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Surface motions of the Beta Cephei stars.


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Stamford, Peter Alan 1980 , 'Surface motions of the Beta Cephei stars.', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Stamford, Peter Alan
Keywords: Cepheids, Pulsating stars
Copyright Holders: The Author
Copyright Information:

Copyright the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s). 1980.

Additional Information:

Thesis (PhD)--University of Tasmania, 1981. Bibliography: l. 187-193

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