Speaker
Description
The observed evolutionary decrease of the $^{12}$C/$^{13}$C isotopic ratio,
Li and C abundances, accompanied by the increase of the N
abundance, in low-mass stars on the upper RGB is caused by
extra mixing in their radiative zones. Multi-dimensional
hydrodynamic simulations of thermohaline convection have
demonstrated that its rate of mixing is almost two orders of
magnitude as low as what is required to reproduce the observational
data. Therefore, the search for an alternative mechanism of
the RGB extra mixing still continues. To simultaneously explain
the observed Li depletion on the lower part of the upper RGB
followed by a presumed fast Li enhancement in the vicinity of
the RGB tip, we consider a model of RGB extra mixing in which
the diffusion coefficient strongly increases with the luminosity.
With analytical prescriptions for the rates of mixing by internal
gravity waves (IGWs) generated by turbulent convection in the envelopes of
RGB stars, that are partly supported by our 3D hydrodynamical
simulations, we can reproduce the high Li abundances recently
revealed in red-clump stars. The results of our simulations
also support the hypothesis that the $^{13}$C pocket in thermally-pulsing
AGB stars, necessary for the main s process to occur under
radiative conditions, could be formed as a result of proton
ingestion by IGW mixing.
| Session | Stellar evolution |
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