NLTE Analysis of High Resolution H-band Spectra. I. Neutral Silicon

TL;DR

This study assesses the impact of NLTE effects on silicon line formation in high-resolution H-band spectra, demonstrating that including NLTE corrections yields more accurate silicon abundances in stars.

Contribution

It provides a validated silicon atomic model and quantifies NLTE corrections for Si I lines in the H-band, improving abundance determinations in stellar spectra.

Findings

NLTE effects reduce Si abundance overestimations in LTE.

NLTE corrections are larger for giants, up to -0.4 dex.

Including NLTE effects improves consistency between H-band and optical Si abundances.

Abstract

We investigated the reliability of our silicon atomic model and the influence of non-local thermodynamical equilibrium (NLTE) on the formation of neutral silicon (Si I) lines in the near-infrared (near-IR) H-band. We derived the differential Si abundances for 13 sample stars with high-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), as well as from optical spectra, both under local thermodynamical equilibrium (LTE) and NLTE conditions. We found that the differences between the Si abundances derived from the H-band and from optical lines for the same stars are less than 0.1 dex when the NLTE effects included, and that NLTE reduces the line-to-line scatter in the H-band spectra for most sample stars. These results suggest that our Si atomic model is appropriate for studying the formation of H-band Si lines. Our calculations show that the NLTE corrections of the Si I H-band lines are negative, i.e. the final Si abundances will be overestimated in LTE. The corrections for strong lines depend on surface gravity, and tend to be larger for giants, reaching ~ -0.2 dex in our sample, and up to ~ -0.4 dex in extreme cases of APOGEE targets. Thus, the NLTE effects should be included in deriving silicon abundances from H-band Si I lines, especially for the cases where only strong lines are available.