SUPERNOVA ELECTRON-CAPTURE RATES FOR CO-55 AND NI-56

We have calculated the Gamow-Teller strength distributions for the gro und states and first excited states in Co-55 and Ni-56. These calculat ions have been performed by shell model diagonalization in the pf shel l using the KB3 interaction;The Gamow-Teller distributions are used to calculate the electron capture rates for typical presupernova conditi ons. Our Co-55 rate is noticeably smaller than the presently adopted r ate as it is dominated by weak low-lying transitions rather than the s trong Gamow-Teller (GT) resonance which is located at a higher excitat ion energy In the daughter than usually parametrized. Although our Ni- 56 rate agrees with the presently adopted rate, we do not confirm the conventional parametrization of the GT centroid. Our results support g eneral trends suggested on the basis of shell model Monte Carlo calcul ations. (C) 1998 Elsevier Science B.V. All rights reserved.