The r -Process in Proto-Neutron Star Winds with Anisotropic Neutrino Emission

The astrophysical origin of the r-process nuclei is still unknown. Even themost promising scenario, the neutrino-driven winds from a nascent neutron star,encounters severe difficulties in obtaining requisite entropy and short dynamictimescale for the r-process. In this study, the effect of anisotropy inneutrino emission from a proto-neutron star surface is examined withsemi-analytic neutrino-driven wind models. The increase of neutrino numberdensity in the wind owing to the anisotropy is modeled schematically byenhancing the effective neutrino luminosity. It is shown that the neutrinoheating rate from neutrino-antineutrino pair annihilation intoelectron-positron pairs can significantly increase owing to the anisotropy andplay a dominant role for the heating of wind material. A factor of fiveincrease in the effective neutrino luminosity results in 50% higher entropy anda factor of ten shorter dynamic timescale owing to this enhanced neutrinoheating. The nucleosynthesis calculations show that this change is enough forthe robust r-process, producing the third abundance peak A = 195 and beyond.Future multi-dimensional studies with accurate neutrino transport will beneeded if such anisotropy relevant for the current scenario (more than a factorof a few) is realized during the wind phase (~1-10 s).Comment: 8 pages, 3 figures, accepted for publication in ApJ Letter