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The Origin of Ejecta Nebulae Surrounding Massive Stars

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The Origin of Ejecta Nebulae Surrounding Massive Stars The Origin of Ejecta Nebulae Surrounding Massive Stars Matthew Paul Stroud Thesis submitted for the Degree of Doctor of Philosophy, in the Faculty of Science of the University of London. UCL c ] UNIVERSITY COLLEGE LONDON December 1997 ProQuest Number: 10106728 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 10106728 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 Dedication To family and friends who made it worthwhile. Acknowledgements I would like to thank Linda Smith for guidance during my PhD and for her help in preparing this thesis, which was very much appreciated, I would also like to thank Paul Crowther whose depth of knowledge of WR stars made for many useful and enlightening conversations. During this PhD I have been financially supported by PPARC, Most of all I would like to acknowledge M att, Kaj, Serena, Orsola, Chris, Richard, Barbara and Phil, who amongst others have made the past three years enjoyable. Abstract. Luminous Blue Variables (LBVs) are a rare class of stars which represent a phase in the transition of Of stars into Wolf-Rayet stars. A ubiquitous feature of LBVs is the presence of a circumstellar nebula. These nebulae bare witness to a phase of enhanced mass loss, the gross characteristics and causes of which are basically unknown. In this thesis we analyse the chemical and dynamical properties of these nebulae to infer the physical conditions present on the surface of the star at the epoch of ejection. We have undertaken abundance studies of three LBV nebulae (AG Car, HR Car and He3-519) and found them to consist of partially CNO processed material. Since the application of atmospheric models to LBV stars has shown them to have a CNO equilibrium abundance pattern, this suggests that the epoch of nebular formation occurred before the present LBV phase. From the low expansion velocities of LBV nebulae we infer that the nebulae were ejected from stars with a low surface gravity. We note that this could arise either when a star approaches its limit or during a RSC phase. Since mass loss near the ÇI limit is inherently asymmetric and LBV nebulae are in general bipolar, we favour the former possibility. In a dynamical study of the nebula surrounding AC Car we find that it has a prolate geometry, with a disk-like outflow external to the nebula. Since these are hallmarks of mass loss from stars nearing the ÇI limit, this reinforces our belief that the mass loss occurred toward the blue. In addition we have performed abundance studies on the nebula RCW 58 which surrounds the WR star WR 40 and NCC 6164/5 surrounding the Of star HD 148937. We find that both these nebulae have very similar abundance patterns to those nebulae surrounding LBV stars. This suggests that the surface abundance pattern plays an important role in the operation of the ejection mechanism. In this context we note the significance of the surface abundance pattern for the opacity of the outer layers which govern the star’s interaction with its modified Eddington limit. Contents Acknowledgements 3 A bstract 4 C ontents 5 List of Tables 9 List of Figures 11 1 Introduction. 15 1.1 Aims................................................................................................................... 15 1.2 The Play - evolutionary scenarios ............................................................... 18 1.3 The Actors - Phases of stellar evolution .................................................... 20 1.3.1 O stars ................................................................................................ 20 1.3.2 Luminous Blue Variables .................................................................... 22 1.3.3 Ofpe/WN s ta r s ................................................................................... 26 1.3.4 WNL stars............................................................................................. 27 1.3.5 Red Supergiants ................................................................................... 29 1.4 The Stage - Mass Loss and the HRD ......................................................... 30 1.4.1 The H-D limit ....................................................................................... 30 1.4.2 The Modified Eddington Limit ......................................................... 32 1.4.3 The Stothers and Chin instability .................................................... 39 1.4.4 The Effects of Shocks .......................................................................... 44 1.4.5 Strange Mode Pulsations ................................................................... 44 6 CONTENTS 1.5 Nebulae Surrounding Massive Stars ............................................................. 47 1.5.1 rj C a rin a e ........................................................................................... 51 2 An abundance study of AG Car’s nebula. 55 2.1 Introduction ...................................................................................................... 55 2.2 Observations and Data Reduction ................................................................ 59 2.3 Analysis ............................................................................................................... 64 2.3.1 Description of the Spectra ................................................................ 64 2.3.2 The Calculation of the Physical Parameters and Abundances. 68 2.4 Results................................................................................................................. 71 2.5 Discussion ........................................................................................................... 77 2.5.1 Introduction ......................................................................................... 77 2.5.2 Dilution of the Nebular Abundance P attern? .......................... 79 2.5.3 The Physical Conditions at the Stellar surface During the Epoch of Nebular Formation ........................................................... 80 2.5.4 The Effect of Supra-Eddington Limit Mass Loss .......................... 83 2.5.5 Implication for the Evolutionary Models ...................................... 86 2.5.6 Conclusions .......................................................................................... 88 3 The nebula surrounding HR Carinae. 91 3.1 Introduction ...................................................................................................... 91 3.2 Observations .................................................................................................. 99 3.3 Analysis ............................................................................................................ 101 3.4 Results............................................................................................................... 110 3.5 Discussion ......................................................................................................... 113 4 The Common Origin of Ejecta Nebulae. 119 4.1 Introduction ..................................................................................................... 119 4.1.1 He3-519................................................................................................. 120 4.1.2 RCW 58 ................................................................................................ 124 4.2 Observations of He3-519 and RCW 5 8 ..................................................... 127 4.3 Results................................................................................................................ 129 CONTENTS 7 4.4 Discussion ....................................................................................................... 138 4.4.1 LBV nebulae as the precursors of WR ring nebulae ......................... 139 4.4.2 The ejecta origin of the nebulae ...................................................... 139 4.4.3 The point of nebula ejection from LBVs below the HD limit. 141 4.4.4 Constraints on the Outburst Mechanisium .........................................144 4.5 Summary ........................................................................................................ 145 5 The Evolutionary History of HD 148937. 147 5.1 Introduction .................................................................................................... 147 5.2 Observations ................................................................................................. 153 5.3 R esults.............................................................................................................. 154 5.4 D iscussion ........................................................................................................ 157 5.4.1 The Location of Nebula Ejection in the HRD ............................. 158 5.4.2 The Mechanisim of Nebula Ejection? ........................................... 161 5.4.3 Is HD 148937 U n iq u e? ..................................................................... 164 5.5 Conclusion ........................................................................................................
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