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

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

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

Relativistic electron-ion recombination in the presence of an intense laser field

Radiative recombination of a relativistic electron with a highly charged ion in the presence of an intense laser field is considered. Various relativistic effects, caused by the high energy of the incoming electron and its strong coupling to the intense laser field, are found to clearly manifest themselves in the spectra of the emitted $\gamma$-photons.Comment: 4 papes, 2 figure

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

Double distributions: Loose ends

We point out that double distributions need not vanish at their boundary. Boundary terms do not change the ambiguity inherent in defining double distributions; instead, boundary conditions must be satisfied in order to switch between different decompositions. We analyze both the spin zero and spin one-half cases.Comment: 4 pages, 0 figures, RevTex 4, Brief Repor

Resonant two-photon single ionization of two atoms

Resonant two-photon ionization in a system consisting of two spatially well-separated atoms is studied. Due to two-center electron-electron correlations, the ionization may also proceed through photo-excitation of both atoms with subsequent interatomic Coulombic decay. We show that this channel may dominate the photoionization process and qualitatively change its dependence on the field intensity and the spectra of emitted electrons.Comment: 4 pages, 4 figure

Non-Radial Instabilities and Progenitor Asphericities in Core-Collapse Supernovae

Since core-collapse supernova simulations still struggle to produce robust neutrino-driven explosions in 3D, it has been proposed that asphericities caused by convection in the progenitor might facilitate shock revival by boosting the activity of non-radial hydrodynamic instabilities in the post-shock region. We investigate this scenario in depth using 42 relativistic 2D simulations with multi-group neutrino transport to examine the effects of velocity and density perturbations in the progenitor for different perturbation geometries that obey fundamental physical constraints (like the anelastic condition). As a framework for analysing our results, we introduce semi-empirical scaling laws relating neutrino heating, average turbulent velocities in the gain region, and the shock deformation in the saturation limit of non-radial instabilities. The squared turbulent Mach number, , reflects the violence of aspherical motions in the gain layer, and explosive runaway occurs for ~0.3, corresponding to a reduction of the critical neutrino luminosity by ~25% compared to 1D. In the light of this theory, progenitor asphericities aid shock revival mainly by creating anisotropic mass flux onto the shock: Differential infall efficiently converts velocity perturbations in the progenitor into density perturbations (Delta rho/rho) at the shock of the order of the initial convective Mach number Ma. The anisotropic mass flux and ram pressure deform the shock and thereby amplify post-shock turbulence. Large-scale (l=2,l=1) modes prove most conducive to shock revival, whereas small-scale perturbations require unrealistically high convective Mach numbers. Initial density perturbations in the progenitor are only of order Ma^2 and therefore play a subdominant role.Comment: revised version, 34 pages, 24 figure

Hadron annihilation into two photons and backward dVCS in the scaling regime of QCD

We study the scaling regime of hadron-(anti)-hadron annihilation into a deeply virtual photon and a real photon, H anti-H -> gamma^* gamma, and deep backward virtual Compton scattering, gamma^* H -> H gamma. We advocate that there is a kinematical region where the scattering amplitude factorizes into a short-distance matrix element and a long-distance dominated object: a transition distribution amplitude which describes the hadron to photon transition.Comment: 4 pages, 1 .eps figure, to be published in Phys. Rev. D Rapid Com