Theoretical models for expanding atmospheres of hot stars

The spectrum of early O and B stars displays a great variety of line profiles, either in absorption or in emission. Due to these variety of profiles it is quite difficult to infer the physical conditions of their atmospheres. Making use of the radiative transfer theory for a NLTE expanding plasma, we study the conditions that give rise to absorption or emission lines. This research is based on the He II λ4686 Å line, since this line is one of the most representative transition in the photographic region of the spectrum of hot stars. Furthermore, we analyse the line behaviour along the spectral sequence.