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White Dwarf Atmospheres and Circumstellar Environments

Donald W. Hoard (Editor)
ISBN: 978-3-527-41031-6
244 pages
October 2011
White Dwarf Atmospheres and Circumstellar Environments (3527410317) cover image
Written by selected astronomers at the forefront of their fields, this timely and novel book compiles the latest results from research on white dwarf stars, complementing existing literature by focusing on fascinating new developments in our understanding of the atmospheric and circumstellar environments of these stellar remnants. Complete with a thorough refresher on the observational characteristics and physical basis for white dwarf classification, this is a must-have resource for researchers interested in the late stages of stellar evolution, circumstellar dust and nebulae, and the future of our own Solar System.
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Preface XI

List of Contributors XV

1 Hot White Dwarfs 1
Edward M. Sion

1.1 Introduction 1

1.2 Remarks on the Spectroscopic Classification of Hot White Dwarfs 2

1.3 The Hot DA Stars 6

1.3.1 DAO White Dwarfs 9

1.4 The PG1159 Stars 11

1.5 DO White Dwarfs 14

1.6 DB White Dwarfs 17

1.6.1 DBA White Dwarfs 18

1.7 Hot DQ White Dwarfs 19

1.8 Conclusion 20

References 21

2 Cool White Dwarfs 25
Mukremin Kilic

2.1 White Dwarf Cosmochronology 25

2.2 Cool White Dwarf Atmospheres 29

2.2.1 Collision Induced Absorption 29

2.2.2 The Missing Opacity Source in the Blue: Lyman-α Absorption 31

2.2.3 Model Atmospheres Versus Observations 32

2.3 Identification of Large Samples of Cool White Dwarfs 34

2.3.1 Photometric Selection 34

2.3.2 Proper Motion Selection 35

2.4 Observational Properties of Cool White Dwarfs 37

2.4.1 Color–Color Diagrams 37

2.4.2 Pure Hydrogen and Pure Helium Atmosphere White Dwarfs 38

2.4.3 Mixed H/He Atmosphere White Dwarfs 40

2.4.4 Ultracool (or Infrared-Faint)White Dwarfs 41

2.5 Spectral Evolution of Cool White Dwarfs 42

2.6 Ages for IndividualWhite Dwarfs 44

2.7 The White Dwarf Luminosity Function 46

2.8 Halo White Dwarfs 48

2.9 Conclusions and Future Prospects 50

References 51

3 Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarfs 53
Patrick Dufour

3.1 Introduction 53

3.2 DQ White Dwarfs 54

3.2.1 Historical Introduction and General Properties 54

3.2.2 Formation Mechanism 59

3.2.3 Relation Between Carbon Abundance and Temperature: An Overview 61

3.2.4 DQ White Dwarfs with Oxygen 64

3.2.5 The Peculiar (“C2H”?) DQ White Dwarfs 65

3.3 Carbon and Oxygen in DBQ White Dwarfs 70

3.4 Hot DQ White Dwarfs 72

3.4.1 Historical Introduction and General Properties 72

3.4.2 Magnetism 76

3.4.3 Oxygen 78

3.4.4 Pulsations 78

3.4.5 Formation and Origin 81

3.4.6 Concluding Remarks on the Hot DQ Stars 83

3.5 Conclusion 84

References 87

4 Planets Orbiting White Dwarfs 89
Rosanne Di Stefano

4.1 Introduction 89

4.2 Expectations 92

4.3 Detecting Radiation from the Planets 94

4.4 Evidence for Minor Planets 97

4.4.1 Survivability 97

4.4.2 Metal Enrichment and Disks 100

4.4.3 Transits of Asteroids 102

4.5 Timing 102

4.6 Mesolensing 105

4.6.1 Gravitational Lensing Basics 105

4.6.2 Nearby Lenses 107

4.6.3 Planet Detection via Lensing 108

4.6.4 IdentifyingWhite Dwarfs in the Lens System 109

4.7 Transits 111

4.7.1 Basics 111

4.7.2 SuperWASP 111

4.7.3 Wide-Field Monitoring 112

4.8 Prospects for the Future 112

References 113

5 White Dwarf Circumstellar Disks: Observations 117
Jay Farihi

5.1 Introduction 117

5.2 History and Background 118

5.2.1 Early Searches 118

5.2.2 The Discovery of Infrared Excess from G29-38 118

5.2.3 The Polluted Nature of Metal-Rich White Dwarfs 120

5.2.4 Interstellar or Circumstellar Matter 122

5.2.5 G29-38 and the Asteroid Accretion Model 123

5.3 Pre-Spitzer and Ground-Based Observations 124

5.3.1 Photometric Searches for Near-Infrared Excess 124

5.3.2 Metal-Polluted White Dwarf Discoveries 125

5.3.3 The Spectacular Case of GD362 126

5.3.4 Spectroscopic Searches for Near-Infrared Excess 127

5.3.5 Spectroscopy at Longer Wavelengths 129

5.4 The Initial Impact of Spitzer 129

5.4.1 Infrared Capabilities of Spitzer 129

5.4.2 First Results 130

5.4.3 The First Spitzer Surveys of White Dwarfs 134

5.5 The Next Wave of Disk Discoveries 138

5.5.1 The Second Class of Polluted White Dwarfs 138

5.5.2 A Highly Successful Spitzer Search 139

5.5.3 The Detection of Gaseous Debris in a Disk 141

5.5.4 Dust Deficiency of DAZ Stars – Collisions? 142

5.5.5 Expanding Searches to the DBZ Stars 143

5.5.6 Additional Disks with Gaseous (and Solid) Debris 145

5.6 Studies and Statistics 147

5.6.1 Spectroscopic Confirmation of Rocky Circumstellar Debris 147

5.6.2 First Statistics and the Emerging Picture 151

5.6.3 Dust-Deficiency in DAZ Stars – Narrow Rings? 153

5.6.4 The Composition and Masses of Asteroids around GD362 and GD40 158

5.6.5 Evidence for Water in Debris Orbiting White Dwarfs 161

5.6.6 A Last Look at the Interstellar Accretion Hypothesis 162

5.7 Related Objects 164

5.7.1 White Dwarfs Polluted by Companions? 164

5.7.2 Dust in the Helix? 165

5.8 Outlook for the Present and Near Future 166

References 168

6 The Origin and Evolution of White Dwarf Dust Disks 173
John H. Debes

6.1 Introduction 173

6.2 Orders of Magnitude around a White Dwarf 174

6.3 Structure and Evolution of a White Dwarf Dust Disk 178

6.3.1 Optically Thin Dust Disks? 182

6.3.2 Subsequent Evolution of the Dust Disk 182

6.4 Origins of White Dwarf Dust Disks 185

6.4.1 The Unstable Planetary Perturbation Model 185

6.4.2 Dust and Accretion, or Just Accretion? 190

6.4.3 The Mean Motion Perturbation Model 192

6.4.4 Observability of Ring Progenitors 196

6.5 Conclusion 198

References 200

7 Planetary Nebulae around White Dwarfs: Revelations from the Infrared 203
You-Hua Chu

7.1 Introduction: Expectations of Nebulae aroundWhite Dwarfs 203

7.2 Planetary Nebulae aroundWhite Dwarfs 204

7.3 High-Excitation Nebulae around Hot White Dwarfs 208

7.4 Mid-Infrared Emission from Circumstellar Nebulae of White Dwarfs 209

7.5 Conclusion 212

References 215

Index 217

Object Index 223

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D. W. Hoard, after earning his Ph.D. at the University of Washington, worked at Cerro Tololo Inter-American Observatory in Chile, the southern hemisphere branch of the United States? National Optical Astronomy Observatory. In 2002, he joined NASA?s Spitzer Science Center, part of the Infrared Processing and Analysis Center (IPAC) at the California Institute of Technology. He is an IPAC Research Scientist and, among other activities, a member of the Spitzer Space Telescope Science User Support Team. His research focuses on white dwarfs and their environments. He has published more than 80 reviewed papers in refereed journals.
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