Accurate Metallicities for Very Metal-poor Stars from the Ca ii Infrared Triplet

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