The NIR Ca II triplet at low metallicity - Searching for extremely low-metallicity stars in classical dwarf galaxies

TL;DR

This study investigates the Ca II triplet method for measuring metallicities in dwarf galaxy stars, revealing biases at low metallicities and proposing a new calibration that suggests more extremely metal-poor stars exist than previously detected.

Contribution

The paper introduces a new calibration for the Ca II triplet technique valid at low metallicities, improving the detection of extremely metal-poor stars in dwarf galaxies.

Findings

Synthetic spectral analysis shows deviations from empirical calibrations below [Fe/H]=-2.

New calibration extends validity to -0.5<[Fe/H]<-4.

Classical dwarf galaxies may host more extremely metal-poor stars than previously believed.

Abstract

The NIR Ca II triplet absorption lines have proven to be an important tool for quantitative spectroscopy of individual red giant branch stars in the Local Group, providing a better understanding of metallicities of stars in the Milky Way and dwarf galaxies and thereby an opportunity to constrain their chemical evolution processes. An interesting puzzle in this field is the significant lack of extremely metal-poor stars, below [Fe/H]=-3, found in classical dwarf galaxies around the Milky Way using this technique. The question arises whether these stars are really absent, or if the empirical Ca II triplet method used to study these systems is biased in the low-metallicity regime. Here we present results of synthetic spectral analysis of the Ca II triplet, that is focused on a better understanding of spectroscopic measurements of low-metallicity giant stars. Our results start to deviate strongly from the widely-used and linear empirical calibrations at [Fe/H]<-2. We provide a new calibration for Ca II triplet studies which is valid for -0.5<[Fe/H]<-4. We subsequently apply this new calibration to current data sets and suggest that the classical dwarf galaxies are not so devoid of extremely low-metallicity stars as was previously thought.