Spectropolarimetric Analysis of Circumstellar Mass Flows

Wood, Kenneth (1993) Spectropolarimetric Analysis of Circumstellar Mass Flows. PhD thesis, University of Glasgow.

Abstract

Polarimetry is not a difficult subject. Unfortunately, as it is a second order effect (typical stellar polarisations are of the order of a few percent), in many undergraduate courses on stellar atmospheres and the interstellar medium it is either ignored entirely or mentioned as an aside if there is time at the end of the course. Through this lack of teaching no standard introductory texts on the subject have been adopted and many student's experience of polarimetry is the intense and rather obscure mathematical formulation of the Stokes parameters presented in Chandrasekhar's book on radiative transfer. As a result most theoreticians and observers embark upon research careers with the knowledge that there exist four Stokes parameters, but unaware of their physical significance or diagnostic potential. For these reasons I have devoted the first chapter of this thesis to giving a brief introduction to the subject of polarimetry and its astronomical applications. The parameters required to study polarised radiation - the Stokes parameters - are presented mathematically and their physical interpretations in terms of sums and differences of intensity components of the radiation field are outlined. Following on from a summary of sources of polarised radiation in astronomy the theoretical analysis predicting the "broad-band" polarisation from the optically thin Thomson scattering extended envelopes surrounding hot single stars is presented. Chapters 2, 3, 4 and 5 extend this theoretical framework to treat the problem of line polarisation through scattering of unpolarised stellar lines in moving atmospheres. Due to the complexity of polarised radiative transfer this problem has not been treated in a systematic fashion before and the few theoretical investigations to date were built around existing radiative transfer codes which did not allow the basic physical processes contributing to the line polarisation to be isolated. While the theory presented in these chapters is much simpler than a full blown radiative transfer approach it clearly demonstrates the role of scattering and Doppler redistribution, due to the scatterer's motion, in the production of spectropolarimetric line profiles. It is further shown that adopting this approach allows determination of the inclination, velocity and density structure of circumstellar discs from analysis of the scattered spectropolarimetric line profiles - information which cannot be determined from spectrometry alone. At the end of Chapter 5 an outline is given as to the future work required to develop a comprehensive theoretical understanding of stellar line polarisation, noting at each stage the effects on the resulting line profiles of the various ammendments to the theory. Temporal variations in the continuum polarisation of Be stars have been observed to occur over a wide range of timescales and have been attributed to variations in the shape or rate of stellar mass loss which changes the number of scattering electrons in the stellar envelope. With this interpretation of the polarimetric variations Chapter 6 illustrates a method for determining these episodic mass loss functions from analysis of polarimetric and absorption line strength data during the outbursts. The formulation of the equations in this chapter as an inverse problem was the initial problem suggested to me by my supervisor, John Brown, and the background reading I did on circumstellar polarisation while working on this problem led me to investigate the line polarisation variations presented in the previous chapters. The stellar wind speeds considered in the above chapters were such that any relativistic effects could be ignored. However, Chapter 7 considers the scattering of radiation off relativistic electrons yielding a very simple result for the degree of scattered polarisation involving the aberrational angle. This is part of an on-going investigation into polarisation from scattering in relativistic jets, which I am conducting in collaboration with John Simmons, of which the initial results are presented.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: John Brown
• Keywords: Astronomy
• Date of Award: 1993
• Depositing User: Enlighten Team
• Unique ID: glathesis:1993-76359
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 15:22
• Last Modified: 19 Nov 2019 15:22
• URI: https://theses.gla.ac.uk/id/eprint/76359