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A Study of Chemistry, Dynamics and Dust in Nova Outflows

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A Study of Chemistry, Dynamics and Dust in Nova Outflows A Study of Chemistry, Dynamics and Dust in Nova Outflows Matthew Pontefract Thesis submitted for the degree of Doctor of Philosophy of the University of London. UCL Department of Physics & Astronomy U n i v e r s i t y C o l l e g e L o n d o n London WClE 6BT September 1999 ProQuest Number: U642247 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 U642247 Published by ProQuest LLC(2015). 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 A bstract Since the celebrated outburst of nova DQ Herculis (1936), observations of which suggested th at dust grains were being formed with high efficiency, research into the chemical pro­ cesses leading to dust nucléation has flourished. Observations show that dust formation is not only highly efficient but a common occurrence in nova outbursts, yet the chemical pathways leading to dust are poorly understood. A study of chemical processes within outflowing ejectum material through closely linked, quasi self-consistent chemical models from just a few days post-outburst until the formation of dust nucléation sites is presented. Chemical modelling, incorporating new and updated chemistry within a realistic radiation field, is followed by a study of dynamics with a new fluid dynamic model incorporating chemistry explicitly within the conservation equations. In contrast to previous studies, we find that a rich chemistry of small molecules develops within a few days after outburst. This leads to the formation of a large abundance of nucléation sites from many model configurations. We find also that CO does not saturate within these chemical scenarios; this negates previous assumptions and helps to explain observations not only in novae, but also in objects such as planetary nebulae. Constraints are placed on physical conditions within the ejecta. Two outflow configurations are modelled with a simplified chemistry embedded in a fluid dynamic code. We suggest an explanation for the time-dependent behaviour of CO molecular lines observed in nova V705 Cas, and further constrain the physical parameters for this nova. It is found that, as a result of the dynamics, a rich chemistry may occur only within localised regions of the principal eject um and that the results are consistent with observation. It is suggested that further development of this model, particularly with respect to the radiation field, will lead to a deeper understanding of both physical and chemical processes within nova outflows. Acknowledgements I would like to thank Jonathan Rawlings above all for providing the inspiration for this work and maintaining his cool in the face of one who would insist on writing code in the devil’s own tongue: C. Having rather directly ascertained whether he drank or not, Jonathan took responsibility for a student who trembled at the very thought of ‘doing chemistry’. Four years later the fear is gone and I am left in no doubt as to the correct answer to that first, testing question! Chapter 5 would not have been possible without the superb code of Andrew Lim. His patience when answering my many questions, and assistance with the elimination of gremlins, was invaluable at a time when to start a novel project was to take a considerable risk. I hope he is pleased with the product of his baby. From the time I started this work four years ago, my friend, confidente and one-time flat-mate, Serena Viti, has always been close to help me find the door when things go black and swooney. A more understanding, generous and humorous friend one could not hope to find, and to her I offer tanti bacil To Tony Lynas-Gray I also extend my heart-felt gratitude for his regular encourage­ ment, liquid refreshment and good humour. His critical assessment of this manuscript and moral support have, without doubt, made this a better work. As for the myriad buddies, friends and family who have played such an important rôle in my life of the past few years, I thank you for all the fun and, of late, the floor space. In particular I feel that my appreciation of both fine gin and tonic and deep penetration would not be what it is today without your encouragement. My appreciation of new and talented female vocalists clearly owes nothing to any of you, however! The assistance of the Particle Physics and Astronomy Research Council (PPARC) by way of financial support is gratefully acknowledged. To Grandpa Contents A bstract 3 Acknowledgements 5 Table of Contents 9 List of Figures 13 List of Tables 17 1 Novae: Physics, Chemistry and the Making of Dust 19 1.1 Introduction .................................................................................................................. 19 1.2 The Physics of a Nova Eruption ...................................................................................21 1.3 The Spectral Evolution of a Nova ............................................................................... 24 1.3.1 The Pre-Nova S t a g e ..........................................................................................25 1.3.2 The Rise to Visual M axim um ..........................................................................26 1.3.3 Early D eclin e .......................................................................................................28 1.3.4 Final D e c lin e .......................................................................................................29 1.4 The Ejecta: Properties, Structure and Evolution .................................................. 29 1.4.1 Physical P roperties ............................................................................................. 29 1.4.2 Ionisation S tr u c tu r e .......................................................................................... 31 1.5 C h em istry ..........................................................................................................................33 1.5.1 Pre-Dust F o rm a tio n ..........................................................................................40 1.5.2 Dust and G e o m e try ..........................................................................................40 1.6 Recent O bservations ...................................................................................................... 43 1.6.1 Molecules in N ovae ............................................................................................. 43 1.7 A Brief Catalogue of Recent N o v ae ............................................................................ 47 10 CONTENTS 2 Modelling Techniques 51 2.1 Introduction ................................................................................................................... 51 2.1.1 Previous W o r k ............................................................................................... 52 2.2 Basic Reaction D a ta .......................................................................................................52 2.3 Photorate Coefficients: Realistic Radiation Fields ............................................... 54 2.3.1 Photoreactions where no cross-section data are available .........................60 2.4 Representing Chemistry in C o d e ............................................................................ 61 2.4.1 A Schematic Model ..................................................................................... 63 2.4.2 Jo: Generating D i f f u n and J a c o b i a n ................................................................ 65 2.5 The C h e m is try ............................................................................................................ 68 2.6 The Model .......................................................................................................................70 2.7 Limitations .......................................................................................................................73 3 Early Chemical Evolution 75 3.1 Introduction ................................................................................................................... 75 3.2 C h em istry ...................................................................................................................... 76 3.2.1 Additions and modifications to Rate95 ..................................................... 78 3.2.2 Excited Hydrogen: Hn = 2 ............................................................................... 86 3.3 A Complete M o d e l ...................................................................................................... 89 3.4 Model Results ............................................................................................................... 91 3.4.1 An O v erv iew ................................................................................................... 91 3.4.2 Reaction Networks in Practice .................................................................. 99 3.5 Concluding Remarks .....................................................................................................109 4 Late Chemical Evolution 113 4.1 Introduction
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