(Re)Solving Red Giants: interferometry and model atmospheres

Asymptotic Giant Branch (AGB) stars, i.e. the advanced evolutionary stages of stars with masses of up to about 8M, are prime targets for optical interferometry since they offer challenging science cases like the formation of molecules and cosmic dust, the (dynamic) interplay between stellar pulsation and the stellar atmosphere, the structure and evolution of surface inhomogeneities and the amount, geometry and mechanism of mass loss. During the past decade considerable progress has been made in modeling and understanding AGB star atmospheres. This came from improved basic data for molecules and dust, systematic and extensive comparisons with spectroscopic observations and, most importantly, maturing dynamic model atmospheres. At the same time, also the capabilities of interferometry are significantly improved by higher efficiency, longer baselines, multi-beam combination, the extension towards the mid-IR and the implementation of higher spectral resolution. Given the dusty and highly time-dependent nature of AGB star atmospheres, the latter two technical improvements are of particular importance since they give access to the dust component and they provide two independent and complementary sets of information simultaneously (spectra and intensitiy profiles). The proposed project will thus take advantage of the progress in atmospheric modeling and in optical interferometry by performing a thorough comparison of new spectro-interferometric data with the predictions of state of the art hydrostatic and dynamic model atmospheres. For this comparison, special emphasis will be given to carbon stars (i.e. giants with C/O>1) for several reasons: (i) these stars are of special relevance for stellar and galactic evolution, (ii) the available dynamic atmospheric models are more advanced compared to M-type AGB stars and (iii) there is a notable lack of interferometric data for them. The work on C-stars specifically aims at linking model parameters with interferometric observables, deriving atmospheric characteristics and stellar parameters from new observations with MIDI and AMBER at the ESO VLTI and comparing interferometric evidence for surface inhomogeneities and asymmetries with available exploratory models. As a complement to two other ongoing FWF-projects dealing with elemental abundances and dust properties of AGB-stars, some work on M-type giants is also planned. This concerns (i) a comparison of the interferometric predictions of different types of dynamic models and of simpler models (based on adding shells to hydrostatic structures) with available observations and (ii) the first estimation of pulsation radii for AGB stars in globular clusters. In order to increase the expertise in interferometric observations and data analysis within the project team and to have early access to data from a next generation instrument with advanced imaging capabilities, the project foresees also participation in the development of the VLTI spectro imager (VSI), a near-IR VLTI second generation instrument recently selected by ESO for a phase A study. The project will be carried out in collaboration with colleagues in Bonn, Garching, Grenoble Leuven, Uppsala, and Vienna.

Asymptotic Giant Branch (AGB) stars, i.e. the advanced evolutionary stages of stars with masses of up to about 8M, are prime targets for optical interferometry since they offer challenging science cases like the formation of molecules and cosmic dust, the (dynamic) interplay between stellar pulsation and the stellar atmosphere, the structure and evolution of surface inhomogeneities and the amount, geometry and mechanism of mass loss. During the past decade considerable progress has been made in modeling and understanding AGB star atmospheres. This came from improved basic data for molecules and dust, systematic and extensive comparisons with spectroscopic observations and, most importantly, maturing dynamic model atmospheres. At the same time, also the capabilities of interferometry are significantly improved by higher efficiency, longer baselines, multi-beam combination, the extension towards the mid-IR and the implementation of higher spectral resolution. Given the dusty and highly time-dependent nature of AGB star atmospheres, the latter two technical improvements are of particular importance since they give access to the dust component and they provide two independent and complementary sets of information simultaneously (spectra and intensitiy profiles). The proposed project will thus take advantage of the progress in atmospheric modeling and in optical interferometry by performing a thorough comparison of new spectro-interferometric data with the predictions of state of the art hydrostatic and dynamic model atmospheres. For this comparison, special emphasis will be given to carbon stars (i.e. giants with C/O>1) for several reasons: (i) these stars are of special relevance for stellar and galactic evolution, (ii) the available dynamic atmospheric models are more advanced compared to M-type AGB stars and (iii) there is a notable lack of interferometric data for them. The work on C-stars specifically aims at linking model parameters with interferometric observables, deriving atmospheric characteristics and stellar parameters from new observations with MIDI and AMBER at the ESO VLTI and comparing interferometric evidence for surface inhomogeneities and asymmetries with available exploratory models. As a complement to two other ongoing FWF-projects dealing with elemental abundances and dust properties of AGB-stars, some work on M-type giants is also planned. This concerns (i) a comparison of the interferometric predictions of different types of dynamic models and of simpler models (based on adding shells to hydrostatic structures) with available observations and (ii) the first estimation of pulsation radii for AGB stars in globular clusters. In order to increase the expertise in interferometric observations and data analysis within the project team and to have early access to data from a next generation instrument with advanced imaging capabilities, the project foresees also participation in the development of the VLTI spectro imager (VSI), a near-IR VLTI second generation instrument recently selected by ESO for a phase A study. The project will be carried out in collaboration with colleagues in Bonn, Garching, Grenoble Leuven, Uppsala, and Vienna.