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

TL;DR

This study investigates the NLTE effects on Y II line formation in cool stars and assesses their impact on using the [Y/Mg] ratio as a reliable Galactic chemical clock, especially in different stellar types and metallicities.

Contribution

It introduces a new NLTE atomic model for Y and evaluates its effects on Y abundance measurements across various stellar populations, confirming the [Y/Mg] ratio's robustness in Sun-like stars.

Findings

NLTE effects on Y are modest in Sun-like stars but substantial in metal-poor red giants.

The solar NLTE Y abundance is 0.04 dex higher than LTE.

NLTE effects do not significantly alter the [Y/Mg] ratio in main-sequence stars.

Abstract

Yttrium (Y), a key s-process element, is commonly used in nucleosynthesis studies and as a Galactic chemical clock when combined with magnesium (Mg). We study the applicability of the previously assumed LTE line formation assumption in Y abundance studies of main-sequence and red giant stars, and probe the impact 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 48 solar analogue stars from high-resolution spectra acquired within the Gaia-ESO survey. For Y, we present a new NLTE atomic model. We determine a solar NLTE abundance 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 are frequently 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 II lines are substantial, potentially exceeding $+0.5$ dex. For the Gaia/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 from different Y II lines. NLTE has a differential effect on Y abundance diagnostics in late-type stars. They notably affect Y II lines in red giants and very metal-poor stars, which are typical Galactic enrichment tracers of neutron-capture elements. For main-sequence stars, NLTE effects on optical diagnostic Y II lines remain minimal across metallicities. This affirms the [Y/Mg] ratio's reliability as a cosmochronometer for Sun-like stars.