Influence of Inelastic Collisions with Hydrogen Atoms on the Formation of Al I and Si I Lines in Stellar Spectra

TL;DR

This study investigates how inelastic collisions with hydrogen atoms affect the formation of aluminum and silicon lines in stellar spectra, using advanced quantum calculations to improve abundance determinations in different stellar environments.

Contribution

First application of Belyaev et al.'s quantum cross sections for H I collisions in non-LTE modeling of Al I and Si I lines in stellar atmospheres.

Findings

Including H I collisions reduces non-LTE effects on Al I lines.

Accounting for H I collisions aligns Al I abundance measurements across different lines.

Non-LTE corrections depend on stellar parameters like temperature, gravity, and metallicity.

Abstract

The non-LTE line formation for Al I and Si I was calculated with model atmospheres corresponding to F-G-K type stars of different metallicity. To account for inelastic collisions with H I, for the first time we applied the cross sections calculated by Belyaev et al. using model approaches within the formalism of the Born-Oppenheimer quantum theory. For Al I non-LTE leads to overionization in the line formation layers and to weakened spectral lines, in line with earlier non-LTE studies. However, in contrast to the previuos studies, our results predict smaller magnitude of the non-LTE effects for the subordinate lines. Owing to large cross sections, the ion-pair production and mutual neutralization processes Al I(nl) + H I(1s) $\leftrightarrow$ Al~II(3s^2) + H^- provide a close coupling of high-excitation Al I levels to the Al II ground state, which causes smaller deviations from the TE populations compared to the case of pure electron collisions. For three metal-poor stars, the Al abundance was determined from seven lines in different models of their formation. In LTE and in non-LTE calculations with pure electron collisions, the Al I 3961 A resonance line gives a 0.25-0.45 dex lower abundance than the mean from the subordinate lines. The difference is removed by accounting for the collisions with H I, and the rms error of the abundance from all seven lines decreases by a factor of 1.5-3 compared to LTE. We calculated the non-LTE abundance corrections for six Al I lines depending on Teff (4500-6500 K), log g (3.0-4.5), and [Fe/H] (0, -1, -2, -3). For Si I including the collisions with H I leads to TE populations in the line formation layers and to minor departures from LTE in spectral lines.