Nuclear Reactions in Stars. IV. Buildup from Carbon

Abstract

Reactions leading to the formation of C, O, and Ne at temperatures T of about 10/sup 8/ deg K, in the core of red giant stars, were studied previously. The nuclear reactions and resulting nucleogenesis are now studied for a gas consisting of C/sup 12/, O/sup 16/, and Ne/sup 20/ at 6 x 10/sup 8/ to 10 x 10/s up 8/ deg K and densities around 10/sup 4/ g/cc. The basic reaction rates are calculated and a set of simultaneous differential equations for various abundances as a function of time is solved numerically. The C/sup 12/ is destroyed by compound nucleus formation from C + C collisions in about 10/sup 5/ and 1 years, respectively, at 6 x 10/sup 8/ and 8.5 x 10/sup 8/ deg K. The net result is the production of some additional amounts of O/sup 16/ and Ne/sup 20/; appreciable amounts of Na/sup 23/ and the three stable magnesium isotopes (mainly Mg/sup 24/) and decreasing amounts of Al/sup 27/, Si, etc. The ratio of Na/sup 23/ to Mg/sup 24/ produced is almost 1/2, appreciably larger than the cosmic ratio. During the carbon burning, protons and alphas are released. At temperatures below 7 x 10/sup 8/ degmore » K, neutrons are produced very copiously through the sequence C/sup 12/(p, gamma )N/sup 13/; N/sup 13/ yields C/sup 13/; C/sup 13/( alpha ,n)O/sup 16/. If small amounts of metals i n the Fe-region (up to about one metal nucleus per 1000 C/sup 12/ nuclei) were present originally, each metal nucleus will absorb about 30 neutrons, At temperatures above 8 x 10/sup 8/ deg K, N/sup 13/ is photodisintegrated and the neutron production is appreciably less. (auth)« less