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.