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Nucleosynthesis in Stellar Models Across Initial Masses and Metallicities and Implications for Chemical Evolution

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Nucleosynthesis in Stellar Models Across Initial Masses and Metallicities and Implications for Chemical Evolution Nucleosynthesis in Stellar Models across Initial Masses and Metallicities and Implications for Chemical Evolution by Christian Heiko Ritter B.Sc., Goethe University Frankfurt, 2011 M.Sc., Goethe University Frankfurt, 2013 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the Department of Physics and Astronomy c Christian Heiko Ritter, 2017 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author. ii Nucleosynthesis in Stellar Models across Initial Masses and Metallicities and Implications for Chemical Evolution by Christian Heiko Ritter B.Sc., Goethe University Frankfurt, 2011 M.Sc., Goethe University Frankfurt, 2013 Supervisory Committee Dr. Falk Herwig, Supervisor (Department of Physics and Astronomy, University of Victoria) Dr. Kim Venn, Departmental Member (Department of Physics and Astronomy, University of Victoria) Dr. Adam Monahan, Outside Member (School of Earth and Ocean Sciences, University of Victoria) Dr. Jeremy Heyl, External Member (Department of Physics and Astronomy, University of British Columbia) iii ABSTRACT Tracing the element enrichment in the Universe requires to understand the el- ement production in stellar models which is not well understood, in particular at low metallicity. In this thesis a variety of nucleosynthesis processes in stellar models across initial masses and metallicities is investigated and their relevance for chemical evolution explored. Stellar nucleosynthesis is investigated in asymptotic giant branch (AGB) models and massive star models with initial masses between 1 M and 25 M for metal frac- tions of Z = 0:02; 0:01; 0:006; 0:001; 0:0001. A yield grid with elements from H to Bi is calculated. It serves as an input for chemical evolution simulations. AGB models are computed towards the end of the AGB phase and massive star models are calculated until core collapse followed by explosive core-collapse nucleosynthesis. The simula- tions include convective boundary mixing in all AGB star models and feature efficient hot-bottom burning and hot dredge-up in AGB models as well the predictions of both heavy elements and CNO species under hot-bottom burning conditions. H-ingestion events in the low-mass low-Z AGB model with initial mass of 1 M at Z = 0:0001 result in the production of large amounts of heavy elements. In super-AGB models H ingestion could potentially lead to the intermediate neutron-capture process. To model the chemical enrichment and feedback of simple stellar populations in hydrodynamic simulations and semi-analytic models of galaxy formation the SYGMA module is created and its functionality is verified through a comparison with a widely adopted code. A comparison of ejecta of simple stellar populations based on yields of this work with a commonly adopted yield set shows up to a factor of 3.5 and 4.8 less C and N enrichment from AGB stars at low metallicity which is attributed to complete stellar models, the modeling of the AGB stage and hot-bottom burning in super- AGB stars. Analysis of two different core-collapse supernova fallback prescriptions show that the total amount of Fe enrichment by massive stars differs by up to two at Z = 0:02. Insights into the chemical evolution at very low metallicity as motivated by the observations of extremely metal poor stars require to understand the H-ingestion events common in stellar models of low metallicity. The occurrence of H ingestion events in super-AGB stars is investigated and identified as a possible site for the production of heavy elements through the intermediate neutron capture process. The peculiar abundance of some C-Enhanced Metal Poor stars are explained with simple iv models of the intermediate neutron capture process. Initial efforts to model this heavy element production in 3D hydrodynamic simulations are presented. For the first time the nucleosynthesis of interacting convective O and C shells in massive star models is investigated in detail. 1D calculations based on input from 3D hydrodynamic simulations of the O shell show that such interactions can boost the production of odd-Z elements P, Cl, K and Sc if large entrainment rates associ- ated with O-C shell merger are assumed. Such shell merger lead in stellar evolution models to overproduction factors beyond 1 dex and p-process overproduction factors above 1 dex for 130;132Ba and heavier isotopes. Chemical evolution models are able to reproduce the Galactic abundance trends of these odd-Z elements if O-C shell merger occur in more than 50% of all massive stars. v Contents Supervisory Committee ii Abstract iii Table of Contents v List of Tables viii List of Figures x CO-AUTHORSHIP xvi Acknowledgements xvii Dedication xviii 1 Introduction 1 1.1 Motivation and goals . 2 1.1.1 Stellar yields . 2 1.1.2 Chemical evolution . 4 1.1.3 Reactive-convective nucleosynthesis . 5 1.2 Stellar nucleosynthesis . 8 1.2.1 Stellar modeling . 8 1.2.2 Stellar phases . 10 1.2.3 Nucleosynthesis . 14 1.2.4 Stellar hydrodynamics . 15 1.3 Chemical evolution . 17 1.3.1 Simple stellar populations . 17 1.3.2 Simple galaxy models . 18 1.3.3 Cosmological simulations . 19 vi 1.4 Thesis outline . 19 2 Yields for chemical evolution 21 2.1 Introduction . 23 2.2 Methods . 26 2.2.1 Stellar evolution . 26 2.2.2 Explosion . 30 2.2.3 Nucleosynthesis code and processed data . 31 2.3 Results of stellar evolution and explosion . 38 2.3.1 General properties . 38 2.3.2 Features at low metallicity . 42 2.4 Post-processing nucleosynthesis results . 55 2.4.1 Dredge-up and dredge-out . 55 2.4.2 HBB nucleosynthesis . 56 2.4.3 C/Si zone and n process . 56 2.4.4 Shell merger nucleosynthesis . 57 2.4.5 Fe-peak elements . 58 2.4.6 H-ingestion nucleosynthesis . 58 2.4.7 α process . 59 2.4.8 Weak s-process . 59 2.4.9 Main s-process . 60 2.4.10 p-process . 61 2.5 Discussion . 80 2.5.1 Resolution of AGB models . 80 2.5.2 Resolution of massive star models . 80 2.5.3 Comparison with stellar yields in literature . 82 2.6 Summary . 87 3 Applications of yields in chemical evolution studies 89 3.1 Chemical enrichment and stellar feedback of simple stellar populations for galaxy models . 89 3.1.1 Introduction . 91 3.1.2 Code details . 93 3.1.3 Results . 103 3.1.4 Discussion . 109 vii 3.1.5 Online availability . 118 3.1.6 Yield set database . 119 3.1.7 Summary and Conclusions . 120 3.2 Effect of convective boundary mixing on O production and [O/Fe] in SSPs . 121 3.3 Galactic chemical evolution with the NuPyCEE framework . 123 3.4 Outreach . 125 3.5 Summary . 125 4 H-ingestion flashes and i process 127 4.1 Introduction . 127 4.2 H ingestion in super-AGB stars . 129 4.3 CEMP-r/s stars reveal i process signature . 131 4.3.1 CEMP-r/s stars . 131 4.3.2 A simple i-proces model for the CEMP-r/s stars . 131 4.4 Summary and Outlook . 133 5 O-C shell merger in massive stars 136 5.1 Introduction . 137 5.2 Methods . 140 5.3 Results . 141 5.3.1 Convection and feedback in 3D . 141 5.3.2 Nucleosynthesis in 1D . 141 5.3.3 Relevance for galactic chemical evolution . 148 5.4 Discussion . 150 5.4.1 Towards full shell merger . 150 5.4.2 Model dependence of shell merger nucleosynthesis . 151 5.5 Summary and Conclusions . 153 6 Summary and Conclusion 155 6.1 Advances in theory of element production . 155 6.2 Prospects . 156 viii List of Tables 1.1 Nuclear burning times ∆t of a low-mass stellar model with initial mass of 2 M and massive star model with initial mass of 20 M at solar metallicity. 13 2.1 Mass fractions of α-enhanced isotopes for Z = 0:0001 derived from Reddy, Lambert, and Allende Prieto (2006) and Kobayashi et al. (2006). 32 2.2 CBM efficiencies f for the diffusive CBM mechanism applied in AGB models. 32 2.3 The final yields for the stellar model with initial mass of 4 M at Z = 0:0001 in comparison with yields of H04, K10 and C15. 33 2.4 Fe core mass of massive star models presented in this work. 33 2.5 Remnant masses of massive star models according to Fryer et al. (2012) for the two delayed and rapid explosion prescriptions. 33 2.6 Final core masses Mfinal and total lifetime τtotal for Z = 0:0001. 46 75% 2.7 Comparison of the He core mass (Mα ), CO core mass (MCO) and final core mass (Mfinal) of this work with J15. 46 75% 2.8 Comparison of the He core mass (Mα ), CO core mass (MCO) and Si core mass MSi of this work with M02 and P16. 46 2.9 Core masses for massive star models. 47 2.10 Lifetimes of major central burning stages of massive star models. 47 2.11 Model properties of the TP-AGB phase for Z = 0:006, 0:001 and 0:0001. 48 2.12 TP-AGB properties for models at Z = 0:0001. The complete table is available online. 48 2.13 Yields derived from stellar winds, pre-SN and SN ejecta for Z = 0:0001. 63 2.14 Comparison of the final yields of the stellar models with initial mass of 2 M at Z = 0:0001 from this work with H04, K10 and S14.
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