No abstract

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli-blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A = 65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations. PACS numbers: 26.50.+x, 97.60.Bw, At the end of their lives, stars with masses exceeding roughly 10 M ⊙ reach a moment in their evolution when their iron core provides no further source of nuclear energy generation. At this time, they collapse and, if not too massive, bounce and explode in spectacular events known as type II or Ib/c supernovae. As the density, ρ, of the star's center increases, electrons become more degenerate and their chemical potential µ e grows (µ e ∼ ρ 1/3 ). For sufficiently high values of the chemical potential electrons are captured by nuclei producing neutrinos, which for densities 10 11 g cm −3 , freely escape from the star, removing energy and entropy from the core. Thus the entropy stays low during collapse ensuring that nuclei dominate in the composition over free protons and neutrons. During the presupernova stage, i.e. for core densities 10 10 g cm −3 and proton-to-nucleon ratios Y e 0.42, nuclei with A = 55-65 dominate. The relevant rates for weak-interaction processes (including β ± decay and electron and positron capture) were first estimated by Fuller, Fowler and Newman [1] (for nuclei with A < 60), considering that at such conditions allowed (Fermi and Gamow-Teller) transitions dominate. The rates have been recently improved based on modern data and state-of-the-art many-body models [2], considering nuclei with A = 45-65. (This rate set will be denoted LMP in the following.) Presupernova models utilizing these improved weak rates are presented in [3]. In collapse simulations, i.e. densities 10 10 g cm −3 , a much simpler description of electron capture on nuclei is used. Here the rates are estimated in the spirit of the independent particle model (IPM), assuming pure Gamow-Teller (GT) transitions and considering only single particle states for proton and neutron numbers be- During core collapse, temperatures and densities are high enough to ensure that nuclear statistical equilibrium (NSE) is achieved. This means that for sufficiently low entropies, the matter composition is dominated by the nuclei with the highest binding energy for a given Y e . Electron capture reduces Y e , driving the nuclear composition to more neutron rich and heavier nuclei, including those with N > 40, which dominate the matter composition for densities larger than a few 10 10 g cm −3 . As a consequence of the model applied in previous collapse simulations, electron capture on nuclei ceases at these densities and the capture is entirely due to free proto...

The most important weak nuclear interaction to the dynamics of stellar core collapse is electron capture, primarily on nuclei with masses larger than 60. In prior simulations of core collapse, electron capture on these nuclei has been treated in a highly parameterized fashion, if not ignored. With realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as the ramifications for the post-bounce evolution in core collapse supernovae.

An improved prescription for choosing a transformed harmonic oscillator (THO) basis for use in configuration-space Hartree-Fock-Bogoliubov (HFB) calculations is presented. The new HFB+THO framework that follows accurately reproduces the results of coordinate-space HFB calculations for spherical nuclei, including those that are weakly bound. Furthermore, it is fully automated, facilitating its use in systematic investigations of large sets of nuclei throughout the periodic table. As a first application, we have carried out calculations using the Skyrme Force SLy4 and volume pairing, with exact particle number projection following application of the Lipkin-Nogami prescription. Calculations were performed for all even-even nuclei from the proton drip line to the neutron drip line having proton numbers Z = 2, 4, . . . , 108 and neutron numbers N = 2, 4, . . . , 188. We focus on nuclei near the neutron drip line and find that there exist numerous particle-bound even-even nuclei (i.e., nuclei with negative Fermi energies) that have at the same time negative two-neutron separation energies. This phenomenon, which was earlier noted for light nuclei, is attributed to bound shape isomers beyond the drip line.