Panchromatic calibration of Ca II triplet luminosity dependence

Abstract

(ABRIDGED) Context. The line strength of the Ca II triplet (CaT) lines are a proxy to measure metallicity from individual stellar spectra of bright red giant stars. It is a mandatory step to remove the magnitude (proxy for gravity, temperature and luminosity) dependence from the equivalent width (EW) of the lines before converting them into metallicities. The working empirical procedure used for decades is to use the relative magnitude with respect to the horizontal branch level. Aims. The V filter is broadly adopted as the reference magnitude, although a few works have used different filters (I and Ks, for example). In this work we investigate the dependence of the CaT calibration using griz filters from the DECam and the GMOS, G from Gaia, BVI filters from the MCPS, YJKs filters from VIRCAM. We use as a reference FORS2 V filter used in the original analysis of the sample. Methods. Red giant stars from clusters with known metallicity and available CaT equivalent widths are used as reference. Public photometric catalogues are taken from SMASH DR2, VMC, Gaia, MCPS surveys plus VISCACHA-GMOS data, for a selection of Small Magellanic Cloud clusters. The slopes are fitted using two and three lines to be applicable to most of the metallicity scales. Results. The magnitude dependence of the CaT EWs is well described by a linear relation using any filter analysed in this work. The slope increases with wavelength of the filters. The zero point (a.k.a. reduced equivalent width), that is the metallicity indicator, remains the same. Conclusions. If the same line profile function is used with the same bandpasses and continuum regions, and the total EW comes from the same number of lines (2 or 3), then the reduced EW is the same regardless the filter used. Therefore, any filter can be used to convert the CaT equivalent widths into metallicity for a given CaT calibration.

12 pages, 22 figures, accepted for publication at Astronomy and Astrophysics

References in corpus (10)

• The Gaia mission
• The NIR Ca II triplet at low metallicity - Searching for extremely low-metallicity stars in classical dwarf galaxies
• Analysis and calibration of CaII triplet spectroscopy of Red Giant Branch stars from VLT/FLAMES observations
• A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 Kiloparsecs
• The Infrared Ca II triplet as metallicity indicator
• The X-Shooter Spectral Library (XSL): Data Release 2
• Stellar atmospheric parameters for 754 spectra from the X-shooter Spectral Library
• Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. III. Abundances and Velocities for a Sample of 14 Clusters
• The VISCACHA survey - I. Overview and First Results
• The Chemical Abundances of the Stellar Populations in the Leo I and Leo II dSph Galaxies

Cited by in corpus (7)

• The VISCACHA survey -- IV. The SMC West Halo in 8D
• The VISCACHA survey -- III. Star clusters counterpart of the Magellanic Bridge and Counter-Bridge in 8D
• Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. V. Abundances and Velocities for 12 Massive Clusters
• Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. VI. Analysis of chemical properties of the Main Body
• Ca Triplet Metallicities and Velocities for twelve Globular Clusters towards the Galactic Bulge
• The VISCACHA survey -- VIII. Chemical evolution history of Small Magellanic Cloud West Halo cluster
• Infrared photometry and CaT spectroscopy of the most metal-poor in-situ globular cluster VVV-CL001