M. R. Strayer

The Role of Electron Captures in Chandrasekhar-Mass Models for Type Ia Supernovae

The Chandrasekhar-mass model for Type Ia supernovae (SNe Ia) has received increasing support from recent comparisons of observations with light-curve predictions and modeling of synthetic spectra. It explains SN Ia events via thermonuclear explosions of accreting white dwarfs in binary stellar systems, being caused by central carbon ignition when the white dwarf approaches the Chandrasekhar mass. As the electron gas in white dwarfs is degenerate, characterized by high Fermi energies for the high-density regions in the center, electron capture on intermediate-mass and Fe group nuclei plays an important role in explosive burning. Electron capture affects the central electron fraction Ye, which determines the composition of the ejecta from such explosions. Up to the present, astrophysical tabulations based on shell model matrix elements were available only for light nuclei in the sd-shell. Recently, new shell model Monte Carlo and large-scale shell model diagonalization calculations have also been performed for pf-shell nuclei. These lead in general to a reduction of electron capture rates in comparison with previous, more phenomenological, approaches. Making use of these new shell model-based rates, we present the first results for the composition of Fe group nuclei produced in the central regions of SNe Ia and possible changes in the constraints on model parameters like ignition densities ρign and burning front speeds vdef.

Basis-Spline collocation method for the lattice solution of boundary value problems

Abstract We study a particular utilization of the basis-spline collocation method (BSCM) for the lattice solution of boundary value problems. We demonstrate the implementation of a general set of boundary conditions. Among the selected problems are the Schrodinger equation in radial coordinates, the Poisson, and the generalized Helmholtz equations in radial and three-dimensional Cartesian coordinates.

Relativistic theory of fermions and classical fields on a collocation lattice

Abstract We present a novel numerical method for solving dynamical strong field problems in quantum mechanics and classical field theory based on expansion of functions in terms of splines. The method differs from traditional approaches by the introduction of a mapping onto a collocation lattice, which is generally nonuniform and time dependent depending on the particular physical application. This approach results in a set of finite matrix transformations of a type which can be evaluated rapidly on supercomputers possessing either vector or matrix coprocessors. As an example of the method, we present a study of the relativistic quantum-mechanical many-electron problem interacting via very strong time-dependent classical fields.

Electron capture on iron group nuclei

We present Gamow-Teller strength distributions from shell model Monte Carlo studies of fp-shell nuclei that may play an important role in the precollapse evolution of supernovas. We then use these strength distributions to calculate the electron-capture cross sections and rates in the zero-momentum transfer limit. We also discuss the thermal behavior of the cross sections. We find large differences in these cross sections and rates when compared to the naive single-particle estimates. These differences need to be taken into account for improved modeling of the early stages of type-II supernova evolution. thinsp {copyright} {ital 1998} {ital The American Physical Society}

Probing the vacuum with highly charged ions

Abstract The physics of the Fermion vacuum is briefly described, and applied to pair production in heavy ion collisions. We consider in turn low energies (

GIANT RESONANCES FROM TDHF

A method of calculating giant resonance strength functions using Time-Dependent Hartree-Fock techniques is described. An application to isoscalar giant monopole resonances in spherical nuclei is made, thus allowing a comparison between independent 1-, 2- and 3-Dimensional computer codes.

Many-body perturbation calculation of spherical nuclei with a separable monopole interaction

We present calculations of ground state properties of spherical, doubly closed-shell nuclei from {sup 16}O to {sup 208}Pb employing the techniques of many-body perturbation theory using a separable density-dependent monopole interaction. The model gives results in Hartree-Fock order that are of similar quality to other effective density-dependent interactions. In addition, second and third order perturbation corrections to the binding energy are calculated and are found to contribute small, but non-negligible corrections beyond the mean-field result. The perturbation series converges quickly, suggesting that this method may be used to calculate fully correlated wave functions with only second or third order perturbation theory. We discuss the quality of the results and suggest possible methods of improvement.

Coherent Higgs and W± pair production

Cross sections for the coherent electromagnetic production of the intermediate vector bosons and the Higgs boson in ultrarelativistic heavy-ion collisions are calculated from standard electroweak theory. The distributions of the resulting decay products are also considered. Particular attention is given to the energy regime of the Superconducting Supercollider and the CERN Large Hadron Collider.

Nuclear shape-isomeric vibrations

Abstract It is argued that the correlated resonances observed in low-energy collisions of light heavy-ions may be vibrational-rotational states built on the shape-isomeric configurations of the composite nuclear system. These states are calculated for the shape-isomer of the 24 Mg nucleus by studying the response of the system to an external perturbation.

Study of nuclear dissipation via muon-induced fission. A relativistic lattice calculation

Abstract Excited muonic atoms in the actinide region may induce prompt nuclear fission via inverse internal conversion. We solve the time-dependent Dirac equation for a state describing a muon in the Coulomb field of the fissioning nucleus on a three-dimensional lattice and demonstrate that the muon attachment probability to the light fission fragment decreases with the energy dissipated during fission.