Gaia / DPAC / CU8

Project title: Gaia / DPAC / CU8
Main applicant: Ulrike Heiter, Division of Astronomy and Space Physics
Co-applicants: Andreas Korn, Division of Astronomy and Space Physics
Grant amount: SEK 6 697 871 for the period 2023-2026

Gaia started its regular operations in 2014 and is delivering precision astrometry, photometry and spectrophotometry for more than one billion stars, extending current knowledge about the stellar content of the Milky Way by many orders of magnitude. Gaia's astrometric data alone will provide a picture of the structure of the Milky Way Galaxy from both stellar positions and velocities with unprecedented detail. Gaia data, when suitably calibrated, will also revolutionise our knowledge of the basic stellar astrophysical parameters – surface temperatures and gravities, chemical composition, masses and ages – for hundreds of millions of stars, providing the basis for understanding both the structure and the evolution of a typical luminous galaxy and the evolutionary properties of stars of all ages and compositions.

A thorough astrophysical calibration of Gaia observations (spectrophotometry and spectroscopy) has become a central aspect for Gaia Data Processing and Analysis Consortium (DPAC) Coordination Unit 8 (CU8). First CU8-derived parameters were included in Gaia Data Release 2 (DR2, 25 April 2018), e.g., effective temperatures for 161 million stars. In Gaia DR3 (13 June 2022) about 40 different parameters derived by CU8 will be published for up to 500 million stars. An even larger number of advanced data products are foreseen to be released for all Gaia sources in forthcoming data releases culminating in the final Gaia catalogue. To achieve this goal, all Gaia data have to pass rigorous validation and calibration procedures making sure that the data produce astrophysically meaningful results.

The Uppsala Gaia group is deeply involved in a variety of aspects of this work within CU8. We have provided the backbone grids of stellar fluxes and spectra (MARCS and LL models) for the vast majority of Gaia's targets and will continue to update these synthetic observables. We have established a set of Gaia FGK benchmark stars which represent the pillars of the calibrations, and we will extend this sample. Larger samples of validation and calibration stars are drawn from existing stellar surveys (e.g., Gaia-ESO Survey, GALAH, APOGEE) which show sufficient variation in stellar parameters and interstellar extinction, with parameters anchored to those of the benchmark stars. In the coming years our focus will be to enable the Gaia DPAC to derive and publish the best possible astrophysical results in Gaia DR4 (foreseen to take place in 2025). We are also involved in work within CU9 ("Catalogue Access") to safeguard that CU8 products are represented in the Gaia archive in a user-friendly and well-documented way.