Local mixing length theory with compositional effects: First application to asymptotic giant branch evolution

Abstract

Context. During the evolution of stars on the asymptotic giant branch (AGB), thermal pulses lead to the formation of strongly stratified layers in the outer regions of the CO core, which might lead to inversions in the chemical gradient. Such inversions would produce instabilities beyond the ones predicted by the Schwarzschild criterion and the standard use of mixing length theory (MLT).Aims. We aim to broaden the scope of MLT in a simple way to consider Rayleigh-Taylor and thermohaline instabilities. We also want to explore the impact of such instabilities during the AGB phase and in the pulsational properties of variable PG1159 (GW Vir) stars.Methods. We used a set of MLT equations that consider the impact of the background chemical gradients. This extension of MLT is referred to in this work as MLT♯, to make a distinction between both prescriptions. We applied MLT♯ in tandem with the more general Ledoux instability criterion. We computed the evolution in the AGB phase and compared the chemical profiles resulting from MLT, MLT♯ and the double diffusive GNA theory. We continued the evolution through a post-AGB thermal pulse and performed a pulsational analysis of the resultant GW Vir models to asses g-mode pulsation periods. Finally, we tested our results with pulsation properties of known GW Vir stars derived from recent observations.Results. We find that the much simpler MLT♯ set of equations closely reproduces the results from the GNA theory. As such, MLT♯ offers a simple way to include chemically driven convection in stellar evolution computations. Stellar evolution simulations show that Rayleigh-Taylor and thermohaline instabilities can play an important role during the TP-AGB. We obtained significantly different chemical profiles using a standard MLT approach compared to those resulting from our MLT♯ and GNA computations. Our adiabatic pulsational analysis shows that these differences in the chemical stratification leave clear mode-trapping signatures in the pulsation spectrum of the GW Vir models. Even though the comparison with current available GW Vir pulsation periods does not conclusively favor any of the three models of mixing explored in this work, our analysis demonstrates that asteroseismology of GW Vir stars has significant potential for probing chemically driven mixing processes during the TP-AGB phase.