The use of the IR and radio continuum as a clumping indicator for the stellar wind of early-type stars is investigated.

The analysis of the UV resonance transitions of superionized species observed in early-type stars suggests the presence of a high temperature region and an outward flow of matter (Snow & Marlborough 1976).

Turbulence is known to be important in the stellar wind of early-type stars. We explore the influence of turbulence that depends on the distance to the star.

Narrow Absorption Components (NACs) have been detected in essentially all unsaturated P Cygni profiles of O-type stars (Howarth & Prinja, 1989). Time-series of observations have revealed their migration through the profile's absorption trough (Prinja et al., 1987, Henrichs et al., 1988). The interpretation of the NACs is essential for the understanding of the dynamic structure of the winds of O stars and of the driving mechanisms behind it. The P Cygni profile substructures caused by isolated reverse and forward shocks were calculated in a ‘line only’ approximation for a spherically symmetric, isothermal stellar wind. The Comoving Frame method was used for an accurate treatment of the complex radiation transfer in the shock environment. The underlying velocity structure follows a β-law (except in the innermost layers where a polynomial law was used) while the ionization was taken to be constant throughout the entire wind.

Our study, of which we give here a progress report, addresses two problems. The first is to develop methods and software permitting to compare a wide range of theoretical models of stars with envelopes with observational data produced or expected to be produced by high angular resolution optical interferometry combined with spectroscopy. The second problem is to find out the modes of interferometric observations (base length, spectral resolution etc) that are most informative for determining the physical parameters of stellar envelopes.

As has been recently established, the atmospheres of early-type stars are strongly disheveled. These atmospheres consist of dense clumps and a more rarefied interclump medium. We consider the forces that determine the motion of the clumps. We show that the main force is gravitational attraction of the clumps to the star. For stars with a high effective temperature (Teff ≥ 6 · 104 K) it appears that the clump/interclump gas number density ratio is ≥ 30. In this case we can conclude that the clumps are formed by shock waves. Some evidence as to the existence of gas jets in the atmospheres of Wolf-Rayet stars have recently been presented. We propose that such jets may be a common feature of all early-type stars. Profiles of HeI lines in spectra of clumped atmospheres with jets have been calculated.

Spectroscopic observations are discussed which indicate that the winds of luminous OB stars are highly structured in space and variable in time. Blueward migrating discrete optical depth enhancements in the absorption troughs of P Cygni profiles are the characteristic signatures of the evolving structure. Constraints on physical mechanisms are discussed, provided by the observed accelerations of the migrating features, the evidence for rotationally modulated variability, and the behaviour of the winds at very low velocities.

The search for the cause of the discrete absorption components (DACs) in P Cygni profiles in UV spectra of O stars continues: pulsation, rotation and magnetic fields are probably all in various degrees important. We highlight some key observations which show that DACs start close to the stellar surface, and emphasize the possible role of localized magnetic fields (which are still to be detected).

From 1986 through 1992, wide-band optical (B or V filter) linear polarization measurements of eight Be stars and seven O stars were obtained simultaneously with ultraviolet observations from IUE and worldwide ground-based optical spectroscopy and photometry in a series of campaigns designed to study the short-term variability of these objects. Each campaign consisted of intensive monitoring of a few carefully chosen stars over a period of several days and nights, with the greatest possible continuity subject to the limitations of instrument scheduling, weather, and the longitudes of the observing sites.

Galactic Luminous Blue Variables and A- and B-type supergiants were monitored spectroscopically with high resolution in wavelength and time. Line profile variations on different timescales are found.

The resonance spectral lines of high-resolution IUE spectra of χ2 Ori (B2I) have been studied with the aim to determine in more detail the physical status of the stellar wind and its variability due to shell outbursts and clumps superimposed on the isotropic stellar wind. Some general features of resonance spectral line profiles have been studied and explained in physical terms using χ2 Ori as a specimen.

A-type supergiants are just at the boundary between the early type supergiants, with strong stellar winds driven by radiation pressure and the cool supergiants in which the origin of mass loss is the dissipation of mechanical energy. The mechanisms involved in the mass loss processes in A supergiants as a whole, are still uncertain.