S-PROCESS Nucleosynthesis in Massive Stars: Core Helium Burning

We have investigated the nucleosynthesis which takes place via neutron capture during core helium burning in massive stars (M > 15 M0). The neutrons are produced by the 22Ne(a, n)25Mg reaction. It was found that complete conversion of 22Ne to 25Mg can take place only in the most massive stars (M > 30 M0). Little buildup of elements heavier than A = 90 can occur; this is due to the competition of lighter neutron poisons, particularly from the 25Mg produced by the neutron source reaction. Although the shape of the abundance distribution of s-process nuclei in the range 60 <% A <% 90 is fairly well reproduced, total production of these nuclei appears to be inadequate by a factor of 3 to 8. The abundances of many rare light nuclei such as 365, 40Ar, 40K, 43Ca, 405c, and 58Fe are considerably enhanced. It also appears possible that significant production of the more abundant nuclei 22Ne, 25Mg, and 26Mg may occur in these burning regions. This study indicates that it is particularly important to obtain experimental data on the neutron-capture cross-sections of 22Ne, 25Mg, and 26Fe. Subject headings: massive stars - nucleosynthesis