Carbon Thermonuclear Reactions and the Formation of Heavy Elements.

When helium is consumed at a high rate of energy generation in massive stars and in shell sources of less massive stars, the principal product is C'2. Following helium exhaustion, the next source of energy generation involves carbon thermonuclear reactions. In these reactions both protons and alpha-particles are produced. In the earlier stages of carbon-burning, the protons are captured by C12, forming C13, and the alpha-particles react with the C'3 to produce neutrons. These neutrons are captured on a slow time scale and can produce heavy elements, provided that the number of protons is comparable to the number of alpha-particles, as is estimated to be the case. The heavy-element abundances produced from material consisting initially of pure hydrogen are approximately normal for population I stars. However, contamination of the initial material by comparatively small amounts of intermediate elements results in heavy-element overabundance factors of several thousand. This process can take place in unmixed stellar material and does not suffer from the uncertainties of the Ne21 hypothesis. It may be the principal source in nature of products formed by neutron capture on a slow time scale.