Electron-capture rates of nuclei at stellar environments and nucleosynthesis

Electron-capture rates in nuclei at stellar environments are evaluated by using new shell-model Hamil-tonians. A new Hamiltonian for pf-shell, GXPF1J, can describe Gamow-Teller (GT) strengths in Ni isotopes very accurately. In particular, large spreading in the GT strength in ⁵⁶Ni and ⁵⁵Co obtained for GXPF1J has been confirmed by a recent experiment. The updated e-capture rates as well as beta-decay rates obtained in a large region of pf-shell nuclei are applied to study nucleosynthesis in Type-Ia supernova explosions and core-collapse supernova explosions. The e-capture rates with GXPF1J, which are generally smaller than those with conventional shell-model Hamiltonians such as KB3G and lead to less production of neutron-rich nuclei such as ⁵⁸Ni and ⁵⁴Cr, can thus solve the problem of over-production of neutron-rich isotopes in the Fe region compared to the solar abundance. For more neutron-rich isotopes such as ⁷⁸Ni, extension of the configuration space outside the pf-shell is essential. Here, GT strength and spin-dipole strengths in ⁷⁸Ni are evaluated within pf+g_9/2+d_5/2 shells. Core-collapse supernova explosions are sensitive to the e-capture rates for nuclei around this region. Electron-capture rates in ⁷⁸Ni are discussed with shell-model as well as with RPA calculations.