Nuclear Reactions Governing the Nucleosynthesis of44Ti

Large excesses of Ca44 in certain presolar graphite and silicon carbidegrains give strong evidence for Ti44 production in supernovae. Furthermore,recent detection of the Ti44 gamma-line from the Cas A SNR by CGRO/COMPTELshows that radioactive Ti44 is produced in supernovae. These make the Ti44abundance an observable diagnostic of supernovae. Through use of a nuclearreaction network, we have systematically varied reaction rates and groups ofreaction rates to experimentally identify those that govern Ti44 abundance incore-collapse supernova nucleosynthesis. We survey the nuclear-rate dependenceby repeated calculations of the identical adiabatic expansion, with peaktemperature and density chosen to be 5.5xE9 K and 1E7 g/cc, respectively, toapproximate the conditions in detailed supernova models. We find that, forequal total numbers of neutrons and protons (eta=0), Ti44 production is mostsensitive to the following reaction rates: Ti44(alpha,p)V47,alpha(2alpha,gamma)C12, Ti44(alpha,gamma)Cr48, V45(p,gamma)Cr46. We tabulatethe most sensitive reactions in order of their importance to the Ti44production near the standard values of currently accepted cross-sections, atboth reduced reaction rate (0.01X) and at increased reaction rate (100X)relative to their standard values. Although most reactions retain theirimportance for eta > 0, that of V45(p,gamma)Cr46 drops rapidly for eta >=0.0004. Other reactions assume greater significance at greater neutron excess:C12(alpha,gamma)O16, Ca40(alpha,gamma)Ti44, Al27(alpha,n)P30, Si30(alpha,n)S33.Because many of these rates are unknown experimentally, our results suggest themost important targets for future cross section measurements governing thevalue of this observable abundance.Comment: 37 pages, LaTex, 17 figures, 8 table