A non-local thermodynamic equilibrum study of scandium in the Sun

¹ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China e-mail: zhw@bac.pku.edu.cn
² Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, 81679 München, Germany e-mail: gehren@usm.lmu.de
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China

Received: 24 October 2007
Accepted: 14 January 2008

Abstract

Aims. We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed at derive solar (Sc) values for scandium lines, which will later be used in differential abundance analyses of metal-poor stars.

Methods. Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for Sc I and 79 for 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.

Results. The statistical equilibrium of scandium is dominated by a strong underpopulation of 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 = 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.