Convenient and accurate expressions for the radiative flux deep inside an arbitrarily shaped, differentially moving, optically thick medium are derived. Spectral lines are included either ina deterministic way or stochastically by assuming a Poisson point process. It is shown that the lines lead to decreasing fluxes when in the medium the absolute value of the velocity gradient is increased. The flux vector needs no longer to be parallel to the temperature gradient in contrast to the radiative acceleration, which to a very good approximation is not influenced by lines. Formulae for the corresponding mean values of the opacity are presented and discussed.

Analytical and numerical solutions of the radiative transfer equation recently obtained in Heidelberg are reviewed. In particular, a multidimensional finite-element algorithm for unstructured grids that can be adaptively refined is discussed in detail.

In the mathematical computation of many physical problems we have to workwith the numerical solution of non-linear problems. Even in casethat a direct solution does in principle exist, its feasibility is howeveractually restricted to a very limited number of instances. Therefore, inthis study we will disregard this possibility. Consequently, we assume thatan iterative procedure shall be necessary in order to tackle, in practice,all kind of non-linear problems.

Element diffusion in the atmospheres of magnetic, chemically peculiar upper main sequence (CP) stars is expected to produce complex surface abundance distributions. In view of modelling the buildup of these abundance inhomogeneities, we have developed a new numerical tool based on accurate polarised radiative transfer: the object-oriented diffusion code CARAT. Its novel approach to thread-parallel high-performance computing is discussed in some detail. Our first results show that radiative accelerations experienced by chemical elements in a stellar atmosphere can be strongly affected by Zeeman splitting due to magnetic fields of moderate strength (a few Teslas). Amplifications of the accelerations - relative to the zero-field case - are found to be strongly dependent on field strength and direction, reaching a maximum near the inclination of 60° between field vector and the vertical. They are also very sensitive to the Zeeman patterns of the spectral lines and to magneto-optical effects which can by no means be neglected as has often been done in the past.

The aim is to determine the role and the influence of assumptions concerning both dynamics and radiative transfer in models of winds and mass loss of evolved stars, when the radiative force on dust grains plays a major role in the structuration of the circumstellar envelope of the star. The flow is described successively using three models coupling the grain and the gas dynamics in a self-consistent way with radiative transfer for three different approaches of the dynamics: the Position Coupling, the Momentum Coupling and the Full Problem. A complete radiative transfer including multiple scattering, absorption and thermal emission is taken into account to determine the temperature of dust grains which in turn governs their thermal emission. The medium is not necessarily optically thin. In all cases, numerical iterations couple dynamics with transfer. Thus two codes are used alternately, starting with an initial profile of radiation pressure, until convergence to a self-consistent solution. This emphasizes the importance of the drift velocity between the grains and the gas, and the inertia of dust together with hydrodynamics/transfer coupling. When the medium is optically thick, an opaque zone is located at the base of the wind. This zone governs the whole envelope structure. Finally, the exact number of solutions was determined for the one-fluid model.

The onset of chromospheric activity appears at late-A and early-F stars where theories predict atmospheres in radiative equilibriumand shallow or non-existent convective zones.The detection of Ly-α emission coresin several A and F stars, first with the IUE satellite and then withthe HST, gives evidence for the presence of chromospheric layersin these stars up to B-V=0.m19 (Catalano et al. 1986). Semiempirical chromospheric models for Altair allowed us(Freireet al. 1995) to explain the observed emission profilestaking into account normal HI IS absorption.However, due to the very high rotational velocity we analyzedalternative hypotheses like the formation of Ly-α emissionsinto a corotating expanding wind, but we ruled out this alternativebecause we obtained inconsistent results. In addition, X-ray emission(originated surely in a corona) strengthenthe presence of a chromosphere.Here we place the problem of chromospheric activity oflate-A and early-F stars in the general context of the formationof over-photospheric stellar layers, comparing them with late-type star and solar cases.

Optical long baseline interferometry (OLBI) is a powerful tool to study detailed stellar structures. Present optical interferometers attain the milliarcsecond angular resolution allowing this study. In the present contribution I present a physically coherent interferometry oriented model which includes a radiation transfer code. An application of this model to rapid rotators is presented where the effects of gravity darkening and geometrical deformation are included. The high angular resolution signatures due to rapid rotation are shown for several interferometric observables.

Using the SIMECA code developped by Stee & Araùjo (1994), we report theoretical HI visible and near-IR line profiles, i.e. Hα (6562 Å), Hβ (4861 Å) and Brγ (21 656 Å), and intensity maps for a large set of parameters representative of early to late Be spectral types. We have computed the size of the emitting region in the Br γ line and its nearby continuum which both originate from a very extended region, i.e. at least 40 stellar radii which is twice the size of the Hα emitting region. We predict the relative fluxes from the central star, the envelope contribution in the given lines and in the continuum for a wide range of parameters characterizing the disk models. Finally, we have also studied the effect of changing the spectral type on our results and we obtain a clear correlation between the luminosity in Hα and in the infrared.

The SAC method is a semi-empirical method for analysing the emission lines of ions, having a complex spectrum, like Fe+. The method and the assumptions it makes, are discussed. Radiative transfer calculations are needed to check their ranges of validity.