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The Ca~II H \& K lines are among the few features available to infer themetallicity of extremely metal-poor stars from medium-resolution spectroscopy.Unfortunately, these lines can overlap with absorption produced in theintervening interstellar medium, introducing systematic errors in the derivedmetallicities. The strength of the Ca~II infrared triplet lines can also bemeasured at extremely low metallicities, and it is not affected by interstellarabsorption, but suffers significant departures from Local ThermodynamicalEquilibrium (LTE). We investigate the feasibility of adopting the Ca~IIinfrared triplet as a metallicity indicator in extremely metal-poor stars usingstate-of-the art Non-LTE models including the most recent atomic data. We findthat the triplet lines exhibit Non-LTE abundance corrections that can exceed0.5\,dex. When interstellar absorption affecting the Ca~II resonance lines isaccounted for using high-resolution observations, the agreement between Non-LTEabundances for the triplet and those for the resonance lines, with only minordepartures from LTE, is excellent. Non-LTE effects strengthen the Ca~II IRtriplet lines, facilitating measurements at very low metallicities, comparedwith LTE estimates, down to $\rm [Fe/H]=-6.0$. This result has importantimplications for the discovery of primitive stars in our Galaxy and others,since instruments are most sensitive at red/near-infrared wavelengths, and tensof millions of spectra covering the Ca~II IR triplet will soon become availablefrom the Gaia, DESI, WEAVE, and PFS missions.

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