On the Role of 14N in Helium Burning Phase in Stellar Evolution

The cross section for the 14N (a, r) 18F reaction has turned out to be1 drastically small, u so that Mitalas2> has proposed the reactions 14N (e, v) 14C (a, r) 180 in electron degenerate matter and concluded that the reaction is unimportant for igniting the helium flash in Population II red giants. Unfortunately, however, he made a great mistake (underestimation in magnitude of several orders) in caluculation of the energy generation rate of the resonant reaction. Moreover, it is considered not to be suitable that the effect of electron screening has been neglected in such dense matter. In this 1 note ~e shall derive more reasonable results and discuss briefly some possible effects of the reaction on the evolution of relatively low mass st11-rs. When the density increases to the critical value 1.15 X 106 g em3, at which the electron Fermi energy is equal to the threshold of the 14N (e, v) 14C reaction, a considerable amount of 14C can be produced. Subsequently the nucleus 14C begins to capt\lre a-particle rapidly to form 180. The excited state (E = 6.341 MeV) of 180 has been found8> to lie at a resonance energy E, = 113 ke V above the combined mass of 14C +4He. Therefore the reaction 14C(a, r) 180 is considered to be resonant, although the value of Jn of the 6.341-Me V state remains to be seen. Using the resonant reaction rate formulas,4> we ca.n easily calculate ~he energy generation rate for the 14N (e, v) 14C (a, r) 180 Table I. Numerical values of l-factor. One of them is applicable to J.