Stellar atmosphere models for Gaia
A major task of modern Astrophysics is to determine the origin, structure, and evolutionary history of our Galaxy. Progress in this task depends crucially on our knowledge of the physics of stars as building blocks of the Galaxy, in particular their atmospheres. Low-mass stars play a very important role in this context, because most of them exist for longer than the present age of the Universe. They also display, to a high degree, their original chemical composition at their surfaces.The ESA cornerstone mission Gaia will provide astronomers with a detailed astrometric and spectrophotometric census of the Galaxy and the local Universe. For one billion stars brighter than an apparent visual magnitude of V≈20, astrometry with an accuracy of 10 microarcseconds and optical and near-IR spectrophotometry will be obtained. This will allow a detailed mapping of the content of our Galaxy, both in space and in terms of astrophysical parameters: surface temperature (Teff), surface gravity (log g) and metal content ([Fe/H]). For brighter objects, the astrometry and spectrophotometry will be supplemented by near-IR spectroscopic observations, which will allow, besides radial velocities, some individual elemental abundances to be measured.
The scientific goals of Gaia range from understanding the origin and history of our Galaxy (e.g. its star formation history, its chemical and dynamical evolution, and the distribution of dust) to understanding the formation and evolution of stars (e.g. the dynamics of star forming regions, a complete and detailed local census, and a census of multiple stars). Additional scientific products include the detection of extra-solar planetary systems, as well as comprehensive surveys of Solar System and extragalactic objects.
The responsibility for processing and analysing Gaia data has been taken over by the Gaia Data Processing and Analysis Consortium (DPAC). This consortium consists of over 300 scientists and software engineers from the European astronomical community. The work of the Gaia-SAM collaboration will contribute to the objectives of DPAC Coordination Unit 8 (CU8), "Astrophysical Parameters", which is developing classification and parametrisation algorithms for Gaia.
The aim is to identify the most realistic atmospheric models by comparing calculated observables to highly accurate and well-calibrated observed data. The observables considered include narrow-band photometry, low-resolution spectra and resolved spectral line profiles at high resolution. The observations are obtained for a number of carefully selected cool dwarf and giant stars, so-called benchmark stars, for which fundamental parameters are determined with high precision. Interferometric observations are used to determine stellar radii. The improved atmospheric models will be based on realistic assumptions for the modelled physics – hydrodynamics in 3D for convection modelling and accounting for deviations from local thermodynamic equilibrium ("non-LTE") for spectral line formation. An important part of the project is devoted to compile and improve the wealth of basic physics data needed as input for such models.