Revisiting the near-infrared calcium triplet as metallicity indicator

ABSTRACT The near-infrared calcium ii triplet (CaT), around 850 nm, is a key metallicity indicator for red giant stars. We present a revised [Fe/H] calibration as a function of CaT line strengths and four luminosity indicators, including the Gaia G band, together with the classical V, I, and $K_s$ bandpasses. For this purpose, we used a sample of 366 red giant stars belonging to 25 globular and open clusters, complemented by 52 extremely metal-poor field giant stars. The CaT line strengths are determined by fitting Gaussian–Lorentzian combination profiles using the python lmfit package, which utilizes the algorithms implemented therein. The derived calibration is valid for a wide metallicity range, $-4$ dex$\lesssim \mathrm{[Fe/H]} \lesssim +0.15$ dex, and for ages older than $\sim$200 Myr. In addition, we performed a detailed assessment of how factors such as spectral resolution, spectral quality (expressed through the signal-to-noise ratio), and the algorithms used to constrain the line profiles affect the measured line strengths and the resulting metallicities.