Radial and Non-Radial Pulsation Modelling in Early-Type Stars

In Chapter I we present a brief summary of the properties and evolutionary status of most known classes of pulsating variables, of the basic properties and classification of non-radial pulsations and of the related formalism. The observational evidence of the connection between stellar pulsation and mass loss is also reported and discussed. The model atmospheres that will be used in this study, in order to analyze the observational data, are described in Chapter II. The assumptions and the basic equations on which the classical atmosphere models are based are presented, their limits are discussed, and non-stationary models are introduced. We conclude that the classical models provide only a simplified representation of the physical processes occurring in a real stellar photosphere, but the classi"cal atmosphere predictions can be used for the pulsating star energy distribution and line-profile studies. In Chapter III we describe the algorithm developed in this work in order to compute line profiles of radial and non-radial pulsating stars. We discuss the advantages of the assumptions made when compared with previous works, and a comparison is made with published results. In order to compute line profiles broadened by rotation and pulsation we must calculate the intrinsic profile appropriate to the star of interest. The line profiles used are HeIdelta-44 71, -5876, -6678 and MgIdelta-4481. A study is made of the reliability of the adopted intrinsic profiles, computed by the available classical model atmosphere codes. In the same chapter the question is raised about the identification of the non-radial pulsation free parameters based on the proposed procedure. In addition, the k problem, that is the disagreement between the observed and the theoretical values of the ratio of horizontal to vertical oscillation velocities, is extensively discussed. The recent growth of interest in the short-term photometric and/ or spectroscopic variability of Be stars and its possible explanation in terms of non-radial pulsation is due to the possible link between non-radial pulsation and the Be phenomenon. In Chapter IV the short term variability in three Be stars, a Eri, P Car, and E Cap is studied. The three Be stars are in three different phases: a Eri shows a normal B spectrum, P Car shows a typical Be spectrum, E Cap is in a Be-shell phase. The non-radial pulsation models provide reasonably good fits of many of the observed spectra in two of the stars studied. The observations are also discussed in the light of the rotational modulation and the binary hypothesis. In Chapter V another aspect of stellar pulsation is treated: that is pulsation as seen from space observations in the ultraviolet wavelengths'. Only a few UV observations covering the whole pulsation cycle of a f3 Cephei star, exist so far. We have analyzed Voyager ultraviolet spectrometer (500-1700 A), low resolution, observations of the large amplitude f3 Cephei variable, v Eri (B2III). The observations, covering approximately 6 pulsation cycles of the star, allow us to derive the UV light as well as the intensity variations of high excitation lines and to infer on new aspects of the pulsation characteristics of v Eri.