Panchromatic calibration of Ca II triplet luminosity dependence
  (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.
