Active-Sterile Neutrino Transformation and r-Process Nucleosynthesis

The type II supernova is considered as a candidate site for the production of heavy elements. Since the supernova produces an intense neutrino flux, neutrino scattering processes will impact element formation. We examine active-sterile neutrino conversion in this environment and find that it may help to produce the requisite neutron-to-seed ratio for synthesis of the r-process elements. The r-process of nucleosynthesis accounts for the most neutron rich of the heavy elements. The most likely environment for this type of synthesis is the late time (t > 10 s post-core bounce) supernova environment. Many studies have explored this ‘neutrino driven wind’ as a candidate environment and found it to be potentially viable [1,2]. However, to date, no model correctly reproduces the observed abundance pattern. In the neutrino driven wind, material in the form of free nucleons is ‘lifted’ off of the surface of the neutron star by energy deposited by neutrino interactions. Analytic and semianalytic parameterizations of the thermodynamic and hydrodynamic conditions in the wind can be obtained [3,4]. Models of this type may be used to explore the range of conditions within the context of the wind which will produce the solar system distribution of r-process elements. The key determinant of whether a given scenario will produce the r-process is the neutron to seed nucleus ratio at the onset of the neutron capture phase. This ratio must be quite high (R > 100) in order to produce the very neutron-rich r-process elements. The factors which determine the neutron-to-seed ratio are the entropy of the material, the hydrodynamic outflow timescale and the electron fraction, Ye = 1/(1 + n/p) where n/p is the neutron-to-proton ratio. A study of many possible model parameters shows that one must decrease the electron fraction, and/or increase the entropy and/or decrease the hydrodynamic outflow timescale, relative to the conditions found in typical wind models, in order to produce the neutron-to-seed ratio necessary for the r-process [5,6]. Including the effects of neutrino interactions in general tends to make the requisite conditions for r-process element production more extreme [7,8]. In particular, neutrino capture on free nucleons during alpha particle formation increases the electron fraction [8]. This is the “alpha effect”. Other neutrino process are discussed in [2,9]. There are three possible solutions to this problem. The first is that the supernova is the site of r-process synthesis, but it does not occur in the neutrino driven wind as it is currently modeled. The second is that the r-process elements are made at some other site such as neutron star-neutron star mergers. However, timescale arguments combined with isotopic abundance measurements and observations of old halo star metallicity show that this site is unlikely to account for the entire r-process distribution [10,11]. The third solution is the one that is investigated here: active-sterile (νe ↔ νs, ν̄e ↔ ν̄s) neutrino transformation. The νs in our study is defined as a particle which mixes with the νe (and possibly also with νμ, and/or ντ ) but does not contribute to the width of the Z boson. If we neglect ν-ν forward scattering contributions to the weak potentials, then the equation which governs the evolution of the neutrinos as they pass though the material in the wind can be written as: