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Solving the TP-AGB STAR Conundrum: a KEY to Galaxy Evolution

Mid-Term Report Summary - STARKEY (Solving the TP-AGB STAR Conundrum: a KEY to Galaxy Evolution)

The last evolutionary stages of low-mass stars (with initial masses up to 6-8 times the mass of the Sun), the so-called Thermally-Pulsing Asymptotic Giant Branch phase, are characterized by an extraordinary richness of physical processes that include a unique nucleosynthesis and mixing episodes, radial pulsation, dust condensation and growth, mass loss. These complex processes produce peculiar features that are identifiable in spectro-photometric studies of resolved stellar populations, ans affect the integrated properties of unresolved galaxies up to high redshift. In the last decade is has become evident how the uncertainties intrinsic to population synthesis models, and especially those related to the TP-AGB contribution, often dominate over the observational errors when deriving galaxy properties (e.g. masses and ages) across cosmic times.
The STARKEY project aims at remedying this persistent condition of controversy and uncertainty through a global calibration of the TP-AGB phase as a function of age and metallicity. To meet this challenge we first selected the research team whose initial efforts were mainly devoted to develop and refine the components of a modular tool that interconnects all key aspects of the TP-AGB phase (stellar evolution, pulsation, dynamical atmosphere, dust formation, mass loss, radiative transfer).
Once assembled, the theoretical machinery was linked with high-quality observations of resolved TP-AGB stars in galaxies and in stars clusters by means of quantitative comparisons based on the stellar population synthesis technique. The major calibration cycle has recently started. Among the main achievements obtained so far we shortly outline: new constraints to TP-AGB lifetimes coming from low-metallicity galaxies containing recent star formation; a large grid of new radial pulsation models for long-period variables suitable for both M and C stars; the modelling of non-radial modes in pulsating AGB stars; new grids of publicly-available model atmospheres and synthetic spectra for cool giants; a global reproduction of the observed properties of AGB stars in the Large Magellanic star cluster NGC1978; new constraints on the size and composition of circumstellar carbon-rich dust (Nanni et al. 2016); new dynamical atmosphere models for C stars at subsolar metallicities; new predictions for neon and sodium ejecta from AGB stars undergoing hot-bottom burning and their relevance to the interpretation of the chemical anticorrelation in observed in Galactic globular clusters; a new theoretical investigation of the shocked chemistry in the inner winds of pulsating AGB stars; a new generation of PARSEC-COLIBRI isochrones incorporating all of the above ingredients and in a format suitable for many different uses, going from the interpretation of observed color-magnitude diagrams, to providing basic ingredients for population synthesis models of distant galaxies. At present the TP-AGB calibration process is fully operational and significant upcoming developments are foreseen.