The stability of nonrotating supermassive stars (M > 105 M⊙) in the presence of a small dark matter component is examined. We find that the destabilizing effects of first-order nonlinear corrections from general relativity can be neutralized when a significant amount of nonrelativistic dark matter is present. In this case, it is possible for supermassive stars to be stabilized through the hydrogen-burning stage. Significant mass loss in this epoch might result in the ejection of the nucleosynthesis products of hot hydrogen burning. Ultimately, however, the post-Newtonian instability can only be delayed. If this instability occurs after the hydrogen-burning epoch, collapse of the star to a black hole is guaranteed. Subject headings: dark matter - gravitation - instabilities nuclear reactions, nucleosynthesis, abundances - relativity - stars: interiors © 1996. The American Astronomical Society. All rights reserved.

We examine the process of electron neutrino and electron antineutrino capture on nuclei with masses A > 40 in the context of the post-core-bounce supernova environment. We discuss the influence of final-state electron blocking, extended distributions of Gamow-Teller strength, and the Coulomb wave correction factor on neutrino and antineutrino capture rates. We study the effect of discrete state transitions and the thermal population of excited states on these rates. We estimate the strength of forbidden capture channels and discuss their importance in neutrino capture rate estimates. We find that forbidden strength can dominate the anti-neutrino capture rates on neutron-rich nuclei which are blocked and have no allowed transitions. Forbidden weak strength may be important in other cases of neutrino or antineutrino capture as well, depending on the excitation energy distribution of this strength. In addition, the importance of neutrino capture relative to anti-neutrino capture on both heavy nuclei and free nucleons is discussed. Formulae for calculating these rates in the context of the post-core-bounce supernova environment are presented. Tables of rates are provided for some key nuclei. A FORTRAN code for calculating rates is available on request.

We discuss how matter-enhanced active-sterile neutrino transformation in the [Formula Presented] and [Formula Presented] channels could enable the production of the rapid neutron capture [Formula Presented]-process) nuclei in neutrino-heated supernova ejecta. In this scheme the lightest sterile neutrino would be heavier than the [Formula Presented] and split from it by a vacuum mass-squared difference of [Formula Presented] with vacuum mixing angle [Formula Presented] © 1999 The American Physical Society.

We examine active-sterile neutrino conversion in the late time post-core-bounce supernova environment. By including the effect of feedback on the Mikheyev-Smirnov-Wolfenstein (MSW) conversion potential, we obtain a large range of neutrino mixing parameters which produce a favorable environment for the r-process. We look at the signature of this effect in the current generation of neutrino detectors now coming on line. We also investigate the impact of the neutrino-neutrino forward-scattering-induced potential on the MSW conversion. © 2002 Elsevier Science B.V. All rights reserved.

Neutrino evolution, of great importance in environments such as neutron star mergers (NSMs) because of their impact on explosive nucleosynthesis, is still poorly understood due to the high complexity and variety of possible flavor conversion mechanisms. In this study, we focus on so-called "fast flavor oscillations", which can occur on timescales of nanoseconds and are connected to the existence of a crossing between the angular distributions of electron (anti)neutrinos. Based on the neutrino radiation field drawn from a three dimensional neutron star merger simulation, we use an extension of the two-moment formalism of neutrino quantum kinetics, and perform a linear stability analysis to determine the characteristics of fast flavor instabilities across the simulation. We compare the results to local (centimeter-scale) three-dimensional two-flavor simulations using either a moment method or a particle-in-cell architecture. We get generally good agreement in the instability growth rate and typical instability lengthscale, although the imperfections of the closure used in moment methods remain to be better understood.