Calcium triplet metallicity calibration for stars in the Galactic bulge

Aims. We present a new calibration of the calcium II triplet equivalent widths versus [Fe/H], constructed upon K giant stars in the Galactic bulge. This calibration will be used to derive iron abundances for the targets of the GIBS survey, and is in general especially well suited for solar and supersolar metallicity giants, which are typical of external massive galaxies. Methods. About 150 bulge K giants were observed with the GIRAFFE spectrograph at the VLT with a resolution of R similar to 20 000 and at R similar to 6000. In the first case, the spectra allowed us to directly determine the Fe abundances from several unblended Fe lines, deriving what we call here high-resolution [Fe/H] measurements. The low-resolution spectra allowed us to measure equivalent widths of the two strongest lines of the near-infrared calcium II triplet at 8542 and 8662 angstrom. Results. By comparing the two measurements, we derived a relation between calcium equivalent widths and [Fe/H] that is linear over the metallicity range probed here, -1 < [Fe/H] < +0.7. By adding a small second-order correction based on literature globular cluster data, we derived the unique calibration equation [Fe/H](CaT) = -3.150 + 0.432W' + 0.006W'(2), with an rms dispersion of 0.197 dex, valid across the whole metallicity range -2.3 < [Fe/H] < + 0.7.