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arXiv · 2512.20574

Revisiting the near infrared Calcium triplet as metallicity indicator

M. Navabi·R. Carrera·N. E. D. Noël·C. Gallart·E. Pancino·M. De Leo
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Abstract

The near-infrared Calcium II Triplet (CaT), around 850nm, 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 utilises the algorithms implemented therein. The derived calibration is valid for a wide metallicity range, $-4$\,dex$ \lesssim \mathrm{[Fe/H]} \lesssim +0.15$, 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.

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M. Navabi, R. Carrera, N. E. D. Noël, C. Gallart, E. Pancino, M. De Leo. 2025-12-23. Revisiting the near infrared Calcium triplet as metallicity indicator. https://doi.org/10.1093/mnras%2Fstag019

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