Galactic giant stars photometric metallicity

We use the calibrations by Calamida et al. (2007ApJ...670..400C) and by Hilker et al. (2000A&A...355..994H), and the standardised synthetic photometry in the v, b, and y Stromgren passbands from Gaia DR3 BP/RP spectra, to obtain photometric metallicities for a selected sample of 694233 old Galactic giant stars having |b|>20.0 and parallax uncertainties lower than 10%. The zero point of both sets of photometric metallicities has been shifted to to ensure optimal match with the spectroscopic [Fe/H] values for 44785 stars in common with APOGEE DR17, focusing on the metallicity range where they provide the highest accuracy.

The metallicities derived in this way from the Calamida et al. (2007ApJ...670..400C) calibration display a typical accuracy of ~0.1dex and 1 sigma precision ~0.2dex in the range -2.2<=[Fe/H]<=-0.4, while they show a systematic trend with [Fe/H] at higher metallicity, beyond the applicability range of the relation. Those derived from the Hilker et al. (2000A&A...355..994H) calibration display, in general, worse precision, and lower accuracy in the metal-poor regime, but have a median accuracy <0.05dex for [Fe/H]>=-0.8.