Study of the departures from LTE in the unevolved stars infra-red spectra

TL;DR

This study investigates non-LTE effects in infrared spectral lines of certain elements in unevolved stars, identifying lines suitable for LTE approximation and analyzing abundance trends with metallicity.

Contribution

It provides a detailed analysis of LTE departures in IR lines of key elements in unevolved stars, highlighting lines that can be reliably used under LTE assumptions.

Findings

Many IR lines of Na I, Mg I, Al I, S I, and K I can be treated in LTE.

Sr II lines show significant departures from LTE, limiting their use.

Confirmed abundance trend correlations with metallicity using high-quality spectra.

Abstract

We present a study of departures from Local Thermodynamic Equilibrium (LTE) in the formation of infra-red lines of Na I, Mg I, Al I, S I, K I and Sr II in unevolved stars of spectral types F,G,K and metallicities around the solar metallicity. The purpose of this investigation is to identify lines of these species that can be safely treated with the LTE approximation in the infra-red spectra of these types of stars. We employ a set of 40 stars observed with the GIANO spectrograph at the 3.5 m Telescopio Nazionale Galileo (TNG) and previously investigated by Caffau et al. We were able to identify many lines that can be treated in LTE for all the above-mentioned species, except for Sr II. The latter species can only be studied using three lines in the J-band, but all three of them display significant departures from LTE. With our small-size, but high-quality sample we can determine robustly the trends of the abundance ratios with metallicity, confirming the trends apparent from a sample that is larger by several orders of magnitude, but of lower quality in terms of resolution and S/N ratio.