We show that the evolutionary track of a low-mass red giant should make an extended zigzag on the Hertzsprung-Russel diagram just after the bump luminosity if fast internal rotation and enhanced extra mixing in the radiative zone bring the temperature gradient close to the adiabatic one. This can explain both the location and peculiar surface chemical composition of Li-rich K giants studied by Kumar et al. We also discuss a striking resemblance between the photometric and composition peculiarities of these stars and giant components of RS CVn binaries. We demonstrate that the observationally constrained values of the temperature gradient in the Li-rich K giants agree with the required rate of extra mixing only if the turbulence that is believed to be responsible for this extra mixing is highly anisotropic, with its associated transport coefficients in the horizontal direction strongly dominating over those in the vertical direction.
Title: A New Twist in the Evolution of Low-mass Stars
Authors: Denissenkov, Pavel A.
Affiliation: AA(Department of Physics & Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada)
Publication: The Astrophysical Journal Letters, Volume 753, Issue 1, article id. L3 (2012). (ApJL Homepage)
Publication Date: 07/2012
Bibliographic Code: 2012ApJ...753L...3D