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.

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