No abstract

55 Co is not only present in abundance in presupernova phase but is also advocated to play a decisive role in the core collapse of massive stars. The spectroscopy of electron capture and emitted neutrinos yields useful information on the physical conditions and stellar core composition. B(GT) values to low-lying states are calculated microscopically using the pn-QRPA theory. Our rates are enhanced compared to the reported shell model rates. The enhancement is attributed partly to the liberty of selecting a huge model space, allowing consideration of many more excited states in our rate calculations. Unlike previous calculations the so-called Brinks hypothesis is not assumed leading to a more realistic estimate of the rates. The electron and positron capture rates are calculated over a wide temperature (0.01 × 10 9 − 30 × 10 9 K) and density (10 − 10 11 gcm −3 ) grid.

Allowed weak interaction rates for sd -shell nuclei in stellar environment are calculated using a generalized form of proton-neutron quasiparticle RPA model with separable Gamow-Teller forces. Twelve different weak rates are calculated for each nucleus as a function of temperature and density. The stellar weak rates are calculated over a wide range of densities (10≤ ρYe (gcm −3 ) ≤ 10 11 ) and temperatures (10 7 ≤ T(K) ≤ 30 × 10 9 ). This project consists of calculation of weak rates for a total of 709 nuclei with masses ranging from A = 18 to 100. This paper contains calculated weak rates for sd-shell nuclei. The calculated capture and decay rates take into consideration the latest experimental energy levels and f t value compilations. The results are also compared with earlier works. Particle emission processes from excited states, previously ignored, are taken into account, and are found to significantly affect some β decay rates.

PACS 23.40.Bw -Weak interaction and lepton (including neutrino) aspects PACS 26.50.+x -Nuclear physics aspects of supernovae PACS 21.60.Jz -Nuclear Density Functional Theory and extensions