Dredge-up and envelope burning in intermediate mass giants of very low metallicity

Abstract

(abbreviated) The evolution of intermediate mass stars at very low metallicity during their final thermal pulse asymptotic giant branch phase is studied in detail. As representative examples models with initial masses of 4Msun and 5Msun with a metallicity of Z=0.0001 ([Fe/H] ~ -2.3) are discussed. The 1D stellar structure and evolution model includes time- and depth dependent overshooting motivated by hydrodynamical simulations, as well as a full nuclear network and time-dependent mixing. Particular attention is given to high time and space resolution to avoid numerical artefacts related to third dredge-up and hot-bottom burning predictions. The model calculations predict very efficient third dredge-up which mixes the envelope with the entire intershell layer or a large fraction thereof, and in some cases penetrates into the C/O core below the He-shell. In all cases primary oxygen is mixed into the envelope. The models predict efficient envelope burning during the interpulse phase. Depending on the envelope burning temperature, oxygen is destroyed to varying degrees. The combined effect of dredge-up and envelope burning does not lead to any significant oxygen depletion in any of the cases considered in this study. The large dredge-up efficiency in our model is closely related to the particular properties of the H-shell during the dredge-up phase in low-metallicity very metal poor stars, which is followed here over many thermal pulses. During the dredge-up phase, the temperature just below the convective boundary is large enough for protons to burn vigorously when they are brought into the C-rich environment below the convection boundary by the time- and depth dependent overshooting. H-burning luminosities of 10^5 to ~2* 10^6L_sun are generated. [...]

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