Calcium triplet metallicity calibration for stars in the Galactic bulge^⋆,⋆⋆

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
e-mail: svasquez@astro.puc.cl
² Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ Laboratoire Lagrange (UMR7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS34229, 06304 Nice Cedex 4, France
⁴ European Southern Observatory, Av. Alonso de Cordova 3107, Casilla 19, 19001 Santiago, Chile
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁶ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁷ Instituto de Astrofísica de Canarias, calle via Láctea s/n, 38205 San Cristobal de La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
⁹ Museo Interactivo Mirador, Dirección de Educación, Av. Punta Arenas, 6711 La Granja, Santiago, Chile

Received: 14 May 2015
Accepted: 29 June 2015

Abstract

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 ~ 20 000 and at R ~ 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 Å.

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.