Effects of Neutrino Trapping on Thermodynamic Properties of Nuclear ``Pasta''

Abstract

Geometrical structure of matter at subnuclear densities is investigated in the presence of a degenerate gas of neutrinos as encountered in stellar collapse. The crystalline phases with spherical, cylindrical and planar nuclei as well as with spherical and cylindrical nuclear bubbles are considered by using a compressible liquid-drop model. This model allows for uncertainties in the lepton fraction $Y_{\rm L}$ in addition to those in the nuclear surface tension $E_{\rm surf}$ and in the proton chemical potential in bulk neutron matter $\mu_{\rm p}^{(0)}$. The phase diagrams obtained at zero temperature show that only the phases with rod-like and slab-like nuclei occur at typical values of $Y_{\rm L}$, $E_{\rm surf}$ and $\mu_{\rm p}^{(0)}$, whereas the bubble phases, especially with spherical bubbles, are at best expected at hypothetically low values of $Y_{\rm L}$ and/or $E_{\rm surf}$. For the rod-like and slab-like nuclei, thermally induced displacements are calculated from their respective elastic constants. It is found that at temperatures appropriate to supernova cores, thermal fluctuations would destroy the layered lattice of slab-like nuclei almost independently of the nuclear models and of the degree of the neutrino degeneracy.