On the temperature dependence of the symmetry energy

We perform large-scale shell model Monte Carlo (SMMC) calculations for many nuclei in the mass range A=56-65 in the complete pfg_{9/2}d_{5/2} model space using an effective quadrupole-quadrupole+pairing residual interaction. Our calculations are performed at finite temperatures between T=0.33-2 MeV. Our main focus is the temperature dependence of the symmetry energy which we determine from the energy differences between various isobaric pairs with the same pairing structure and at different temperatures. Our SMMC studies are consistent with an increase of the symmetry energy with temperature. We also investigate possible consequences for core-collapse supernovae events

Electron screening in molecular fusion reactions

Recent laboratory experiments have measured fusion cross sections at center-of-mass energies low enough for the effects of atomic and molecular electrons to be important. To extract the cross section for bare nuclei from these data (as required for astrophysical applications), it is necessary to understand these screening effects. We study electron screening effects in the low-energy collisions of Z=1 nuclei with hydrogen molecules. Our model is based on a dynamical evolution of the electron wavefunctions within the TDHF scheme, while the motion of the nuclei is treated classically. We find that at the currently accessible energies the screening effects depend strongly on the molecular orientation. The screening is found to be larger for molecular targets than for atomic targets, due to the reflection symmetry in the latter. The results agree fairly well with data measured for deuteron collisions on molecular deuterium and tritium targets.Comment: 15 Page RevTeX document, twelve postscript figures, now in a uufile packag

Improved estimate of electron capture rates on nuclei during stellar core collapse

Electron captures on nuclei play an important role in the dynamics of the collapsing core of a massive star that leads to a supernova explosion. Recent calculations of these capture rates were based on microscopic models which account for relevant degrees of freedom. Due to computational restrictions such calculations were limited to a modest number of nuclei, mainly in the mass range A=45-110. Recent supernova simulations show that this pool of nuclei, however, omits the very neutron-rich and heavy nuclei which dominate the nuclear composition during the last phase of the collapse before neutrino trapping. Assuming that the composition is given by Nuclear Statistical Equilibrium we present here electron capture rates for collapse conditions derived from individual rates for roughly 2700 individual nuclei. For those nuclei which dominate in the early stage of the collapse, the individual rates are derived within the framework of microscopic models, while for the nuclei which dominate at high densities we have derived the rates based on the Random Phase Approximation with a global parametrization of the single particle occupation numbers. In addition, we have improved previous rate evaluations by properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics

Long-term evolution of massive star explosions

We examine simulations of core-collapse supernovae in spherical symmetry. Our model is based on general relativistic radiation hydrodynamics with three-flavor Boltzmann neutrino transport. We discuss the different supernova phases, including the long-term evolution up to 20 seconds after the onset of explosion during which the neutrino fluxes and mean energies decrease continuously. In addition, the spectra of all flavors become increasingly similar, indicating the change from charged- to neutral-current dominance. Furthermore, it has been shown recently by several groups independently, based on sophisticated supernova models, that collective neutrino flavor oscillations are suppressed during the early mass-accretion dominated post-bounce evolution. Here we focus on the possibility of collective flavor flips between electron and non-electron flavors during the later, on the order of seconds, evolution after the onset of an explosion with possible application for the nucleosynthesis of heavy elements.Comment: 12 pages, 7 figures, conference proceeding, HANSE 2011 worksho

Presupernova collapse models with improved weak-interaction rates

Improved values for stellar weak interaction rates have been recently calculated based upon a large shell model diagonalization. Using these new rates (for both beta decay and electron capture), we have examined the presupernova evolution of massive stars in the range 15-40 Msun. Comparing our new models with a standard set of presupernova models by Woosley and Weaver, we find significantly larger values for the electron-to-baryon ratio Ye at the onset of collapse and iron core masses reduced by approximately 0.1 Msun. The inclusion of beta-decay accounts for roughly half of the revisions, while the other half is a consequence of the improved nuclear physics. These changes will have important consequences for nucleosynthesis and the supernova explosion mechanism.Comment: 4 pages, 2 figure

WKB approximation for multi-channel barrier penetrability

Using a method of local transmission matrix, we generalize the well-known WKB formula for a barrier penetrability to multi-channel systems. We compare the WKB penetrability with a solution of the coupled-channels equations, and show that the WKB formula works well at energies well below the lowest adiabatic barrier. We also discuss the eigen-channel approach to a multi-channel tunneling, which may improve the performance of the WKB formula near and above the barrier.Comment: 15 pages, 4 eps figure

Pairing Reentrance Phenomenon in Heated Rotating Nuclei in the Shell Model Monte Carlo Approach

Rotational motion of heated 72-Ge is studied within the microscopic Shell Model Monte Carlo approach. We investigate the the angular momentum alignment and nuclear pairing correlations associated with J-pi Cooper pairs as a function of the rotational frequency and temperature. The reentrance of pairing correlations with temperature is predicted at high rotational frequencies. It manifests itself through the anomalous behavior of specific heat and level density.Comment: 4 pages; 4 figure

Resonant tunneling in a schematic model

Tunneling of an harmonically bound two-body system through an external Gaussian barrier is studied in a schematic model which allows for a better understanding of intricate quantum phenomena. The role of finite size and internal structure is investigated in a consistent treatment. The excitation of internal degrees of freedom gives rise to a peaked structure in the penetration factor. The model results indicate that for soft systems the adiabatic limit is not necessarily reached although often assumed in fusion of nuclei and in electron screening effects at astrophysical energies.Comment: 7 pages, 7 figure