Ion screening effects and stellar collapse

Description/Abstract

During the collapse of a massive star{close_quote}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 {open_quotes}ion screening,{close_quotes} has been calculated most recently by Horowitz. Using his correction, we investigate its effect on stellar core collapse. Our numerical results show that {nu}{sub 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. In particular, inclusion of ion screening lowers the trapped lepton fraction by only 0.015 in both our 15M{sub {circle_dot}} and a 25M{sub {circle_dot}} models. We confirm this with an analytic model that elucidates ion screening{close_quote}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. {copyright} {ital 1997} {ital The American Physical Society}