Abstract
We discuss the evolution of stars through the asymptotic giant branch, focusing on the physical mechanisms potentially able to alter the surface chemical composition and on how changes in the chemistry of the external regions affect the physical properties of the star and the duration of this evolutionary phase. We focus on the differences between the evolution of low-mass stars, driven by the growth of the core mass and by the surface carbon enrichment, and that of their higher mass counterparts, which experience hot bottom burning. In the latter sources, the variation of the surface chemical composition reflects the equilibria of the proton capture nucleosynthesis experienced at the base of the convective envelope. The pollution expected from this class of stars is discussed, outlining the role of mass and metallicity on the chemical composition of the ejecta. To this aim, we considered evolutionary models of 0.7–8 M⊙ stars in a wide range of metallicities, extending from the ultra-metal-poor domain to super-solar chemistries.
Highlights
The last decades have witnessed a growing interest towards the properties of stars evolving across the asymptotic giant branch (AGB)
We present the main results of an ongoing research project developed by our research team during the last decade, aimed at describing the main aspects characterising the evolution of AGB stars, focusing on the different nucleosynthesis channels taking place in the interior and on the efficiency of the mixing processes
When the core mass of AGB stars exceeds ∼0.8 M [44], the density and temperature gradients become so steep that the innermost regions of the convective envelope are partially overlapped with the CNO-burning shell, which favours the ignition of proton capture nucleosynthesis at the bottom of the envelope, a phenomenon known as hot bottom burning (HBB) [45]
Summary
The last decades have witnessed a growing interest towards the properties of stars evolving across the asymptotic giant branch (AGB). AGB stars are extremely complex systems, composed by regions characterised by considerably different physical properties, ranging from the compact core, supported by the pressure of degenerate electrons, to the tenuous, extended envelope, which is gradually lost via stellar winds These stars are supported on the energy side by the CNO nucleosynthesis taking place in an internal region above the CO core. The evolutionary properties of the stars evolving along the AGB exhibit an outstanding diversity, being extremely sensitive to the initial mass and metallicity The former is relevant in assessing the relative importance of the various mechanisms able to alter the surface chemical composition, whereas metallicity affects the number of thermal pulses experienced by the star and the temperature at which the internal proton capture nucleosynthesis takes place, which in turn affects the equilibrium among the abundances of the different species. Particular care is devoted to discussing the chemical composition of the ejecta of AGB stars of different masses and chemical compositions, to understand the role played by this class of stars as pollutants of the interstellar medium
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