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

Abstract

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). 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$\sharp$, to make a distinction between both prescriptions. We applied MLT$\sharp$ 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$\sharp$ 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. We find that the much simpler MLT$\sharp$ set of equations closely reproduces the results from the GNA theory. As such, MLT$\sharp$ 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$\sharp$ 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.

Figures (9)

• Figure 1: Chemical abundances of He (dotted lines), C (continuous lines) and O (dashed lines) of stellar models of initial mass $M_\text{in}=1M_\odot$ (left panel), $1.5M_\odot$ (middle). and $3M_\odot$ (right) at the end of the AGB phase. The blue lines correspond to the case where MLT was used, the orange ones are for MLT$\sharp$, and the black ones are for GNA.
• Figure 2: Top two panels: Surface luminosity (top) and helium luminosity (bottom) during the ninth thermal pulse of the sequence with $M_\text{in}=1.5 \ M_\odot$. Four stages, labeled as a, b, c, and d, are highlighted, which correspond to the chemical stratifications shown in the four bottom panels. Bottom panels: Formation of the O peak during the thermal pulses. Panels b and c show that the O peak is formed in the same way using both prescriptions, while panel d shows how the additional instabilities considered in MLT$\sharp$ lead to the mixing of the O peak formed in the previous TP. We also highlight the position of the eighth and ninth O peaks.
• Figure 3: Evolutionary track for the $M_\text{in}=1.5M_\odot$ star. The VLTP is highlighted as well as the three points (marked with crosses), where the pulsational calculations were made.
• Figure 4: Chemical profiles (top), Ledoux term B (middle), and the logarithm of the squared Brünt-Väisälä frequency (bottom) for a PG1159 model with a mass of $M=0.58 \ M_\odot$ ($M_\text{in}=1.5 \ M_\odot$) at three different stages after the VLTP. As in Fig. \ref{['Fig1']}, blue lines correspond to MLT, orange lines to MLT$\sharp$, and black lines to GNA.
• Figure 5: Forward period spacing for the PG1159 models with masses of $M=0.58 \ M_\odot \ (M_\text{in}=1.5 \ M_\odot )$ and the three mixing prescriptions at the three different stages selected after the VLTP. As in the previous figures, blue dots correspond to MLT, orange dots to MLT$\sharp$, and black dots to GNA.