Horizontal flows in the atmospheres of chemically peculiar stars

Context. Classical chemically peculiar stars exhibit atmospheres that are often structured by the effects of atomic diffusion. As a result of these elemental diffusion and horizontal abundance variations, the photospheric temperature varies at a given height in the atmosphere. This may lead to horizontal flows in the photosphere. In addition, the suppression of such flows by a magnetic field can alter the elemental transport processes. Aims. Using a simplified model of such a structured atmosphere and 2D magnetohydrodynamic simulations of a typical He-rich star, we examined atmospheric flows in these chemically peculiar stars, which often are strongly magnetic. Methods. We used Zeus-MP, which is a publicly available Fortran 90-based parallel finite element modular code. Results. We find that for non-magnetic stars of spectral type BA, the atmospheric flow related to the horizontal temperature gradient can reach 1.0 km s⁻¹, yielding mixing timescales of the order of tens of days. For the magnetic counterparts, the flow speeds are an order of magnitude lower, allowing for the stratification of chemical elements. Conclusions. Magnetic fields can significantly influence the dynamics in atmospheres. A strong horizontal magnetic field inhibits flow in the vertical direction, while a strong vertical magnetic field can suppress horizontal atmospheric flow and prevent elemental mixing.