Nuclear Reactions Governing the Nucleosynthesis of 44Ti

Large excesses of 44Ca in certain presolar graphite and silicon carbide grains give strong evidence for 44Ti production in supernovae. Furthermore, recent detection of the 44Ti γ line from the Cas A supernova remnant by the Compton Gamma Ray Observatory Compton Telescope shows that radioactive 44Ti is produced in supernovae. These make the 44Ti abundance an observable diagnostic of supernovae. Through use of a nuclear reaction network, we have systematically varied reaction rates and groups of reaction rates to experimentally identify those that govern 44Ti abundance in core-collapse supernova nucleosynthesis. We survey the nuclear-rate dependence by repeated calculations of the identical adiabatic expansion, with peak temperature and density chosen to be 5.5 × 109 K and 107 g cm-3, respectively, to approximate the conditions in detailed supernova models. We find that, for equal total numbers of neutrons and protons (η = 0),44Ti production is most sensitive to the following reaction rates:44Ti(α, p)47V, α(2α, γ)12C,44Ti(α, γ)48Cr, and 45V(p, γ)46Cr. We tabulate the most sensitive reactions in order of their importance to the 44Ti production near the standard values of currently accepted reaction rates, at both a reduced reaction rate (times 0.01) and an increased reaction rate (times 100) relative to their standard values. Although most reactions retain their importance for η > 0, that of 45V(p, γ)46Cr drops rapidly for η ≥ 0.0004. Other reactions assume greater significance at greater neutron excess:12C(α, γ)16O,40Ca(α, γ)44Ti,27Al(α, n)30P,30Si(α, n)33S. Because many of these rates are unknown experimentally, our results suggest the most important targets for future cross section measurements governing the value of this observable abundance.