The Occultation of Polarised Light from Stellar Envelopes

Fox, Geoffrey Kevin (1990) The Occultation of Polarised Light from Stellar Envelopes. PhD thesis, University of Glasgow.

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Following the discovery of the spectral variability in a number of emission line B type stars (so called Be stars) at around the turn of this century, extensive theoretical and observational campaigns have since been an ongoing process in an attempt to understand the controlling physics of the star and its environment. Although it is now known that variability occurs on all time scales the mechanisms responsible for the variability are still not well understood. The consensus is that the stars are rapidly rotating (approximately up to 80% of the critical rotational velocity) which appears to have a major influence upon the characteristics of the star and its environment as is inferred by the observed spectral variability. The observed high intrinsic polarisation of Be stars also implies that the rapidly rotating star rotationally distorts the surrounding circumstellar envelope. This suggests that by using current geometrical model envelopes together with polarisation theory, constraints upon the distribution of scattering material, the geometry and mass of the scattering envelope could be inferred from polarimetric observations. In this thesis the theory of optically thin, Thomson (or Rayleigh) scattering polarisation from stellar envelopes for both single and binary star systems (Brown and McLean, 1977; Brown et al., 1978) are considered and extended to include finite light sources in order to provide more stringent constraints upon Be star envelopes and also to enable inferences to be made as to the density structure of regular polarimetric variations in single and binary star systems. The spectral variability characterising the Be star phenonena is reviewed in chapter 1 with particular reference to gamma Cas. Current spectroscopic geometric models are also discussed in some detail. Following this, the polarimetric theory and observations related to Be stars are discussed which also includes a brief section on binary diagnostics as Be stars are frequently observed in binary systems and a qualitative account of the observational consequences of scatterer occultation in binary systems is also included. Continuing with polarimetric variability in binary systems, in chapter 2 the polarimetric variability of the Be/X-ray transient A0538-66 is investigated with a view to understanding the mass transfer from the primary (Be) star disc envelope to the secondary (neutron) star. This chapter is in a slightly different vein to the remainder of the thesis in that the scattering material is assumed not to be occulted (in fact due to the scanty data set this question cannot even be asked). It is included here partially because Be stars in binary systems seem to exhibit spectral behaviour similar to that of single stars and hence this may throw some light upon possible underlying mechanisms and also as an example of how the density structure within a binary system can be inferred from the polarimetric data which is one of the questions addressed in this thesis. In chapters 3-5 the effects of incorporating a finite size (spherical) light source into the optically thin, single Thomson scattering polarimetric theory is developed for various geometrical models with particular reference to understanding what constraints are needed to be imposed upon Be star models in order to produce the necessary degree of observed intrinsic polarisation. Also addressed is the question to what extent the density structure can be inferred from the polarimetric variability of a system when there is only one important light source. This may be considered as a density perturbation in a stellar envelope around a single star or a binary system in which the secondary is important as a light source (eg. a neutron star). In chapter 6 the effects of occultation in obliquely rotating envelopes are discussed and the error in inferring the inclination and obliquity angle by Fourier analysing the polarimetric data of such systems is assessed when no account of occultation is made but is present within the data. Finally in chapter 7 a brief summary of the conclusions of this thesis is made and suggestions are put forward for future work with particular reference to the application of polarisation theory, as an independent method, to understanding the underlying structure of UV discrete absorption line components is discussed .

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Astronomy
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78090
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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