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The Influence of Low-Mass Companions Around AGB Stars

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The Influence of Low-Mass Companions Around AGB Stars From Binaries to Asymmetric Outflows: The Influence of Low-mass Companions Around AGB Stars by Jason T. Nordhaus Submitted in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Supervised by Professor Eric G. Blackman Department of Physics and Astronomy The College Arts and Sciences University of Rochester Rochester, New York 2008 ii To my mom and dad iii Curriculum Vitae The author was born November 24, 1980 in Concord, Massachusetts. He graduated from the University of Rochester with a Bachelor of Science degree in Physics and Astronomy and a Bachelor of Arts degree in Mathematics in 2003. Upon completion of his undergraduate education, he entered the doctoral program in the Department of Physics and Astronomy at the University of Rochester. In May 2004 the author received a Master of Arts degree in Physics from the University of Rochester. The author received a Department of Energy Frank J. Horton Fellowship (2004-2008) in addition to a Department of Education GAANN fellowship (2003-2006). Selected Publications • Nordhaus, J., Minchev, I., Sargent, B., Forrest, W., Blackman, E. G., De Marco, O., Kastner, J., Balick, B., Frank, A. 2008 MNRAS, submitted • Edgar, R. G., Nordhaus, J., Blackman, E., Frank, A. 2008 ApJL, in press • Nordhaus, J., Blackman, E. G., Frank, A. 2007 MNRAS, 376, 599 • Minchev, I., Nordhaus, J., Quillen, A. 2007 ApJL, 664, 31 • Watson, D. M., Leisenring, J. M., Furlan, E., Bohac, C. J., Sargent, B., Forrest, W. J., Calvet, N., Hartmann, L., Nordhaus, J. T., Green, J. D., Kim, K. H., Sloan, G. C., Chen, C. H., Keller, L. D., d’Alessio, P., Najita, J., Uchida, K. I., Houck, J. R. 2007, ApJS, submitted • Nordhaus, J., Blackman, E. G. 2006, MNRAS370, 2004 • Blackman, E. G., Nordhaus, J., Thomas, J. H. 2006 New Astronomy, 11, 452 iv • Nordhaus, J., Blackman, E. G. 2008 to appear in AIP Proceedings of the IXth Torino Workshop on AGB Nucleosyntheis • Nordhaus, J., Blackman, E. G. 2007, in Asymmetric Planetary Nebulae IV, eds. R. L. M. Corradi, A. Manchado, N. Soker (in ASP Conference Series: San Francisco), in press • Blackman, E. G., Nordhaus, J. 2007, in Asymmetric Planetary Nebulae IV, eds. R. L. M. Corradi, A. Manchado, N. Soker (in ASP Conference Series: San Francisco), in press v Acknowledgments This work would not have been possible without the support and encouragement of my thesis advisor, Dr. Eric Blackman. His guidance, patience and scientific insight were instrumental throughout my graduate career. I also wish to thank my fellow graduate compatriots, Dave Clader and Ivan Minchev for invaluable friendship, stim- ulating scientific & non-scientific discussions and for an endless supply of much needed distractions. I wish to acknowledge institutions which provided financial support and assis- tance. In particular, the Department of Physics and Astronomy at the University of Rochester. Financial support for this work was provided by the Laboratory for Laser Energetics through a U.S. Department of Energy Horton Fellowship and a U.S. Department of Education GAANN Fellowship. I would also like to thank my parents, Kurt and Sherri, and sisters, Miranda and Tiffany, for their love and support. Finally, I wish to thank my wife Lea for her unconditional love and encouragement. vi Abstract The study of intermediate mass, evolved stars is undergoing renewed interest due to recent observational and theoretical results suggesting that binarity is fundamental for shaping post-Asymptotic Giant Branch and Planetary Nebula outflows. Despite ex- tensive research, the physical mechanism responsible for transitioning from a spherical Asymptotic Giant Branch (AGB) star to an asymmetric post-AGB object is poorly understood. In an effort to understand how binaries may produce asymmetries, this thesis presents several theoretical studies which explore the effect of low-mass com- panions on evolved star outflows. This thesis consists of four separate projects: (1.) Close companions may become engulfed by the evolved star and in-spiral during a common envelope phase. Common envelope evolution can lead to three different consequences: (i.) equatorial ejection of material (ii.) spin-up of the envelope resulting in an explosive dynamo-driven jet and (iii.) tidal shredding of the companion into an accretion disk which ejects a poloidal wind. (2.) In addition, we study a dynamical, large-scale α − Ω interface dynamo oper- ating in an AGB star in both an isolated setting and a setting in which a low-mass companion is embedded inside the envelope. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipa- tion and Poynting flux. For the isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. Furthermore, we investigate the energy requirements that convection must satisfy to accomplish this by analogy to the Sun. For the com- mon envelope case, a robust dynamo results, unbinding the envelope under a range of conditions. (3.) Wide binaries can interact with the wind of the evolved primary. The grav- itational influence of the secondary focuses material in the equatorial plane. The vii companion induces spiral shocks which may anneal amorphous grains into crystalline dust. This work presents a physical mechanism to produce crystalline dust in AGB star binaries. (4.) We present a spectral modeling technique which constrains the geometry of evolved star nebulae. We apply our technique to HD 179821 which exhibits a dou- ble peaked spectral energy distribution (SED) with a sharp rise from ∼ 8 − 20 µm. Such features have been associated with dust shells or inwardly truncated circumstel- lar disks. In order to compare SEDs from both systems, we employ a spherically sym- metric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 µm, we find that both models produce equivalent fits to the data. However, longward of 40 µm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and the observations. For HD 179821, our best fit consists of a Teff = 7000 K central source characterized by τV ∼ 1.95 and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a Teff = 5750 K photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modeling. viii Contents CurriculumVitae................................. iii Acknowledgments................................. v Abstract...................................... vi Introduction 1 1.1Post-MainSequenceEvolution...................... 1 1.1.1 AsymptoticGiantBranchEvolution............... 1 1.1.2 Post-AGBPhase.......................... 2 1.2 Support for the Binary Hypothesis . ................. 4 1.2.1 ObservationalIndications..................... 4 1.2.2 MagneticShaping......................... 5 1.3ThesisStructure.............................. 5 References..................................... 7 Low-mass Binary Induced Outflows from Asymptotic Giant Branch Stars 9 2.1Abstract................................... 9 2.2Introduction................................. 10 2.3CommonEnvelopeEvolution....................... 11 2.3.1 EnvelopeBindingEnergy..................... 12 2.3.2 Orbital Energy and Angular Momentum Evolution . 14 2.4CommonEnvelopeEvolutionScenarios................. 17 2.4.1 SecondaryInducedEnvelopeExpulsion............. 17 2.4.2 Secondary Induced Envelope α − ΩDynamo.......... 20 2.4.3 DiscDrivenOutflow........................ 24 CONTENTS ix 2.5DiscussionofObservationalImplications................. 25 2.5.1 ObservationalConsequences.................... 26 2.5.2 ApplicationstospecificPPNeandPNesystems......... 27 2.6Conclusions................................. 29 References..................................... 31 Isolated versus Common Envelope Dynamos in Planetary Nebula Pro- genitors 35 3.1Abstract................................... 35 3.2Introduction................................. 36 3.3 Dynamos, Common Envelopes and Isolated AGB Evolution . 37 3.4DynamicalEquations............................ 39 3.4.1 EvolutionofΩand∆Ω...................... 41 3.4.2 Evolution of α ........................... 43 3.5NumericalResults............................. 44 3.5.1 IsolatedDynamoWithoutReseeding∆Ω............ 45 3.5.2 IsolatedDynamoWithReseeding∆Ω.............. 47 3.5.3 CommonEnvelopeDynamo.................... 49 3.6MorphologyofMagneticOutflows.................... 54 3.7Conclusions................................. 56 References..................................... 58 Towards a Spectral Technique for Determining Material Geometry Around Evolved Stars: Application to HD 179821 61 4.1Abstract................................... 61 4.2Introduction................................. 62 4.3 HD 179821: post-AGB or red supergiant? . .......... 64 4.4PhotosphericModelsandExtinction................... 66 4.5InnerWall,Edge-onDiskModels..................... 68 4.6 Spherical Shell Models ........................... 72 4.7Summary.................................. 77 CONTENTS x The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks 83 5.1Abstract................................... 83 5.2Introduction................................. 83 5.3NumericalStudy.............................
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