power is the gravitational energy of infall, and the core oscillation acts like a transducer to convert this accretion energy into sound. An advantage of the acoustic mechanism is that acoustic power does not abate until accretion subsides, so that it is available as long as it may be needed to explode the star. This suggests a natural means by which the supernova is self-regulating.

A New Mechanism for Core-Collapse Supernova Explosions

Dynamic and diffusive instabilities in core collapse supernovae.

We investigate doubly diffusive instabilities in hot, dense stellar material by examining its behavior in response to small perturbations. These instabilities, particularly "neutron fingers," have been suggested to arise in the postcollapse cores of massive stars, where entropy and lepton gradients are stabilizing and destabilizing, respectively, and to be important for the supernova explosion mechanism. We model neutrino-mediated equilibration of energy and leptons as a linear system characterized by four response functions. These latter are evaluated for a given thermodynamic state by detailed neutrino-transport experiments. Our results indicate that regions in a postcollapse stellar core may be semiconvectively rather than neutron-finger unstable but that these regions shrink with time as more and more of the lepton profile of the core falls below the critical lepton number for which a negative lepton gradient becomes stabilizing. Because these results differ from what has been assumed before, the role of doubly diffusive instabilities in the supernova explosion mechanism will have to be reexamined.

The Role of Doubly Diffusive Instabilities in the Core-Collapse Supernova Mechanism

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Dynamic and diffusive instabilities in core collapse supernovae.

The Role of Doubly Diffusive Instabilities in the Core-Collapse Supernova Mechanism