The 5200-Å flux depression of chemically peculiar stars – II. The cool chemically peculiar and λ Bootis stars

Abstract

After establishing the synthetic Δa photometric system in the first paper of this series, we now present model atmospheres computed with individual abundances for a representative sample of chemically peculiar (CP) stars and either confirm or redetermine their input parameters through comparisons with photometric, spectrophotometric and high-resolution spectroscopic data. The final models obtained from this procedure were used to compute synthetic Δa indices which were compared with observations. The observed behaviour of Aa is reproduced for several types of CP stars: models for Am stars show negligible (or marginally positive) values of a few mmag, while for λ Bootis stars - and for metal deficient A stars in general - we obtain negative values (as low as -12 mmag in one case). For the coolest CP2 stars with effective temperatures below about 8500 K, we obtain mild (∼ +10 mmag) to moderately large (∼ +30 mmag) flux depressions in agreement with observations. However, Δa values for slightly hotter members of the CP2 group (for which still T eff < 10 000 K) are underestimated from these new models. The effect of the microturbulence parameter on the Δa index is revisited and its different role in various types of CP stars for reproducing the flux depression at 5200 A is explained. We also provide reasons why models based on enhanced microturbulence and scaled solar abundance could not explain the observed flux depression for all types of CP stars. We discuss potential improvements of the current models including the possibility of still missing line opacities (unidentified and autoionization lines), modifications due to an explicit account of a global stellar magnetic field, and the effect of vertical abundance stratification.