Compulsory Deep Mixing of ³He 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 ³He(³He,2p)⁴He 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 ³He 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 three decades standing. This mixing mechanism, which we call "δ μ mixing," operates rapidly (relative to the nuclear timescale of overall evolution, ~10⁸ yr) once the hydrogen-burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Population I stars between 0.8 and 2.0 M_☉ develop ¹²C/¹³C ratios of 14.5 ± 1.5, while Population II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M_☉, this mechanism also destroys 90%-95% of the ³He produced on the main sequence.