The Calcium Triplet metallicity calibration for galactic bulge stars

TL;DR

This paper introduces a new calibration method for estimating metallicity in galactic bulge stars using Calcium II Triplet lines, applicable across a wide metallicity range, aiding studies of stellar populations.

Contribution

The authors develop a novel linear and quadratic calibration for Calcium II Triplet equivalent widths to [Fe/H], specifically tailored for bulge stars and applicable to external galaxies.

Findings

Calibration equation with rms dispersion of 0.197 dex.

Linear relation between Calcium equivalent widths and [Fe/H] over -1 to +0.7.

Effective for metallicities from -2.3 to +0.7.

Abstract

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 in general it is especially suited for solar and supersolar metallicity giants, typical of external massive galaxies. About 150 bulge K giants were observed with the GIRAFFE spectrograph at VLT, both at resolution R~20,000 and at R~6,000. In the first case, the spectra allowed us to perform direct determination of 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 A. 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 a rms dispersion of 0.197 dex, valid across the whole metallicity range -2.3<[Fe/H]<+0.7.