Core Collapse and Then? The Route to Massive Star Explosions

The rapidly growing base of observational data for supernova explosions of massive stars demands theoretical explanations. Central of these is a self-consistent model for the physical mechanism that provides the energy to start and drive the disruption of the star. We give arguments why the delayed neutrino-heating mechanism should still be regarded as the standard paradigm to explain most explosions of massive stars and show how large-scale and even global asymmetries can result as a natural consequence of convective overturn in the neutrino-heating region behind the supernova shock. Since the explosion is a threshold phenomenon and depends sensitively on the efficiency of the energy transfer by neutrinos, even relatively minor differences in numerical simulations can matter on the secular timescale of the delayed mechanism. To enhance this point, we present some results of recent one- and two-dimensional computations, which we have performed with a Boltzmann solver for the neutrino transport and a state-of-the-art description of neutrino-matter interactions. Although our most complete models fail to explode, the simulations demonstrate that one is encouragingly close to the critical threshold because a modest variation of the neutrino transport in combination with postshock convection leads to a weak neutrino-driven explosion with properties that fulfill important requirements from observations.Comment: 14 pages; 3 figures. Invited Review, in: ``From Twilight to Highlight: The Physics of Supernovae'', Eds. W. Hillebrandt and B. Leibundgut, Springer Series ``ESO Astrophysics Symposia'', Berli

Supernova neutrinos: Flavor-dependent fluxes and spectra

Transporting nu_mu and nu_tau in a supernova (SN) core involves several processes that have been neglected in traditional simulations. Based on a Monte Carlo study we find that the flavor-dependent spectral differences are much smaller than is often stated in the literature. A full-scale SN simulation using a Boltzmann solver and including all relevant neutrino reactions confirms these results. The flavor-dependent flux differences are largest during the initial accretion phase.Comment: Proceedings NOON 03, Kanazawa, 10-14 Feb 200

Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars

Differential rotation in stars generates toroidal magnetic fields whenever an initial seed poloidal field is present. The resulting magnetic stresses, along with viscosity, drive the star toward uniform rotation. This magnetic braking has important dynamical consequences in many astrophysical contexts. For example, merging binary neutron stars can form "hypermassive" remnants supported against collapse by differential rotation. The removal of this support by magnetic braking induces radial fluid motion, which can lead to delayed collapse of the remnant to a black hole. We explore the effects of magnetic braking and viscosity on the structure of a differentially rotating, compressible star, generalizing our earlier calculations for incompressible configurations. The star is idealized as a differentially rotating, infinite cylinder supported initially by a polytropic equation of state. The gas is assumed to be infinitely conducting and our calculations are performed in Newtonian gravitation. Though highly idealized, our model allows for the incorporation of magnetic fields, viscosity, compressibility, and shocks with minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our evolution calculations show that magnetic braking can lead to significant structural changes in a star, including quasistatic contraction of the core and ejection of matter in the outermost regions to form a wind or an ambient disk. These calculations serve as a prelude and a guide to more realistic MHD simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap

Gravitational waves from relativistic rotational core collapse

We present results from simulations of axisymmetric relativistic rotational core collapse. The general relativistic hydrodynamic equations are formulated in flux-conservative form and solved using a high-resolution shock-capturing scheme. The Einstein equations are approximated with a conformally flat 3-metric. We use the quadrupole formula to extract waveforms of the gravitational radiation emitted during the collapse. A comparison of our results with those of Newtonian simulations shows that the wave amplitudes agree within 30%. Surprisingly, in some cases, relativistic effects actually diminish the amplitude of the gravitational wave signal. We further find that the parameter range of models suffering multiple coherent bounces due to centrifugal forces is considerably smaller than in Newtonian simulations.Comment: 4 pages, 3 figure

Supernova Simulations with Boltzmann Neutrino Transport: A Comparison of Methods

Accurate neutrino transport has been built into spherically symmetric simulations of stellar core collapse and postbounce evolution. The results of such simulations agree that spherically symmetric models with standard microphysical input fail to explode by the delayed, neutrino-driven mechanism. Independent groups implemented fundamentally different numerical methods to tackle the Boltzmann neutrino transport equation. Here we present a direct and detailed comparison of such neutrino radiation-hydrodynamical simulations for two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the Garching group. The former solves the Boltzmann equation directly by an implicit, general relativistic discrete angle method on the adaptive grid of a conservative implicit hydrodynamics code with second-order TVD advection. In contrast, the latter couples a variable Eddington factor technique with an explicit, moving-grid, conservative high-order Riemann solver with important relativistic effects treated by an effective gravitational potential. The presented study is meant to test both neutrino radiation-hydrodynamics implementations and to provide a data basis for comparisons and verifications of supernova codes to be developed in the future. Results are discussed for simulations of the core collapse and post-bounce evolution of a 13 solar mass star with Newtonian gravity and a 15 solar mass star with relativistic gravity.Comment: 23 pages, 13 figures, revised version, to appear in Ap

Differential Rotation in Neutron Stars: Magnetic Braking and Viscous Damping

Diffferentially rotating stars can support significantly more mass in equilibrium than nonrotating or uniformly rotating stars, according to general relativity. The remnant of a binary neutron star merger may give rise to such a ``hypermassive'' object. While such a star may be dynamically stable against gravitational collapse and bar formation, the radial stabilization due to differential rotation is likely to be temporary. Magnetic braking and viscosity combine to drive the star to uniform rotation, even if the seed magnetic field and the viscosity are small. This process inevitably leads to delayed collapse, which will be accompanied by a delayed gravitational wave burst and, possibly, a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the braking of differential rotation by magnetic fields and viscosity. The star is idealized as a differentially rotating, infinite cylinder consisting of a homogeneous, incompressible conducting gas. We solve analytically the simplest case in which the gas has no viscosity and the star resides in an exterior vacuum. We treat numerically cases in which the gas has internal viscosity and the star is embedded in an exterior, low-density, conducting medium. Our evolution calculations are presented to stimulate more realistic MHD simulations in full 3+1 general relativity. They serve to identify some of the key physical and numerical parameters, scaling behavior and competing timescales that characterize this important process.Comment: 11 pages. To appear in ApJ (November 20, 2000

Electron Neutrino Pair Annihilation: A New Source for Muon and Tau Neutrinos in Supernovae

We show that in a supernova core the annihilation process nu_e nu_e-bar -> nu_{mu,tau} nu_{mu,tau}-bar is always more important than the traditional reaction e^+ e^- -> nu_{mu,tau} nu_{mu,tau}-bar as a source for muon and tau neutrino pairs. We study the impact of the new process by means of a Monte Carlo transport code with a static stellar background model and by means of a self-consistent hydrodynamical simulation with Boltzmann neutrino transport. Nucleon bremsstrahlung NN -> NN nu_{mu,tau} nu_{mu,tau}-bar is also included as another important source term. Taking into account nu_e nu_e-bar -> nu_{mu,tau} nu_{mu,tau}-bar increases the nu_mu and nu_tau luminosities by as much as 20% while the spectra remain almost unaffected. In our hydrodynamical simulation the shock was somewhat weakened. Elastic nu_{mu,tau} nu_e and nu_{mu,tau} nu_e scattering is not negligible but less important than nu_{mu,tau} e^+ or e^- scattering. Its influence on the nu_{mu,tau} fluxes and spectra is small after all other processes have been included.Comment: 11 pages, 9 eps-figs, submitted to Ap

General-Relativistic MHD for the Numerical Construction of Dynamical Spacetimes

We assemble the equations of general relativistic magnetohydrodynamics (MHD) in 3+1 form. These consist of the complete coupled set of Maxwell equations for the electromagnetic field, Einstein's equations for the gravitational field, and the equations of relativistic MHD for a perfectly conducting ideal gas. The adopted form of the equations is suitable for evolving numerically a relativistic MHD fluid in a dynamical spacetime characterized by a strong gravitational field.Comment: 8 pages; scheduled for March 10 issue of Ap

About the detection of gravitational wave bursts

Several filtering methods for the detection of gravitational wave bursts in interferometric detectors are presented. These are simple and fast methods which can act as online triggers. All methods are compared to matched filtering with the help of a figure of merit based on the detection of supernovae signals simulated by Zwerger and Muller.Comment: 5 pages, proceedings of GWDAW99 (Roma, Dec. 1999), to appear in Int. J. Mod. Phys.