Evolution through Alpha-Burning (M=3→15 M[sun]).

Abstract

Models of 3, 5, 9, and 15 solar masses and Population I initial composition are followed from gravitationally contracting phases through hydrogen and helium burning stages up to the development of an 160 core. The elements `2C, `4N, 160, `H, 3He, and 4He are followed in detail. Alpha-particle burning involves the reactions 5Be (a,~)'2C, `2C(a,~)'6O, and `4N(a,~)'5F. It is found that, due to the extension of a convective envelope down to regions in which `2C has been converted almost completely to `4N, the surface ratio of `4N to `2C immediately preceding and following giant phases is considerably larger than this ratio in the contraction and early hydrogen-burning phases. From an initial value of 0.333, the 14N/12C mass ratio increases to 1.27, 0.88, and 1.09 for M/Mo = 9, 5, and 3, respectively. This ratio is not changed further during core-helium burning stages. Since, as the calculations show, a star in the mass range 3 < M/Mo ~ 9 spends approximately 25% of its life in the core helium burning stage relative to its life near the main sequence in the core-hydrogen burning stage, a statistically large fraction of stars which lie in the Hertzsprung-Russell diagram where corehelium burning is expected to occur should show the predicted enhancement. This work was supported in part by the Office' of Naval Research and the National Aeronautics and Space Administration.