Most of the stars (M < 8 solar masses, Ms) end their lives with a phase of strong mass loss on the Asymptotic Giant Branch (AGB), just before they form planetary nebulae (PNe). The strong mass loss efficiently enriches the interstellar medium (ISM) with gas and dust; AGB stars are one of the main contributors to the enrichment of the ISM where new stars and planets are born, and thus to the chemical evolution of galaxies. More specifically, the more massive (M > 4-5 Ms) AGB stars form very different isotopes (such as 87Rb, 7Li, 14N) from the isotopes formed by lower mass AGB stars and Supernova explosions, as a consequence of different dominant nuclear reaction mechanisms. However, single stars cannot form the asymmetric structures than are seen in post-AGB stars and PNe, thus binaries may play an important role in stellar evolution. Stars evolving from the AGB phase to the PN stage also form complex organic molecules such as polycyciclic aromatic hydrocarbons (PAHs), fullerenes, and graphene, representing a most fascinating laboratory for Astrochemistry. We aim at studying the late stages of stellar evolution with a special emphasis in the nucleosynthesis of light elements/isotopes and heavy neutronrich elements in AGB stars and in the formation process of fullerene and graphene nanostructures around stars evolving from the AGB phase throughout the PN stage.
We propose the use of our novel dynamical model atmospheres to explore the circumstellar effects on the abundance determination (both from atomic and molecular lines) in AGB stars with very different progenitor masses, and to obtain the chemical patterns of large samples of Galactic and extra-galactic AGB stars observed by the SDSS/APOGEE survey. This will impose severe observational constraints to the present theoretical models of AGB nucleosynthesis, a fundamental step forward to understand the progressive enrichment of the ISM and the chemical evolution of stellar systems such as galaxies and globular clusters.
We will also explore the formation process of fullerene and graphene nanostructures in the transition phase between AGB stars and PNe. Such complex organic compounds are major constituents of planetary atmospheres and Solar System solid bodies, and their study has important implications for the enrichment of the Early Solar System and possibly on the origin of life on Earth. We propose to explore this new and fertile field of research involving the synergistic combination of novel laboratory and observational/theoretical studies, addressing fundamental questions about the formation mechanism of fullerenic and graphenic nanostructures in evolved stars and the long-standing problem of the composition and structure of the carriers of certain unidentified astronomical features widely observed in these sources. The proposed laboratory/theoretical studies of new fullerenic and graphenic nanostructures undoubtedly will lead to new discoveries in this unexplored field, enabling the search of these molecules in space, which would represent a major breakthrough in Astrochemistry.
Finally, we will also explore how binarity affects the AGB evolution. We will use the GALEX database to search for binary central stars in PNe; both searching for double spectral energy distributions and for variability. Then their properties such as morphology, Galactic distribution, and chemical abundances will be analyzed in order to pose constraints on the evolution of binary systems.