Magnetic and Chemical Structures in Stellar Atmospheres

We present an investigation of the magnetic field geometries and inhomogeneous distribution of chemical elements in the atmospheres of peculiar A and B stars. Our study combines high-quality spectroscopic and spectropolarimetric stellar observations with the development and application of novel techniques for theoretical interpretation of the shapes and variability of stellar line profiles. In particular, we extend the method of Doppler imaging to the analysis of spectra in the four Stokes parameters, making it possible to derive detailed and reliable stellar magnetic maps simultaneously with the imaging chemical inhomogeneities. The magnetic Doppler imaging is applied to study of magnetic topologies and distributions of chemical elements in the peculiar stars α2 CVn and 53 Cam. We found that the magnetic field geometry of 53 Cam is considerably more complex than a low-order multipolar topology, commonly assumed for magnetic A and B stars. Our Doppler imaging analysis also led to a discovery and study of spots of enhanced mercury abundance in the atmosphere of α And, a star where the presence of a global magnetic field is unlikely. The ESO 3.6-m telescope is used to collect unique, very high spectral- and time-resolution observations of rapidly oscillating peculiar A (roAp) stars and to reveal line profile variations due to stellar pulsations. We present a detailed characterization of the spectroscopic pulsational behaviour and demonstrate a remarkable diversity of pulsations in different spectral lines. The outstanding variability of the lines of rare-earth elements is used to study propagation of pulsation waves through the stellar atmospheres and identify pulsation modes. This analysis led to a discovery of a non-axisymmetric character of pulsations in roAp stars. Our study of chemical stratification in the atmosphere of the roAp star γ Equ provides a compelling evidence for significant variation of the chemical composition with depth. We find a combined effect of extreme chemical anomalies and a growth of pulsation amplitude in the outermost atmospheric layers to be the most likely origin of the high-amplitude pulsational variations of the lines of rare-earth elements. Observations of cool magnetic CP stars are obtained with the ESO Very Large Telescope and are used for empirical investigation of the anomalies in the atmospheric temperature structure. We show that the core-wing anomaly of the hydrogen Balmer lines observed in some cool CP stars can be attributed to a hot layer at an intermediate atmospheric depth