Radiation driven diffusion in magnetic stellar atmospheres

The atmospheres of many upper main sequence stars with peculiar chemical abundances (CP stars) are permeated by surface magnetic fields ranging from a few mT up to some 4T and characterised by large, sometimes variable, over-abundances of some elements, in particular the rare earths, but also by moderate depletion of elements like carbon, nitrogen and oxygen. Such behaviour is generally ascribed to an inhomogeneous distribution of the chemical elements over the stellar surface, caused by radiative diffusion. In a sufficiently stable environment (no convection), photo-absorption in lines and continua will push up certain elements against gravity, leading to abundance gradients with depth, depletion of chemical elements occurring in some layers, over-abundances in others. Alecian & Stift (2002), using the newly developed object-oriented, parallel code CARAT, have recently discovered that Zeeman splitting of spectral lines leads to a substantial increase in radiative accelerations over the zero-field value, depending strongly on magnetic field strength and direction. This project proposes a detailed investigation of radiative levitation controlled by large-scale dipole-quadrupole magnetic fields in view of modelling the local build-up of vertical stratification of chemical elements and the ensuing emergence of surface inhomogeneities. For this purpose, the input physics have to include photo-ionisation cross sections, the redistribution of momentum between the various stages of ionisation, and the influence of magnetic field lines on the diffusion. Since chemical abundances vary both horizontally and vertically with time, it will be necessary to develop a new object-oriented, parallel code for stratified magnetic stellar atmospheres, CAMAS, which will take into account the effects of magnetic forces, the increase in line-blocking due to Zeeman splitting, and provide accurate treatment of polarised radiative transfer. The ultimate goal is the self-consistent modelling of time-dependent diffusion in magnetic stars. Concurrently, spectropolarimetric observations of selected CP stars and detailed modelling of the polarised line profiles with the spectral synthesis code COSSAM will provide important clues as to the occurrence and nature of abundance stratification.