Full spectrum of turbulence convective mixing II. Lithium production in AGB stars

We present results from new, detailed computations of lithium production by hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass (3.5 ≤ M ≤ 6M ). The dependence of lithium production on stellar mass, metallicity, mass loss rate, convection and overshooting are discussed. In particular, nuclear burning, turbulent mixing and convective overshooting (if any) are self–consistently coupled by a diffusive algorithm, and the Full Spectrum of Turbulence (FST) model of convection is adopted, with test comparisons to Mixing Length Theory (MLT) stellar models. All the evolutions are followed from pre–main sequence down to late AGB, when stars do not appear any longer lithium rich. A “reference mass” of 6 M has been chosen since, although relatively close to the upper limit for which degenerate 12C ignition occurs, all the studied mechanisms show up more clearly. HBB is always found above ∼ logL/L = 4.4, but the range of (initial) masses reaching HBB is largely dependent on convection model, overshooting and metallicity. For solar chemistry, masses ≥ 4M evolve through HBB in the FST case and including core overshooting whereas, with solarly tuned MLT models and no overshooting, only masses ≥ 6M can reach HBB. These constraints can give feedbacks about the more correct convection model and/or the extent of overshooting, thanks to the signatures of HBB in AGB stars in clusters of known turnoff masses and metallicity. Overshooting (when included) is addressed as an exponentially decreasing diffusion above formally convective regions. It makes convective cores during the main sequence to grow larger, and also starting masses and luminosities in AGB are then larger. However, also preliminary results obtained when allowing displacement of convective elements below convective regions in AGB are shown. In the “reference” case (6M ), we find that overshooting from below the convective envelope totally suppresses thermal pulses and ultimately leads to the formation of massive (∼ 1M ) white dwarfs rich in Carbon and Oxygen immediately below the photosphere.