Nucleosynthesis and Characteristics in Liquid-Gas Coexistence Phase of Supernova Matter

Abstract

For the study of supernova explosion and neutron star structure, it is important to know the equation of state (EoS) as well as the nuclear composition of matter. At present, one of the standard approaches is based on the mean field treatment, in which the matter is assumed to be composed of one kind of large nucleus surrounded by nucleon and alpha gas. 1) On the other hand, recent heavy-ion collision studies have shown that nuclear matter should be regarded as a statistical ensemble of various fragment configurations at around the critical temperature of the liquid-gas phase transition at sub-saturation densities. Based on this understanding, we have proposed a nucleosynthesis process in which various nuclei are formed through the liquid-gas phase transition (LG process) as a preprocess of the standard r-process. 2) We have also demonstrated that it would be possible for a part of ejected paths in the actual supernova explosion to experience the liquid-gas coexisting region before freeze-out. In addition, if the freeze-out temperature is around the critical temperature, many fragments are formed even at very low densities, and then we can describe bulk structure of the solar abundance. With this nuclear distribution as the initial condition, it may be easier to proceed the r-process up to the third peak. In this paper, we turn our attention to one of the characteristic features of the liquid-gas coexistence phase in supernova matter, which allows many fragments to be formed even at very low densities. First we define a new quantity, gas baryon ratio Yg, Yg = (baryons in gas phase)/(baryons in whole system), as a measure of bulk fragment yield. For example, all baryons are bound in nuclei at Yg = 0. In Fig. 1, we show this gas baryon ratio of asymmetric nuclear matter as a function of the baryon density and the asymmetry, A = 1 − 2Yp, calculated in RMF. The behavior of Yg in asymmetric nuclear matter is smooth. In symmetric matter, both of the liquid and gas phase are symmetric, and the density in each phase is constant during the coexistence. Then the gas baryon ratio can be expressed by these liquid, gas, and the given average densities (ρ0, ρg, ρB) as, Yg = ρg(ρ0 − ρB)/ρB(ρ0 − ρg). This is a monotonically decreasing function of the baryon density, and very small in the density range under consideration. When the asymmetry increases, the liquid phase loses the symmetric energy and nucleons are emitted to gas phase, while Yg is still a decreasing function of ρB. The behavior of Yg in supernova matter is very different from that in asym-