The Near Infrared Ca II Triplet as Metallicity Indicator: II Extension to extremely metal-poor metallicity regimes

TL;DR

This paper extends the calibration of the infrared Ca II triplet as a metallicity indicator to extremely metal-poor stars, providing a more accurate relation across a broad metallicity range from -4.0 to +0.5.

Contribution

It introduces a new calibration method incorporating non-linear terms to accurately measure metallicity in extremely metal-poor regimes.

Findings

Calibration extends from -4.0 to +0.5 in metallicity.

Achieves a tight correlation between high-resolution and CaT-derived [Fe/H].

Validates the calibration across four luminosity indicators.

Abstract

We extend our previous calibration of the infrared Ca II triplet as metallicity indicator to the metal-poor regime by including observations of 55 field stars with [Fe/H] down to -4.0 dex. While we previously solved the saturation at high-metallicity using a combination of a Lorentzian plus a Gaussian to reproduce the line profiles, in this paper we address the non-linearity at low-metallicity following the suggestion of Starkenburg et al 2010 of adding two non-linear terms to the relation among the [Fe/H], luminosity, and strength of the Calcium triplet lines. Our calibration thus extends from -4.0 to +0.5 in metallicity and is presented using four different luminosity indicators: V-V_{HB}, M_V, M_I, and M_K. The calibration obtained in this paper results in a tight correlation between [Fe/H] abundances measured from high resolution spectra and [Fe/H] values derived from the CaT, over the whole metallicity range covered.