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Asymptotic Giant Branch Stars: Their Influence on Binary Systems and the Interstellar Medium

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Asymptotic Giant Branch Stars: Their Influence on Binary Systems and the Interstellar Medium Asymptotic Giant Branch Stars: their influence on binary systems and the interstellar medium Amanda I Karakas, BSc (Hons) A thesis submitted for the degree of Doctor of Philosophy. School of Mathematics & Statistics, Monash University, Australia. July, 2003 i Quotes Since September it’s just gotten colder and colder. There’s less daylight now, I’ve noticed too. This can only mean one thing – the Sun is going out. In a few more months the Earth will be a dark and lifeless ball of ice. Dad says the Sun isn’t going out. He says it’s colder because the Earth’s orbit is taking us farther from the Sun. He says winter will be here soon. Isn’t it sad how some people’s grip on their lives is so precarious that they’ll embrace any preposterous delusion rather than face an occasional bleak truth? “Calvin & Hobbes”, by Bill Watterson The Cat's Eye Nebulae, NGC 6543, imaged by the Hubble Space Telescope. Planetary nebulae such as NGC 6543 are believed to consist of several tenths of a solar mass of gas and dust expelled during the latter stages of the asymptotic giant branch phase of stellar evolution. Image from http://www.astro.washington.edu/balick/WFPC2/. Contents Abstract . vi Statement . vii Publications . viii Acknowledgments . x 1 Introduction 1 1.1 Evolution of Low and Intermediate Mass Stars . 2 1.2 Chapter Synopsis . 6 1.2.1 The Barium Stars . 6 1.2.2 Parameterizing the Third Dredge-Up . 7 1.2.3 Stellar Yields . 8 2 The Formation of the Barium Stars 10 2.1 Introduction . 10 2.2 Observations of the Barium Stars . 12 2.3 Description of the Model . 15 2.3.1 Mass Loss . 15 2.3.2 Stellar Wind Accretion . 16 2.3.3 Nucleosynthesis Model . 17 2.3.4 Monte-Carlo Simulation Parameters . 19 2.4 Tidal Evolution Model . 20 2.5 Results and Discussion . 22 2.5.1 Number of Ba stars to Red Giants . 24 2.5.2 Dependence of Results on Free Parameters . 25 2.5.3 Barium Over-abundances . 27 2.5.4 The Mass Distributions . 29 2.6 Summary and Further Work . 29 3 Pre–AGB Evolution and Nucleosynthesis 32 3.1 Numerical Details . 32 3.1.1 The Stellar Evolution Code . 32 3.1.2 The Nucleosynthesis Code . 36 3.2 Stellar Models . 41 3.3 Stellar Lifetimes . 41 3.3.1 Uncertainties Affecting the Stellar Lifetimes . 42 ii CONTENTS iii 3.3.2 Model Results . 43 3.3.3 Comparison With Other Work . 47 3.4 First and Second Dredge-Up: the CNO Isotopes . 49 3.4.1 Conflict Between Theory and Observation . 50 3.4.2 Depth of the First and Second Dredge-Up . 51 3.4.3 Results from the FDU and SDU: Carbon . 53 3.4.4 Nitrogen . 54 3.4.5 Oxygen . 59 3.5 First and Second Dredge-Up: non-CNO Elements . 60 3.5.1 Lithium and Fluorine . 61 3.5.2 Neon and Sodium . 62 3.5.3 Magnesium and Aluminium . 63 3.6 The Core Helium Flash . 64 3.6.1 Results From the Core-helium Flash . 66 3.7 Summary . 68 4 AGB evolution 69 4.1 Evolution on the AGB . 69 4.1.1 The Third Dredge-Up . 70 4.2 Evolution During a Thermal Pulse . 75 4.2.1 Mass and Duration of the Convective Pocket . 75 4.2.2 Temperature in the Helium-burning Shell . 79 4.2.3 Secondary Convective Pockets . 82 4.3 Evolution During the Interpulse Phase . 84 4.3.1 The Interpulse Period . 85 4.3.2 Temperature at the Base of the Convective Envelope . 87 4.3.3 The Core–mass–luminosity Relationship . 91 4.4 Summary . 99 5 AGB nucleosynthesis 101 5.1 Introduction . 101 5.2 Nucleosynthesis Resulting From the Third Dredge-Up . 102 5.2.1 The Hydrogen-burning Shell . 103 5.2.2 The Helium-burning Shell . 106 5.3 Nucleosynthesis Resulting from Hot Bottom Burning . 110 5.3.1 The Creation of 7Li via HBB . 110 5.3.2 Nucleosynthesis from HBB: CNO Isotopes and Fluorine . 113 5.3.3 The Production of Primary 14N . 114 5.3.4 The Neon, Magnesium and Aluminium Isotopes . 117 5.4 Comparison of Model Predictions to Observations . 118 5.4.1 Observations of the Mg Isotopes in IRC+10216 . 119 5.4.2 Observations of the Mg Isotopes in Other Stars . 121 5.5 Neon in Planetary Nebulae . 126 5.5.1 The Observations . 127 CONTENTS iv 5.5.2 Model Results . 129 5.5.3 Discussion . 132 5.6 Summary and Further Work . 134 6 Parameterizing the Third Dredge-Up 138 6.1 Introduction . 138 6.2 Stellar Models Without Mass Loss . 140 6.2.1 Model Details . 140 6.2.2 Convection and the Third Dredge-Up . 140 6.2.3 Mass Loss . 141 6.2.4 Model Results . 141 min 6.3 The Fit for Mc(1) and Mc . 148 6.3.1 The Core Mass at the First Thermal Pulse . 148 6.3.2 The Core Mass at the First TDU Episode . 150 6.4 The Fit for λmax . 151 6.4.1 The Dredge-Up Parameter, λ as a Function of Time . 154 6.5 Discussion . 156 6.5.1 The Core Mass at the First Pulse . 156 min 6.5.2 The Third Dredge-Up: Mc and λmax . 157 6.5.3 The Carbon Star Luminosity Function . 158 6.6 Summary and Further Work . 159 7 Stellar Yields 160 7.1 Stellar Yield Calculation . 160 7.1.1 Estimating the Surface Enrichment from the Last Thermal Pulses 161 7.2 Stellar Yield Uncertainties . 164 7.2.1 Uncertainties From Mass Loss . 166 7.2.2 The Initial–final Mass Relation . 167 7.2.3 Uncertainties in the Nuclear Reaction Rates . 168 7.3 Stellar Yields . 170 7.3.1 Dependence Upon the Initial Abundances . 171 7.4 Comparison to Other Authors . 171 7.4.1 Physical Assumptions . 172 7.5 Yield Results . 174 7.5.1 Hydrogen . 175 7.5.2 Helium: 4He . 178 7.5.3 Lithium: 7Li . 179 7.5.4 Carbon: 12C . 180 7.5.5 Carbon: 13C . 182 7.5.6 Nitrogen: 14N . 184 7.5.7 Nitrogen: 15N . 184 7.5.8 Oxygen: 16O . 186 7.5.9 Oxygen: 17O . 188 7.5.10 Oxygen: 18O . 189 CONTENTS v 7.5.11 Fluorine . 189 7.5.12 The Neon Isotopes . 191 7.5.13 Sodium . 193 7.5.14 The Magnesium Isotopes . 195 7.5.15 The Aluminium Isotopes . 196 7.5.16 The Silicon Isotopes . 199 7.6 Summary and Further Work . 200 8 Conclusion 203 8.1 Future Directions . 206 A HR Diagrams 207 B Surface Abundance Results From the First and Second Dredge-Up 211 C Surface Abundance Results for the Asymptotic Giant Branch 221 D Stellar Yields 261 Bibliography 272 CONTENTS vi Abstract In this thesis we investigate a diverse range of topics related to asymptotic giant branch (AGB) stars by using detailed stellar models with the latest.
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