The Gaia-ESO Survey: Calibration strategy

The Gaia-ESO survey (GES) is now in its fifth and last year ofobservations and has produced tens of thousands of high-quality spectraof stars in all Milky Way components. This paper presents the strategybehind the selection of astrophysical calibration targets, ensuring thatall GES results on radial velocities, atmospheric parameters, andchemical abundance ratios will be both internally consistent and easilycomparable with other literature results, especially from other largespectroscopic surveys and from Gaia. The calibration of GES isparticularly delicate because of (i) the large space of parameterscovered by its targets, ranging from dwarfs to giants, from O to Mstars; these targets have a large wide of metallicities and also includefast rotators, emission line objects, and stars affected by veiling;(ii) the variety of observing setups, with different wavelength rangesand resolution; and (iii) the choice of analyzing the data with manydifferent state-of-the-art methods, each stronger in a different regionof the parameter space, which ensures a better understanding ofsystematic uncertainties. An overview of the GES calibration andhomogenization strategy is also given, along with some examples of theusage and results of calibrators in GES iDR4, which is the fourthinternal GES data release and will form the basis of the next GES publicdata release. The agreement between GES iDR4 recommended values andreference values for the calibrating objects are very satisfactory. Theaverage off sets and spreads are generally compatible with the GESmeasurement errors, which in iDR4 data already meet the requirements setby the main GES scientific goals.