A New Twist In the Evolution of Low-Mass Stars

TL;DR

This paper proposes that low-mass red giants undergo an extended zigzag evolution on the Hertzsprung-Russell diagram due to internal mixing and rotation, explaining their chemical peculiarities and surface properties.

Contribution

It introduces a new evolutionary scenario involving zigzag tracks caused by anisotropic turbulence and enhanced mixing, explaining Li-rich K giants' features.

Findings

Zigzag evolutionary tracks can explain Li-rich K giants' location and composition.

Anisotropic turbulence with dominant horizontal transport is necessary for observed mixing rates.

The model aligns with observed photometric and chemical peculiarities of these stars.

Abstract

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, Reddy, & Lambert (2011). 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 which 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.