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Dust Around Main-Sequence and Supergiant Stars

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Dust Around Main-Sequence and Supergiant Stars Dust around Main-Sequence and Supergiant Stars A Thesis submitted for the Degree of Doctor of Philosophy of the University of London by Roger James Sylvester UCL Department of Physics & Astronomy University College London University of London 1995 ProQuest Number: 10044337 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 10044337 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 A bstract This thesis is a study of the properties of the dust around two rather different types of star. The first part is concerned with the mid-infrared emission from a sample of 16 M-type supergiants. As weU as silicate emission features, seven of the stars showed the UIR (unidentified infrared) emission bands, associated with carbonaceous material. According to standard theory, aU the carbon in the outflows from these oxygen-rich stars should be bound up in CO molecules, preventing the formation of carbonaceous dust. The results were interpreted in terms of a non-equilibrium chemical model, which invoked chromospheric UV photons to dissociate CO, allowing carbonaceous material to form, and to excite the observed UIR-band emission. The larger part of the thesis considers Vega-excess stars — main sequence stars with excess infrared emission from circumstellar dust discs. Photometric and spectroscopic observations were carried out. A number of the stars displayed excess near-IR emission, indicating the presence of hot material. Mid-infrared spectroscopy enabled the grain composition to be identified: both silicates and carbona­ ceous species were detected. Millimetre and submiUimetre photometry indicated that large grains are present around many of our sources, implying that significant grain coagulation has occurred. Most of the sources were modelled using a radiative transfer code, with disc geome­ try and multiple grain sizes. Two grain materials, astronomical silicate and amorphous carbon, were considered. Successful fits to the spectral energy distributions at mid-IR and longer wavelengths were found. The temperatures needed to produce near-IR excess emission were too high for grains in thermal equilibrium to survive. A model was therefore developed with very small grains undergoing thermal spiking due to single-photon absorp­ tion, which provided satisfactory fits for the hottest stars; the others had insufficient UV flux to excite the small grains. Acknowledgements First, I would like to thank my supervisor, Professor Mike Barlow, for all his help and guidance thoroughout the course of my PhD work, and for his useful comments on the actual text of this thesis. Mike is a co-author of Chapter 2, which appeared in a slightly different form in Monthly Notices. Chris Skinner also helped me a great deal, particularly with observing and the dust-disc modelling. I would also like to thank my family for their constant interest and encouragement throughout this period. I am grateful to SERC and PPARC for three years’ funding, to the Department of Physics and Astronomy for funding during (what I believed would be) my final term, and to the staff at UKIRT, JCMT and La Palma for their assistance, and for making Service observations. And finally, a big thanks to all the inhabitants at various times of the Corridor of Gloom, which is no more... Mike H., Robin, Bill, Bill, John, Kay, Raul, Alex, Sean, Annelie, Sarah, Vince, Ian, Ian, Xiao-Wei, Richard, Martin, Martin, and everyone I’ve forgotten. C ontents Title Page 1 A bstract 2 Acknowledgements 3 Table of Contents 4 List of Tables 8 List of Figures 10 1 Introduction 13 1.1 Overview ........................................................................................................................... 13 1.2 Stardust.............................................................................................................................. 14 1.3 Infrared and Submillimetre Astronomy .................................................................... 15 1.4 Circumstellar D ust......................................................................................................... 17 1.5 Vega-Excess Stars ......................................................................................................... 18 1.5.1 The Prototypes ................................................................................................... 18 1.5.2 (3 P ic to r is ..................................................................................................................20 1.5.3 Other sources ...........................................................................................................22 1.6 Evolutionary Status of Vega-excess System s ..............................................................24 1.6.1 Target Selection .................................................................................................... 27 2 Dust Emission from M-type Supergiants 29 2.1 Introduction........................................................................................................................... 29 2 . 2 Spectroscopy with CGS3 ....................................................................................................30 2.3 The Observed Spectra ................................................................. 33 2.4 Mass loss ra te s ......................................................................................................................42 2.5 Discussion................................................................................................................................ 44 2.5.1 Non-Equilibrium Dust Formation...................................................................... 46 2.5.2 Incidence of UIR-Band Emission in the Sam ple .............................................48 3 Observations of Vega-Excess Stars 54 3.1 Introduction.............................................................................................................................54 3.2 Optical Photom etry ............................................................................................................55 3.2.1 Results ...................................................................................................................... 56 3.3 Near-Infrared P h o to m e try ..................................................................................................60 3.3.1 UKT 9 ..........................................................................................................................60 3.3.2 IRCAM ......................................................................................................................62 3.3.3 Results ...................................................................................................................... 62 3.4 Mid-Infrared Spectroscopy with CG S 3 ........................................................................... 67 3.4.1 Results ...................................................................................................................... 6 8 3.4.2 Spectral Features..................................................................................................... 77 3.4.3 Notes on Individual Sources.................................................................................82 3.5 Near-Infrared Spectroscopy with CGS4 ............................................................................87 3.6 Millimetre-Wave Photometry using UK T 14 ................................................................. 8 8 3.6.1 Results ...................................................................................................................... 91 3.6.2 Spectral Indices .........................................................................................................97 3.7 Spectral Energy Distributions .........................................................................................102 3.8 Tabulated D ata ....................................................................................................................118 4 Radiative Transfer Modelling of Vega-Excess Systems 122 4.1 The Model ...........................................................................................................................122 4.2 The Central Stars ............................................................................................................. 122 4.3 The G rains ...........................................................................................................................124 4.3.1 Size Distribution................................................................................................... 124 4.3.2 Radiative Equilibrium and Dust Temperature...............................................126 4.3.3 Grain Optical Properties .................................................................................. 127 4.4 Spatial Distribution of the D ust..................................................................................... 131 4.5 Model 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