From canonical to enhanced extra mixing in low-mass red giants: Tidally locked binaries

Stellar models that incorporate simple diffusion or shear-induced mixing are used to describe canonical extra mixing in low-mass red giants of low and solar metallicity. These models are able to simultaneously explain the observed Li and CN abundance changes along the upper red giant branch (RGB) in field low-metallicity stars and match photometry, rotation, and C-12/C-13 ratios for stars in the old open cluster M67. The shear mixing model requires that main-sequence ( MS) progenitors of upper RGB stars possessed rapidly rotating radiative cores and that specific angular momentum was conserved in each of their mass shells during their evolution. We surmise that solar-type stars will not experience canonical extra mixing on the RGB because their more efficient MS spin-down resulted in solid-body rotation, as revealed by helioseismological data for the Sun. Thus, RGB stars in the old, high-metallicity cluster NGC 6791 should show no evidence for mixing in their C-12/C-13 ratios. We develop the idea that canonical extra mixing in a giant component of a binary system may be switched to its enhanced mode with much faster and somewhat deeper mixing as a result of the giant's tidal spin- up. This scenario can explain photometric and composition peculiarities of RS CVn binaries. The tidally enforced enhanced extra mixing might contribute to the star-to-star abundance variations of O, Na, and Al in globular clusters. This idea may be tested with observations of C-12/C-13 ratios and CN abundances in RS CVn binaries.