Ion screening effects and stellar collapse

During the collapse of a massive star's stellar core Coulomb effects maintain the ions in a highly correlated state. This has an important consequence: Neutrino-nucleus elastic scattering, which dominates the neutrino opacity, is substantially reduced for low-energy neutrinos. This results from phase interference effects that occur when the neutrino wavelength becomes larger than the interion spacing, and is analogous to a crystal becoming transparent to x rays when the change in wave number from scattering is smaller than the reciprocal lattice spacing. This reduction in the neutrino-nucleus elastic scattering cross section, referred to as "ion screening," has been calculated most recently by Horowitz. Using his correction, we investigate its effect on stellar core collapse. Our numerical results show that nu(e) downscattering with electrons is sufficiently rapid to fill the low-energy neutrino window created by ion screening, but the window width is insufficient for ion screening to have a significant effect on core deleptonization. Zn particular, inclusion of ion screening lowers the trapped lepton fraction by only 0.015 in both our 15M. and a 25M. models. We confirm this with an analytic model that elucidates ion screening's essential effect. For the sake of comparison, we also investigate the effect on core deleptonization of turning neutrino-nucleus elastic scattering off completely, and of turning off all semileptonic neutral-current neutrino scattering. These latter neutrino opacity modifications have substantially greater effects on core deleptonization than the ion-screening correction. [S0556-2821(97)06922-1].