Accurate Metallicities for Very Metal-Poor Stars from the Ca~II Infrared Triplet

TL;DR

This study demonstrates that the Ca II infrared triplet, when modeled with Non-LTE corrections, provides accurate metallicity estimates for extremely metal-poor stars, overcoming previous limitations of interstellar absorption and LTE assumptions.

Contribution

The paper introduces a Non-LTE modeling approach for the Ca II infrared triplet, enabling reliable metallicity measurements down to [Fe/H] = -6.0 in extremely metal-poor stars.

Findings

Non-LTE corrections can exceed 0.5 dex for the triplet lines.

Agreement between triplet and resonance line abundances is excellent when interstellar absorption is accounted for.

The method allows metallicity measurements at very low levels, aiding primitive star discovery.

Abstract

The Ca~II H \& K lines are among the few features available to infer the metallicity of extremely metal-poor stars from medium-resolution spectroscopy. Unfortunately, these lines can overlap with absorption produced in the intervening interstellar medium, introducing systematic errors in the derived metallicities. The strength of the Ca~II infrared triplet lines can also be measured at extremely low metallicities, and it is not affected by interstellar absorption, but suffers significant departures from Local Thermodynamical Equilibrium (LTE). We investigate the feasibility of adopting the Ca~II infrared triplet as a metallicity indicator in extremely metal-poor stars using state-of-the art Non-LTE models including the most recent atomic data. We find that the triplet lines exhibit Non-LTE abundance corrections that can exceed 0.5\,dex. When interstellar absorption affecting the Ca~II resonance lines is accounted for using high-resolution observations, the agreement between Non-LTE abundances for the triplet and those for the resonance lines, with only minor departures from LTE, is excellent. Non-LTE effects strengthen the Ca~II IR triplet lines, facilitating measurements at very low metallicities, compared with LTE estimates, down to $\rm [Fe/H]=-6.0$. This result has important implications for the discovery of primitive stars in our Galaxy and others, since instruments are most sensitive at red/near-infrared wavelengths, and tens of millions of spectra covering the Ca~II IR triplet will soon become available from the Gaia, DESI, WEAVE, and PFS missions.