Interferometry and the Mass Loss of Red Giants

Asymptotic Giant Branch (AGB) stars are the late evolutionary stages of stars with masses of up to about 8 solar masses. Given their large intrinsic radii and high infrared luminosity AGB stars are prime targets for optical interferometry. But they also offer important science cases like the origin cosmic dust, the mechanism, amount and geometry of mass loss or the structure and evolution of surface inhomogeneities. Interferometry in the near and mid IR has now reached a high level of versatility, efficiency and "user friendliness" especially through the ESO VLT Interferometer. This progress also opened new perspectives for AGB research, in particular through spectro-interferometry and imaging. At the same time, models for the dynamic AGB star atmospheres have also become more realistic and more and more observational data are compared with advanced models. Taking advantage of the progress in interferometry and building upon the results and insights from ongoing observing projects and modeling efforts this proposal now mainly aims at shedding new light on the mass loss process(es) operating in AGB stars and on deepening our understanding of the dynamic processes operating in AGB star atmospheres. The work on carbon stars (i.e. giants with C/O>1) specifically will provide solid observational constraints for dynamic models, especially for stars with low to intermediate mass loss rates which are not yet well reproduced by models. For the M-stars (C/O<1), where mass loss due to classical small silicate dust grains does not seem to work, the hypothesis of large iron-free dust grains as wind drivers shall be tested observationally and the origin of peculiar mm-CO line profiles will be investigated by optical interferometry. The observational data will come from the MIDI and AMBER instruments at the VLTI and are partly already available. Although the comparison with predictions of spherically symmetric models will be the main analyzing tool, the data will also be evaluated in terms of deviations from spherical symmetry. In addition, improvements in the codes used to calculate observables from the models as well as tests with image reconstruction codes are planned. To complement ongoing activities in Vienna with regard to the HERSCHEL satellite, the photospheres and inner envelopes of AGB stars to be observed within a HERSCHEL-PACS key program shall be characterized by using existing and new interferometric data. In an ongoing effort to increase the expertise in interferometric observations and data analysis and to ensure early access to data from a next generation instrument with advanced imaging capabilities, the project foresees also a contribution to the mid-IR VLTI second generation instrument MATISSE which is currently in Phase B. The contribution concerns the establishment of a catalog of calibrator stars with special emphasis on the L-band at 3.5 micrometers and on stars with small predicted apparent diameters. The project will be carried out in collaboration with colleagues in Bonn, Garching, Nice, Padova, Uppsala, and Vienna.

Asymptotic Giant Branch (AGB) stars are the late evolutionary stages of stars with masses of up to about 8 solar masses. Given their large intrinsic radii and high infrared luminosity AGB stars are prime targets for optical interferometry. But they also offer important science cases like the origin cosmic dust, the mechanism, amount and geometry of mass loss or the structure and evolution of surface inhomogeneities. Interferometry in the near and mid IR has now reached a high level of versatility, efficiency and user friendliness especially through the ESO VLT Interferometer. This progress also opened new perspectives for AGB research, in particular through spectro-interferometry and imaging. At the same time, models for the dynamic AGB star atmospheres have also become more realistic and more and more observational data are compared with advanced models. Taking advantage of the progress in interferometry and building upon the results and insights from ongoing observing projects and modeling efforts, this project aimed at deepening our understanding of the mass loss process(es) operating in AGB stars and of the dynamic processes operating in AGB star atmospheres. The work on carbon stars (i.e. giants with C/O>1) provided solid observational constraints for dynamic models. While the spectral energy distributions can be well reproduced by the models, there are notable discrepancies for the interferometric measurements. For the M-stars (C/O<1), where mass loss due to classical small silicate dust grains does not seem to work, the hypothesis of large iron-free dust grains as wind drivers could be confirmed. Furthermore, the VLTI data indicate that circumstellar disks cause the preculiar mm-CO line profiles. Measurements with different interferometric instruments show a manifold morphology of the outer atmospheres ranging from spherical symmetry to disks and clumpy structures. In the course of the project an active participation in MATISSE could be achieved, the mid-IR VLTI second generation instrument which will become operational in 2018. The contribution concerns the catalog of calibrator stars with special emphasis on the L-band at 3.5 micrometers and the simulation and planning of observations of carbon stars. In addition, improvements in the codes used to calculate observables from the models as well as tests with image reconstruction codes were carried out. The project was carried out in collaboration with colleagues in Brussels, Garching (ESO), Nice, Padova, Uppsala, and Vienna.