THE EFFECTS OF THERMONUCLEAR REACTION RATE VARIATIONS ON 26 Al PRODUCTION IN MASSIVE STARS: A SENSITIVITY STUDY

We investigate the effects of thermonuclear reaction rate variations on 26Al production in massive stars. The dominant production sites in such events were recently investigated by using stellar model calculations: explosive neon-carbon burning, convective shell carbon burning, and convective core hydrogen burning. Post-processing nucleosynthesis calculations are performed for each of these sites by adopting temperature-density-time profiles from recent stellar evolution models. For each profile, we individually multiplied the rates of all relevant reactions by factors of 10, 2, 0.5, and 0.1, and analyzed the resulting abundance changes of 26Al. In total, we performed ≈900 nuclear reaction network calculations. Our simulations are based on a next-generation nuclear physics library, called STARLIB, which contains a recent evaluation of Monte Carlo reaction rates. Particular attention is paid to quantifying the rate uncertainties of those reactions that most sensitively influence 26Al production. For stellar modelers our results indicate to what degree predictions of 26Al nucleosynthesis depend on currently uncertain nuclear physics input, while for nuclear experimentalists our results represent a guide for future measurements. We also investigate equilibration effects of 26Al. In all previous massive star investigations, either a single species or two species of 26Al were taken into account, depending on whether thermal equilibrium was achieved or not. These are two extreme assumptions, and in a hot stellar plasma the ground and isomeric states may communicate via γ-ray transitions involving higher-lying 26Al levels. We tabulate the results of our reaction rate sensitivity study for each of the three distinct massive star sites referred to above. It is found that several current reaction rate uncertainties influence the production of 26Al. Particularly important reactions are 26Al(n,p)26Mg, 25Mg(α,n)28Si, 24Mg(n,γ)25Mg, and 23Na(α,p)26Mg. These reactions should be prime targets for future measurements. Overall, we estimate that the nuclear physics uncertainty of the 26Al yield predicted by the massive star models explored here amounts to about a factor of three. We also find that taking the equilibration of 26Al levels explicitly into account in any of the massive star sites investigated here has only minor effects on the predicted 26Al yields. Furthermore, we provide for the interested reader detailed comments regarding the current status of certain reactions, including 12C(12C,n)23Mg, 23Na(α,p)26Mg, 25Mg(α,n)28Si, 26Alm (p,γ)27Si, 26Al(n,p)26Mg, and 26Al(n,α)23Na