Ledoux Convection in Protoneutron Stars—A Clue to Supernova Nucleosynthesis?

Two-dimensional hydrodynamical simulations of the deleptonization of a newlyformed neutron star were performed. Driven by negative lepton fraction andentropy gradients, convection starts near the neutrinosphere about 20-30 msafter core bounce, but moves deeper into the protoneutron star, and after aboutone second the whole protoneutron star is convective. The deleptonization ofthe star proceeds much faster than in the corresponding spherically symmetricalmodel because the lepton flux and the neutrino luminosities increase by up to afactor of two. The convection below the neutrinosphere raises theneutrinospheric temperatures and mean energies of the emitted neutrinos by10-20%. This can have important implications for the supernova explosionmechanism and changes the detectable neutrino signal from the Kelvin-Helmholtzcooling of the protoneutron star. In particular, the enhanced electron neutrinoflux relative to the electron antineutrino flux during the early post-bounceevolution might solve the overproduction problem of certain elements in theneutrino-heated ejecta in models of type-II supernova explosions.Comment: 17 pages, LaTeX, 8 postscript figures, uses epsf.sty. To appear in ApJ 473 (Letters), 1996 December 1