Advanced Evolution of Massive Stars. IV. Secondary Nucleosynthesis During Helium Burning

The thermonuclear processing of 14N during core helium burning in massive stars is examined. A detailed discussion of the relevant reaction rates is given, including the evaluation of recent experimental data, and the presentation of analytic fits for N aV . If, as seems likely, the 12C and 160 that we observe was produced under such conditions, then most, if not all, the 22Ne was produced at the same time by the sequence 14N(a, 7)'8F( +)18O(a, y)22Ne. For stars in the mass range (4 < MIMo 8) it is unlikely that 25Mg, which can be an effective contributor to the Urca process, is greatly enhanced by helium burning.