Stellar Reaction Rates for ^(28)Si

Stellar rates for the reactions ^(27)Al(p,-y)^(28)Si and ^(24)Mg(ɑ,-y)^(28)Si have been calculated from new cross- section data for these reactions. The contributions to the interaction rates, (σv)_(py) and (σv)_(ay), and to the corresponding photodisintegration rates, λ_(yp) and λ_(ya), from the excited states of ^(27)Al, ^(24)Mg, and ^(28)Si are discussed. The calculated interaction rates, N_A(σv), are listed for temperatures from 0.1 X 10^9º to 5.0 X 10^9° K; two- and four-parameter fits are given for N_A(σv) as a function of temperature

The reaction rate sensitivity of nucleosynthesis in type II supernovae

We explore the sensitivity of the nucleosynthesis of intermediate-mass elements (28 less than or equal to A less than or similar to 80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler. Two stellar models are calculated for each of two masses: 15 and 25 M.. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network and then exploded using a piston near the edge of the iron core as described by Woosley & Weaver. The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of 2 over the entire range of nuclei studied. Reasons for the major discrepancies along with the physics underlying the two reaction rate sets employed are discussed in detail. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed