Influence of departures from LTE on determinations of the scandium abundances in A–B-type stars

Abstract

We developed a comprehensive model atom of Sc ii–Sc iii. Abundances of scandium for a sample of eight unevolved A9–B3-type stars with well-determined atmospheric parameters were determined based on the non-local thermodynamical equilibrium (NLTE) line formation for Sc ii–Sc iii and high-resolution observed spectra. For the Sc ii lines, the abundance differences between NLTE and local thermodynamical equilibrium (LTE) grow rapidly with increasing effective temperature (Teff), from slightly negative at Teff = 7250 K to positive ones of up to 0.6 dex at Teff = 10 400 K. For Sc iii in ι Her, NLTE reduces the line-to-line scatter substantially compared to the LTE case. The NLTE abundances of Sc in our five superficially normal stars are consistent within the error bars with the Solar system Sc abundance, while the LTE abundances of the late B-type stars are greatly subsolar. NLTE reduces but does not remove a deficiency of Sc in the Am stars HD 72660 and Sirius. Based on our own and the literature data, the Ca/Sc abundance ratios of the sample of 16 Am stars were found to be close together, with [Ca/Sc] = 0.6–0.7. We propose the Ca/Sc abundance ratio, but not the abundances of individual Ca and Sc elements to be used for classifying a star as Am and for testing the diffusion models. We provide the NLTE abundance corrections for 10 lines of Sc ii in a grid of model atmospheres appropriate for A to late B-type stars.