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Topics in Core-Collapse Supernova Theory: the Formation of Black Holes and the Transport of Neutrinos

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Topics in Core-Collapse Supernova Theory: The Formation of Black Holes and the Transport of Neutrinos Thesis by Evan Patrick O'Connor In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2012 (Defended May 21, 2012) ii c 2012 Evan Patrick O'Connor All Rights Reserved iii Acknowledgements First and foremost, I have the pleasure of acknowledging and thanking my advisor, Christian Ott, for his commitment and contribution to my research over the last four years. Christian, your attention to detail, desire for perfection, exceptional physical insight, and unwavering stance against the phrase direct black hole formation are qualities I admire and strive to reproduce in my own research. From our very first meeting, Christian has been an staunch advocate for open science, a philosophy that he has instilled in me throughout my time at Caltech and one that I plan on maintaining in my future career. I also thank the love of my life. Erin, there are so many reasons to be thankful to you. You are the best part of each and every day. On the science front, thank you for all our neutrino discussions, enlightening me on so many different aspects of neutrinos, and keeping my theoretical meanderings grounded in experimental reality. I am indebted to you for all your support, encouragement, per- sistence, and love throughout the writing of this thesis and I look forward to repaying that debt soon. Thank you to my family, as always, your constant encouragement and support in everything I do is greatly appreciated. I am so glad that most of you had a chance to come visit me in Pasadena at some point throughout my time here. I am very much looking forward to moving closer to home, even if it does mean the return of a real winter. I would like to thank all of my collaborators and scientific mentors. Notably, Christian Ott and Luc Dessart who have help contribute to the work presented in this thesis; but also those whom I have worked/collaborated with throughout my graduate career, in particular, Ernazar Abdikamalov, Basudeb Dasgupta, Gang Shen, and Chuck Horowitz, but of course many others. During my first year at UPEI, Derek Lawther converted me from Engineering to Physics with his timely letter en- couraging me to pursue physics. Later on, Michelle Cottreau, Heather Hughes, and Sean Dougherty slowly converted me to astrophysics, and finally Achim Schwenk brought me away from the dark side and into the realm of theoretical astrophysics. Many of my research techniques, principles, and talents stem from my undergraduate honours advisor Sheldon Opps, for which I will be forever grateful. A special thanks to James Polson, and a later nudge by Achim Schwenk, for suggesting that I reach for the top when searching for graduate schools. iv To my fellow Tapir graduate students, particularly my awesome officemate Jeff Kaplan; the Tapir postdocs; and the Tapir faculty and staff: thanks for all the interesting discussions, Thursday enlightenment sessions, helpful advice, and support. I would also like to thank all of the friends I have made at Caltech over the last five years. TNDs are on my list as one of the best memories of Caltech|I will miss them and you. v Abstract Core-Collapse Supernovae are one of the most complex astrophysical systems in the universe. They deeply entwine aspects of physics and astrophysics that are rarely side by side in nature. To accu- rately model core-collapse supernovae one must self-consistently combine general relativity, nuclear physics, neutrino physics, and magneto-hydrodynamics in a symmetry-free computational environ- ment. This is a challenging task, as each one of these aspects on its own is an area of great study. We take an open approach in an effort to encourage collaboration in the core-collapse supernovae community. In this thesis, we develop a new open-source general-relativistic spherically-symmetric Eulerian hydrodynamics code for studying stellar collapse, protoneutron star formation, and evolution until black hole formation. GR1D includes support for finite temperature equations of state and an efficient and qualitatively accurate treatment of neutrino leakage. GR1D implements spherically-symmetric rotation, allowing for the study of slowly rotating stellar collapse. GR1D is available at http://www. stellarcollapse.org We use GR1D to perform an extensive study of black hole formation in failing core-collapse super- novae. Over 100 presupernova models from various sources are used in over 700 total simulations. We systematically explore the dependence of black hole formation on the input physics: initial zero- age main sequence (ZAMS) mass and metallicity, nuclear equation of state, rotation, and stellar mass loss rates. Assuming the core-collapse supernova mechanism fails and a black hole forms, we find that the outcome, for a given equation of state, can be estimated, to first order, by a single parameter, the compactness of the stellar core at bounce. By comparing the protoneutron star structure at the onset of gravitational instability with solutions of the Tolman-Oppenheimer-Volkof equations, we find that thermal pressure support in the outer protoneutron star core is responsible for raising the maximum protoneutron star mass by up to 25% above the cold neutron star value. By artificially increasing neutrino heating, we find the critical neutrino heating efficiency required for exploding a given progenitor structure and connect these findings with ZAMS conditions. This establishes, albeit approximately, for the first time based on actual collapse simulations, the mapping between ZAMS parameters and the outcome of core collapse. We also use GR1D to study proposed progenitors of long-duration γ-ray bursts. We find that many vi of the proposed progenitors have core structures similar to garden-variety core-collapse supernovae. These are not expected to form black holes, a key ingredient of the collapsar model of long-duration γ-ray bursts. The small fraction of proposed progenitors that are compact enough to form black holes have fast rotating iron cores, making them prone to a magneto-rotational explosion and the formation of a protomagnetar rather than a black hole. Finally, we present preliminary work on a fully general-relativistic neutrino transport code and neutrino-interaction library. Following along with the trends explored in our black hole formation study, we look at the dependence of the neutrino observables on the bounce compactness. We find clear relationships that will allow us to extract details of the core structure from the next galactic supernova. Following the open approach of GR1D, the neutrino transport code will be made open- source upon completion. The open-source neutrino-interaction library, NuLib, is already available at http://www.nulib.org. vii Contents Acknowledgements iii Abstract v Table of Contents x List of Figures xii List of Tables xiii 1 Overview 1 1.1 Main Results of this Thesis................................1 1.1.1 GR1D and Other Open-Source Codes.......................1 1.1.2 Black Holes in Stellar Collapse..........................2 1.2 Related Work Not Contained in this Thesis.......................3 1.3 Thesis Structure......................................4 2 Introduction 6 2.1 Observations of Core-Collapse Supernovae........................6 2.2 Core-Collapse Theory...................................8 2.3 The Quest for the Explosion Mechanism.........................9 2.3.1 The Neutrino-Driven Explosion Mechanism................... 10 2.3.2 The Magneto-Rotational Explosion Mechanism................. 11 2.3.3 Other Explosion Mechanisms........................... 12 2.3.4 Missing Physics................................... 12 2.3.5 Failed Supernovae................................. 13 2.4 Signatures of the Supernova Central Engine....................... 14 2.4.1 Neutrino Signatures................................ 14 2.4.2 Gravitational Wave Signatures.......................... 15 2.4.3 Indirect Probes of the Central Engine...................... 16 viii 3 General Relativistic Hydrodynamics & Core-Collapse Supernova Microphysics 17 3.1 Introduction......................................... 17 3.2 Spacetime Equations.................................... 19 3.3 GR Hydrodynamics in 1D Radial-Gauge, Polar Slicing................. 20 3.4 Extension to 1.5D: Including Rotation.......................... 22 3.5 Equations of State (EOS)................................. 24 3.5.1 Hybrid EOS..................................... 24 3.5.2 Lattimer-Swesty EOS............................... 25 3.5.3 Table-Based EOS.................................. 27 3.5.4 Neutron Star Mass vs. Radius Relation Constraints.............. 28 3.6 Neutrino Leakage and Heating.............................. 29 3.6.1 Deleptonization and Electron Capture in the Collapse Phase......... 29 3.6.2 Postbounce Deleptonization and Neutrino Heating/Cooling.......... 30 3.6.2.1 Neutrino Heating............................. 33 3.6.3 Neutrino Pressure................................. 34 3.7 Code Tests......................................... 36 3.7.1 Relativistic Shocktube............................... 36 3.7.2 Sedov Blast Wave................................. 37 3.7.3 Oppenheimer-Snyder Collapse........................... 38 3.7.4 Hybrid Core Collapse: Convergence....................... 39 3.8 Summary.......................................... 40 4 Black Hole Formation in Failing Core-Collapse Supernovae 42 4.1 Nonrotating Black Hole Formation...........................
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