Fluid Stability Below the Neutrinospheres of Supernova Progenitors and the Dominant Role of Lepto-Entropy Fingers

Abstract

We have developed a formalism for analyzing the stability of a fluid in the presence of neutrinos of all flavors and in the presence of a gravitational field. When applied to an extensive two-dimensional grid of core radii and fluid element sizes for each of several time slices of a number of proto-supernovae, we find no evidence for the neutron finger instability as described by the Livermore group. We find, instead, that the rate of lepton equilibration always exceeds that of thermal equilibration. Furthermore, we find that thermal diffusion driven by a lepton fraction difference and lepton diffusion driven by an entropy difference are both nonzero and that the first of these tends to be large in magnitude. An important consequence of this is the presence of a doubly diffusive instability, which we refer to as ``lepto-entropy fingers,'' in an extensive region below the neutrinosphere where the lepton number, $Y_{\ell}$, is small. This instability is driven by a mechanism very different from that giving rise to neutron fingers, and may play an important role in enhancing the neutrino emission. Deep in the core where the entropy is low and the lepton number higher, our analysis indicates a region unstable to another instability which we refer to as ``lepto-entropy semiconvection.'' These instabilities, particularly lepto-entropy fingers, may have already been seen in some multi-dimensional core collapse simulations described in the literature.