Gaia spectroscopy overview and comparative spectrum modeling for cool giants

The Gaia mission will provide spectroscopic and spectrophotometric observations for a vast number of stars, allowing a characterization in terms of stellar parameters along with the astrometric measurements. The pipeline for astrophysical parameter determination relies on libraries of model spectra for mapping stellar parameters to flux distributions. To maximize the reliability of the parametrization, adequate efforts are required to assess the realism of the model spectra and to calibrate the analysis methods. We describe a spectrum modelling experiment, in which high-resolution optical and infrared spectra of four cool giant stars were analysed by 14 different groups. The resulting variance in derived stellar parameters illustrates the need to be cautious when comparing or combining stellar parameters from different model atmospheres and analysis strategies. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method.