Revisiting the near infrared Calcium triplet as metallicity indicator

TL;DR

This paper revises the calibration of the near-infrared Calcium II Triplet as a metallicity indicator for red giant stars, incorporating multiple luminosity indicators and assessing measurement factors.

Contribution

It provides a new, comprehensive calibration for CaT-based metallicity estimates across a wide range of stellar metallicities and ages, including methodological insights.

Findings

Calibration valid for -4 to +0.15 dex metallicity range

Effective for stars older than ~200 Myr

Assessment of measurement factors affecting line strength

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.