Compulsory Deep Mixing of3He and CNO Isotopes in the Envelopes of Low‐Mass Red Giants

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the {sup 3}He({sup 3}He,2p){sup 4}He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of {sup 3}He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades standing. This mixing mechanism operates rapidly once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M{sub {circle_dot}} develop {sup 12}C/{sup 13}C ratios of 14.5 {+-} 1.5, while Pop II stars process the carbon to ratios of 4.0 {+-} 0.5. In stars less than 1.25 M{sub {circle_dot}}, this mechanism also destroys 90% to 95% of the {sup 3}He produced on the main sequence.