Presupernova evolution and explosive nucleosynthesis in massive stars for main-sequence masses from 13 M⊙ to 70 M⊙ are calculated. We examine the dependence of the supernova yields on the stellar mass, 12C(α,γ)16O rate, and explosion energy. The supernova yields integrated over the initial mass function are compared with the solar abundances.

Presupernova models and the 12C(α,γ)16O rate

Presupernova models are obtained for helium stars with masses of Mα = 3.3, 4, 5, 6, 8, 16, and 32 M⊙ as an extension of the studies by Nomoto & Hashimoto (1988), Thielemann et al. (1993), and Hashimoto et al. (1993). These helium star masses correspond approximately to main-sequence masses of Mms = 13, 15, 18, 20, 25, 40, and 70 M⊙, respectively (Sugimoto & Nomoto 1980). The systematic study for such a dense grid of stellar masses enables us to understand how explosive nucleosynthesis depends on the presupernova stellar structure and to apply the results to the chemical evolution of galaxies. We use the Schwarzschild criterion for convection and neglect overshooting. The initial composition is given by X(4He) = 0.9879 and X(14N) = 0.0121. These helium stars are evolved from helium burning through the onset of the Fe core collapse.

Nuclear reaction rates are mostly taken from Caughlan & Fowler (1988). For the uncertain rate of 12C(α,γ)16O, we use the rate by Caughlan et al. (1985; CFHZ85), which is larger than the rate by Caughlan & Fowler (1988; CF88) by a factor of ∼ 2.4.