NLTE study of scandium in the Sun

TL;DR

This study models the non-local thermodynamic equilibrium (NLTE) formation of scandium lines in the Sun's photosphere to improve abundance measurements, confirming discrepancies with meteoritic values.

Contribution

It provides detailed NLTE calculations for scandium lines in the Sun, enabling more accurate solar and stellar abundance analyses.

Findings

NLTE effects cause significant underpopulation of f1 in the Sun.

Solar scandium abundance is between 3.07 and 3.13, depending on model parameters.

The known difference between photospheric and meteoritic scandium abundances is confirmed.

Abstract

We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed derive solar $\log gf\epsilon_{\odot}$(Sc) values for scandium lines, which will later be used in differential abundance analyses of metal-poor stars. Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for \ion{Sc}{i} and 79 for \ion{Sc}{ii}. Photoionization cross-sections are assumed to be hydrogenic. Synthetic line profiles calculated from the level populations according to the NLTE departure coefficients were compared with the observed solar spectral atlas. Hyperfine structure (HFS) broadening is taken into account. The statistical equilibrium of scandium is dominated by a strong underpopulation of \ion{Sc}{i} caused by missing strong lines. It is nearly unaffected by the variation in interaction parameters and only marginally sensitive to the choice of the solar atmospheric model. Abundance determinations using the ODF model lead to a solar Sc abundance of between $\log\epsilon_\odot = 3.07$ and 3.13, depending on the choice of $f$ values. The long known difference between photospheric and meteoritic scandium abundances is confirmed for the experimental $f$-values.