Revision of the Calcium and Scandium Abundances in Am Stars Based on Non-LTE Calculations and Comparison with Diffusion Stellar Evolution Models

Abstract

A homogeneous set of calcium and scandium abundances has been obtained for 54 A-type stars with strong metal lines (Am stars) by taking into account the departures from local thermodynamic equilibrium. A correlation of the Ca and Sc abundances with the effective temperature \((T_{\textrm{eff}})\) has been revealed, with the Ca and Sc abundances in stars with a surface gravity \(\log g<4\) growing with increasing \(T_{\textrm{eff}}\) faster than in stars with \(\log g\geq 4\). No correlation of the Ca and Sc abundances with the iron abundance and the stellar rotation velocity has been found. Am stars exhibit, on average, a higher value of [Ca/H] than that of [Sc/H] and \([\textrm{Ca}/\textrm{Sc}]=0.41\pm 0.30\). However, for \(T_{\textrm{eff}}>9500\) K there is a hint at a systematic difference between stars with \(\log g\geq 4\) and \(\log g<4\). The iron overabundance is, on average, the same in the range \(7200\leq T_{\textrm{eff}}\leq 10\,030\;\textrm{K}\). It is shown that when atomic diffusion is taken into account, evolution computations with the MESA code for stellar masses of 1.5–2.2 \(M_{\odot}\) give surface abundances that are consistent with the Ca and Fe abundances in Am stars in three open clusters with an age greater than 600 Myr. Additional mechanisms for the separation of chemical elements are required to explain the Am phenomenon in young stars in the Pleiades cluster. Published diffusion models have been tested. The turbulent models of Richer et al. (2000) and Hui-Bon-Hoa et al. (2022) are consistent with the observations of Am stars in open clusters at large values of the free parameter \(\omega\): 1000 for Ca and Fe and 500 for Sc. None of the diffusion models corresponding to the mass and age of the Am star Sirius reproduces the abundances of the elements from He to Ni observed in it. The results obtained are important for a better understanding of the chemical peculiarity mechanisms in Am stars.