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Yttrium (Y), a key s-process element, is commonly used in nucleosynthesisstudies and as a Galactic chemical clock when combined with magnesium (Mg). Westudy the applicability of the previously assumed LTE line formation assumptionin Y abundance studies of main-sequence and red giant stars, and probe theimpact of NLTE effects on the [Y/Mg] ratio, a proposed stellar age indicator.We derive stellar parameters, ages, and NLTE abundances of Fe, Mg, and Y for 48solar analogue stars from high-resolution spectra acquired within the Gaia-ESOsurvey. For Y, we present a new NLTE atomic model. We determine a solar NLTEabundance of A(Y)$_{\rm NLTE}=2.12\pm0.04$ dex, $0.04$ dex higher than LTE.NLTE effects on Y abundance are modest for optical Y II lines, which arefrequently used in Sun-like stars diagnostics. NLTE has a small impact on the[Y/Mg] ratio in such stars. For metal-poor red giants, NLTE effects on Y IIlines are substantial, potentially exceeding $+0.5$ dex. For theGaia/4MOST/WEAVE benchmark star, HD 122563, we find the NLTE abundance ratio of[Y/Fe]$_{\rm NLTE}=-0.55\pm0.04$ dex with consistent abundances obtained fromdifferent Y II lines. NLTE has a differential effect on Y abundance diagnosticsin late-type stars. They notably affect Y II lines in red giants and verymetal-poor stars, which are typical Galactic enrichment tracers ofneutron-capture elements. For main-sequence stars, NLTE effects on opticaldiagnostic Y II lines remain minimal across metallicities. This affirms the[Y/Mg] ratio's reliability as a cosmochronometer for Sun-like stars.

monthly notices of the royal astronomical society

Observational constraints on the origin of the elements. VII. NLTE  analysis of Y II lines in spectra of cool stars and implications for Y as a  Galactic chemical clock