Explosion Mechanisms of Core-Collapse Supernovae (1206.2503v1)

Abstract: 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 ONeMg-core and some Fe-core progenitors. The characteristics of the neutrino emission from new-born 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 two to three dimensions, more realism, new perspectives, and hopefully answers to long-standing questions are coming into reach.

• The paper demonstrates how multidimensional neutrino simulations reveal the pivotal role of instabilities like convection and SASI in reviving stalled shocks.
• It details the magnetorotational mechanism in extreme cases, underscoring its dependency on rapid rotation and strong magnetic fields.
• The study connects explosion asymmetries to observable pulsar kicks and remnant mass distributions, bridging theoretical predictions with astrophysical data.

Overview of Explosion Mechanisms of Core-Collapse Supernovae

The paper by Hans-Thomas Janka offers a comprehensive analysis of the current understanding of core-collapse supernovae (CCSNe), with an emphasis on the mechanisms that potentially drive these stellar explosions. In recent years, significant advancements have been made in our capability to simulate these events, with notable improvements in the modeling of neutrino transport, microphysical processes, and hydrodynamic instabilities. This essay highlights the key findings and implications from Janka's work, focusing on the role of neutrinos, magnetorotational phenomena, and progenitor properties in influencing supernova explosions and their remnants.

Explosion Mechanisms

Neutrino Heating Mechanism

Neutrinos play a crucial role in the delayed explosion mechanism of CCSNe. Neutrino transport simulations suggest that nonradial instabilities such as convection and the standing accretion shock instability (SASI) significantly aid the neutrino-heating process, which is essential for reviving the stalled shock front post-core bounce. This heating is self-regulating, typically generating explosion energies on the order of $10^{51}$ erg, suitable for ordinary supernovae but inadequate for hypernovae. Janka emphasizes the importance of multi-dimensional simulations in capturing turbulent effects and the dynamic range of influences that impact the critical conditions leading to explosion.

Magnetorotational Mechanism (MRM)

The magnetorotational mechanism caters to more energetic explosions, such as those associated with some highly magnetized neutron stars and gamma-ray bursts. Rapid rotation and intense magnetic fields can facilitate the extraction of rotational energy to drive the explosion. However, current stellar evolutionary models indicate progenitor cores tend to rotate too slowly for these effects to dominate in typical supernovae, suggesting MRM may be limited to specific stellar conditions or evolutionary histories possibly involving binary interactions.

Shock and Phase Transition Mechanisms

Traditional bounce-shock mechanisms are largely invalidated by modern simulations, as the outgoing shock loses energy quickly through nuclear dissociation and gravitational counteraction. Conversely, phase-transition-driven explosions can occur if specific conditions reduce the effective adiabatic index during the postbounce accretion phase, a particularly speculative scenario that has yet to align with observational constraints of known neutron-star masses.

Implications and Observational Connections

Neutrino and Gravitational Waves (GWs)

Neutrinos carry direct information about the core collapse and are a diagnostic tool for studying the supernova mechanism. Observable features such as the hydrodynamically modulated neutrino luminosity variations and energy spectra provide invaluable data, especially in a Galactic event. Similarly, gravitational waves produced by aspheric mass motions during the collapse provide complementary insights into the dynamical state of the core and the nature of the instabilities involved.

Pulsar Kicks and Asymmetries

Janka discusses the implications of explosion asymmetries, driven by instabilities like convection and SASI, on pulsar kicks and chemical dispersion within supernova remnants. This asymmetry imparts a natal kick to the neutron star and influences the mixing of elements in the supernova ejecta, offering an explanation for the observed heterogeneity and high velocities of young pulsars.

Remnant Mass Distribution

The paper also explores the correlation between progenitor properties and the resultant neutron star or black hole remnant masses. A comprehensive understanding of this relationship is vital for connecting observations of supernova remnants and known compact object populations to theoretical models of supernova explosions.

Conclusion and Future Directions

Janka's review reinforces the complexity inherent in supernova explosion mechanisms, while reflecting significant strides in understanding driven by advances in computational astrophysics. Continued refinement of multidimensional simulations, along with enhanced transport and microphysical models, is critical. Looking forward, investigating the interplay between neutrino-driven and magnetorotational explosions, understanding the implications for nucleosynthesis, and probing the progenitor parameter space will form an important trajectory for future breakthroughs in CCSNe research. This work sets the stage for ongoing convergence between theoretical predictions and observational data, offering the potential to unravel the multifaceted nature of massive stellar explosions.