Supernova SN1987A Bound on Neutrino Spectra for R-Process Nucleosynthesis

The neutrino driven wind during a core collapse supernova is an attractive site for r-process nucleosynthesis. The electron fraction Ye in the wind depends on observable neutrino energies and luminosities. The mean antineutrino energy is limited by supernova SN1987A data while lepton number conservation constrains the ratio of antineutrino to neutrino luminosities. If Ye, in the wind, is to be suitable for rapid neutron capture nucleosynthesis, then the mean electron neutrino energy may be significantly lower then that predicted in present supernova simulations, or there may be new neutrino physics such as oscillations to sterile neutrinos. Subject headings: Supernovae: core collapse, SN1987A, Nucleosynthesis: r-process The neutrino driven wind above a protoneutron star in a core collapse supernova is an attractive site for r-process nucleosynthesis. In the r-process, seed nuclei rapidly capture free neutrons to produce about half of the heavy elements(Burbidge, Fowler and Hoyle 1957; Wallerstein et al. 1997). Many simulations, for example (Meyer and Brown 1997), have explored a range of physical conditions including entropy, expansion time scale, and electron fraction (number of electrons or protons per baryon) Ye necessary to produce r-process elements with solar system abundances. The electron fraction Ye is an improtant parameter that determines the number of free neutrons. If there are too few free neutrons per seed nucleus, then the heaviest elements may not be produced. A reasonable minimum requirement for an r-process, producing solar system like abundances, is that Ye be less then 1/2. If Ye is greater then 1/2, all of the neutrons may be quickly incorporated into alpha particles leaving only free protons. In a core collapse supernova, high neutrino luminosities eject some baryons from the surface of the protoneutron star into a neutrino driven wind. Many authors have explored r-process nucleosynthesis in this wind (Woosley et al. 1994; Takahashi, Witti and Janka 1994; Qian and Woosley 1996). The electron fraction Ye in the wind is set by the relative rates of the neutrino capture reactions, νe + n → p+ e −, (1) ν̄e + p → n+ e . (2) These rates depend on the known cross sections and the neutrino and antineutrino luminosities and mean energies. The cross section for Eq. (1) is larger than that for Eq. (2) because of important weak magnetism and recoil corrections. The ratio of rates for Eqs. (1) and (2) yields the initial electron fraction Ye (Horowitz and Li 1999),