Surface motions of the Beta Cephei stars.
Creators: name: Stamford, Peter Alan
Publisher: name: University Of Tasmania
Year: 2023

description: The nature of the 8 Cephei surface motions, whether radial or nonradial, has been the subject of considerable controversy. In recent years a number of techniques have been developed for the identification of the pulsation modes operative in variable stars. In this thesis, these mode identification techniques are explored theoretically and applied to some members of the 8 Cephei group. The first technique uses the relative amplitudes and phases of the light, colour and radial velocity variations for mode discrimination. It is demonstrated that, for the B Cephei stars, the colour to visual light amplitude ratio is mode discriminating providing the colour index is derived over a sufficiently long wavelength baseline. Of ten 8 Cephei stars investigated by this method, eight are found to be radial pulsators, while two are probably quadrupole pulsators. However, for the o Scuti and cepheid variables, it is found that the phase difference between light and colour variations is mode discriminating, whereas the phase difference between light and velocity, and the amplitude ratios, are of no value. These techniques are related to the Baade-Wesselink method as applied to nonradial pulsators. It is shown that the underlying assumption of the Balona and Stobie derivation relating surface brightness to colour is invalid in the general nonradial case. A corrected Baade-Wesselink formula is obtained and the mode identification capabilities of the resulting Baade-Wesselink coefficients are investigated. A second mode identification method would involve the monitoring of 8 Cephei stars for variations in linear polarization. Such variations should result from the anisotropic scattering geometry associated with nonradial pulsations. Calculations of the expected polarization variations for nonradial modes up to i = 3 are obtained using accurate B-star model atmospheres. It is demonstrated that although variations in the polarization at visual wavelengths are not likely to be detectable for the B Cephei stars, this technique should be successfully applicable to ultraviolet observations obtained with proposed satellite-borne polarimeters. The polarization variations expected for some of the pulsation models which have been proposed for individual B Cephei stars are then discussed. The third mode diagnostic considered involves the eomparison of observed line profiles with those synthesized theoretically for particular nonradial modes. The line profiles originating from modes with l ~ 3 are discussed and an attempt is made to model the line profile variations of 12 Lacertae using two interfering nonradial modes. From the application of these mode identification methods to a number of B Cephei stars it is inferred that the singly-periodic stars are radial pulsators. Since BW Vulpeculae is in this category, the radial shock interpretation of its spectral variations demands important consideration. To investigate this hypothesis a hydrodynamic envelope pulsation model was constructed. This model indicates that nonlinear wave propagation of underlying sinusoidal motions into the outer envelope of a large amplitude radial pulsator may indeed generate strong atmospheric shock waves. Observed line doubling and light curve bump phenomena can be understood in terms of this process.