Revision of calcium and scandium abundances in Am stars based on NLTE calculations and comparison with diffusion stellar evolution models

TL;DR

This study refines calcium and scandium abundance measurements in Am stars using NLTE calculations, compares them with diffusion models, and assesses the models' ability to explain observed chemical peculiarities.

Contribution

It provides the first homogeneous NLTE-based abundance analysis for a large Am star sample and evaluates diffusion models against these observations.

Findings

Calcium and scandium abundances correlate with effective temperature.

Diffusion models agree with observed abundances at high turbulence parameters.

Standard models fail to reproduce surface abundances of Sirius.

Abstract

The homogeneous data sets for the calcium and scandium abundances accounting for departures from LTE were obtained for a sample of 54 metallic-line (Am) stars. The Ca and Sc abundances were found to correlate with effective temperature Teff, the abundance growth with increasing Teff being higher in stars with surface gravity log g < 4 than in those with log g > 4. No correlation was found between Ca or Sc abundances and the iron abundance or the velocity of axial rotation. Am stars exhibit on average the higher values of [Ca/H] than those of [Sc/H] as well as the abundance ratio [Ca/Sc] = 0.41 +/- 0.30. However, at Teff > 9500 K there is an allusion to the systematic difference between Am stars with surface gravity log g > 4 and log g < 4. The iron excess is nearly the same in the range 7200 K <= Teff <= 10030 K. Evolution diffusion models computed with the code MESA for stars with masses from 1.5 to 2Msun show the surface abundances that are in good agreement with Ca and Fe abundances observed in Am stars of the three open clusters with the age > 600 Myr. Additional mechanisms of chemical separation should be considered for explanation of the Am phenomenon in young stars of the Pleiades cluster. We tested the published diffusion stellar evolution models. The diffusion models by Richer et al. (2000) and Hui-Bon-Hoa et al. (2022) are shown to agree with observations of Am stars in the open clusters at large values of the free turbulence parameter: omega=1000 for Ca and Fe, omega=500 for Sc. There is no model with the mass and age of the Am-type star Sirius that could reproduce its surface abundances from He to Ni. The results presented in the paper may be of importance for understanding the chemical peculiarity of Am stars.