The Core-Collapse Supernova Explosion Mechanism

The explosion mechanism of core-collapse supernovae is a long-standing problem in stellar astrophysics. We briefly outline the main contenders for a solution and review recent efforts to model core-collapse supernova explosions by means of multi-dimensional simulations. We discuss several suggestions for solving the problem of missing or delayed neutrino-driven explosions in three-dimensional supernova models, including -- among others -- variations in the microphysics and large seed perturbations in convective burning shells. Focusing on the neutrino-driven mechanism, we summarise currents efforts to predict supernova explosion and remnant properties based on first-principle models and on more phenomenological approaches.Comment: Invited review to appear in the International Astronomical Union Proceedings Serie (IAU Symposium 329, "The Lives and Death Throes of Massive Stars"). 8 pages, 2 figure

The Status of Multi-Dimensional Core-Collapse Supernova Models

Models of core-collapse supernova explosions powered by the neutrino-driven mechanism have matured considerable in recent years. Explosions at the low-mass end of the progenitor spectrum can routinely be simulated in 1D, 2D, and 3D and allow us to study supernova nucleosynthesis based on first-principle models. Results of nucleosynthesis calculations indicate that supernovae of the lowest masses could be important contributors of some lighter n-rich elements beyond iron. The explosion mechanism of more massive stars is still under investigation, although first 3D models of neutrino-driven explosions employing multi-group neutrino transport have recently become available. Together with earlier 2D models and more simplified 3D simulations, these have elucidated the interplay between neutrino heating and hydrodynamic instabilities in the post-shock region that is essential for shock revival. However, some physical ingredients may still need to be added or improved before simulations can robustly explain supernova explosions over a wide mass range. We explore possible issues that may affect the accuracy of supernova simulations, and review some of the ideas that have recently been explored as avenues to robust explosions, including uncertainties in the neutrino rates, rapid rotation, and an external forcing of non-radial fluid motions by strong seed perturbations from convective shell burning. The perturbation-aided neutrino-driven mechanism and the implications of recent 3D simulations of shell burning in supernova progenitors are discussed in detail. The efficacy of the perturbation-aided mechanism is illustrated by the first successful multi-group neutrino hydrodynamics simulation of an 18 solar mass progenitor with 3D initial conditions. We conclude with speculations about the potential impact of 3D effects on the structure of massive stars through convective boundary mixing. (abridged)Comment: 30 pages, 7 figures. Invited review for Publications of the Astronomical Society of Australia, to be published in special issue on "Electron Capture Supernoave". Accepted version after refereein

Neutrino Emission as Diagnostics of Core-Collapse Supernovae

With myriads of detection events from a prospective Galactic core-collapse supernova, current and future neutrino detectors will be able to sample detailed, time-dependent neutrino fluxes and spectra. This offers enormous possibilities for inferring supernova physics from the various phases of the neutrino signal from the neutronization burst through the accretion and early explosion phase to the cooling phase. The signal will constrain the time evolution of bulk parameters of the young proto-neutron star like its mass and radius as well as the structure of the progenitor, probe multi-dimensional phenomena in the supernova core, and constrain thedynamics of the early explosion phase. Aside from further astrophysical implications, supernova neutrinos may also shed further light on the properties of matter at supranuclear densities and on open problems in particle physics.Comment: 26 pages, 5 figures. Accepted for publication in Annual Review of Nuclear and Particle Science, vol. 69. Non-copyedited version prepared by the autho

Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma

We derive the space-time distribution of energy and momentum deposited by a fast parton traversing a weakly coupled quark-gluon plasma by treating the fast part on as the source of an external color field perturbing the medium. We then use our result as a source term for the linearized hydrodynamical equations of the medium. We show that the solution contains a sonic Mach cone and a dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio

Two-center resonant photo ionization

Photoionization of an atom $A$, in the presence of a neighboring atom $B$, can proceed via resonant excitation of $B$ with subsequent energy transfer to $A$ through two-center electron-electron correlation. We demonstrate that this two-center mechanism can strongly outperform direct photoionization at nanometer internuclear distances and possesses characteristic features in its time development and the spectrum of emitted electrons.Comment: 4 pages, 3 figure

Hydrodynamic noise and Bjorken expansion

Using the Bjorken expansion model we study the effect of intrinsic hydrodynamic noise on the correlations observed in heavy-ion collisions.Comment: Proceedings of Quark Matter 2012, (August 13-18, 2012, Washington DC), 4 pages, 3 figure

The subgroup growth spectrum of virtually free groups

For a finitely generated group $\Gamma$ denote by $\mu(\Gamma)$ the growth coefficient of $\Gamma$, that is, the infimum over all real numbers $d$ such that $s_n(\Gamma)<n!^d$. We show that the growth coefficient of a virtually free group is always rational, and that every rational number occurs as growth coefficient of some virtually free group. Moreover, we describe an algorithm to compute $\mu$