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Spectropolarimetry As a Probe of Stellar Winds

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Spectropolarimetry as a Probe of Stellar Winds Timothy James Harries Thesis submitted for the degree of Doctor of Philosophy in the Faculty of Science of the University of London. UCL Department of Physics & Astronomy U n iv e r s it y • C o l l e g e • L o n d o n February 1995 ProQuest Number: 10045605 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10045605 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract The use of spectropolarimetry as a diagnostic probe of stellar-wind structure is inves­ tigated by using high-quality observations and state-of-the-art analytical and numerical models. The winds of late-type giant stars are studied through the highly polarized 6830Â and 7088Â Raman-scattered emission hnes that are observed in many symbiotic systems. A spectropolarimetric survey of 28 symbiotic stars is presented. A Monte-Carlo code is developed in order to aid interpretation of the hnes and the parameter sensitivity of the Raman hne polarization spectrum is investigated. It is demonstrated that the observed Une polarization morphologies can be reproduced by using reahstic physical parameters and that the hne polarization structure is a powerful diagnostic of the cool stehar wind. The binary phase dependence of the hne structure is studied with the aim of using multi-epoch observations to derive orbital parameters. The polarization spectrum of the symbiotic star BI Crucis is analyzed and discussed in relation to the geometry of its extended bipolar nebula. The first detection of emission-hne (Ho) polarization structure in an 0 supergiant (( Puppis) is presented. Model polarization spectra are computed using statistical equi- hbrium calculations and Monte-Carlo radiative transfer. It is demonstrated that the lat­ itudinal wind density structure predicted by radiation-driven wind theory is incapable of producing the observed polarization signature. Multi-epoch observations of the pathological WN star EZ Canis Majoris are presented. These observations enable the accurate determination of the intersteUar polarization (ISP) vector. The rehabihty of techniques used to estimate the ISP is assessed using tests performed on numerical models. The observed variabihty of the continuum polarization is explained in terms of scattering off density inhomogeneities propagating through the stellar wind. A spectropolarimetric survey of 15 galactic Wolf-Ray et stars is presented. Emission- hne polarization structure is observed in four of the survey stars. These data are combined with results of similar surveys in order to determine the frequency of line polarization structure in galactic WR stars. Detailed analyses are performed on the the polarization spectra of the dust-producing WC star WR 137 and the WN6 star WR 134. Acknowledgments I owe a great debt of gratitude to my supervisor, Ian Howarth, whose boundless en­ thusiasm, guidance and advice (not to mention endless patience) made working on this project a pleasure. Thanks too, Ian, for playing your Eagles CD so regularly while we were observing - “Hotel California” does grow on you, doesn’t it? The A25 Society (Andy, Barry, Keith and Mike G.), that hot-bed of astronomical re­ search and gossip, is warmly thanked for many hours of highly entertaining and highly libellous conversation. Members are also thanked for their very considerable help on mat­ ters scientific (in particular, Keith for p g p ix and the 0-supergiant model atmospheres). Many thanks also go to my other friends at UCL who were fortunate enough not to share an office with me: Wonky, JuHe, Joan, Dave, Mike H., Matt, Raman and Fish. I am indebted to Paul Crowther for supplying his EZ CMa and WR 134 models and for answering my enumerable e-mails on WR line IDs. My thanks also go to John HiUier for allowing me to use his Monte-Carlo code and answering aU my dumb questions about it. I also owe a great deal to Regina Schulte-Ladbeck, particularly for the observations of EZ CMa but also for hUing-in some of the gaps in my knowledge of polarimetry. Thanks, too, to my family, that has so recently become rather larger; Mum, Dad, Andrew and Simon, Malcolm and Sue. I owe them a lot, not least because of their encouragement despite not knowing, to paraphrase Andrew, what the hell it is that I do. Finally I would hke to thank Lorna, who has not only put with me over the last few years (despite having a thesis of her own to write) but also actually married me. What a fool. I gratefully acknowledge a postgraduate studentship from SERC. This work was based on observations obtained at the William Herschel Telescope, La Palma and the Anglo-Australian Telescope, NSW. The computational work was performed on the UCL node of the Starlink project. To Lorna, Mum and Dad. C ontents A b stra ct 2 Acknowledgements 3 Dedication 4 Table of Contents 5 List of Tables 9 List of Figures 11 1 Introduction 16 1.1 A brief history of astronomical polarimetry ......................................................... 16 1.2 T erm inology ...................................................................................................................... 18 1.3 Electron s c a tte r in g ........................................................................................................ 19 1.4 Interstellar polarization ..................................................................................................20 1.5 Intrinsic p o la riz a tio n .........................................................................................................21 1.6 0 s ta r s .................................................................................................................................... 23 1.6.1 Mass-loss via stellar winds .................................................................................. 24 1.7 Be and B[e] s t a r s ................................................................................................................26 1.8 Wolf-Rayet stars and Luminous Blue V ariables ...................................................... 27 1.9 Mass-loss from low gravity late-type sta rs ..................................................................30 1.10 Symbiotic stars ................................................................................................................... 32 1.11 Observational techniques and data reduction ..........................................................33 1.11.1 The polarimetry m odule ......................................................................................33 1.11.2 Observational techniques .................................................................................. 34 1.12 D ata r e d u c tio n ................................................................................................................... 35 1.12.1 Statistical erro rs ................................................................................................... 37 1.12.2 Calibration ..............................................................................................................38 1.13 This study ............................................................................................................................. 39 2 Spectropolarimetric Observations of Symbiotic Stars 41 2.1 Introduction ............................................................................................................................ 41 2.1.1 Raman scattering ................................................................................................... 42 2.1.2 Raman scattering in symbiotic sta rs ................................................................44 2.2 Observations ......................................................................................................................... 45 2.3 Observational techniques .....................................................................................................45 2.4 Data reduction ......................................................................................................................47 2.5 Data analysis .........................................................................................................................47 2.6 Equivalent-width measurements .......................................................................................56 2.7 Polarization measurements ................................................................................................. 59 2.8 Line polarization morphology .......................................................................................... 63 2.9 Temporal variations ........................................................................................................... 67 2.10 The effect of the diluting continuum .............................................................................67 2.11 Symbiotic Mira BI C ru .................................................................................................... 71 2.11.1 Introduction .....................................................................................................
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