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Chemical Modeling of Interstellar Molecules in Dense Cores

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Chemical Modeling of Interstellar Molecules in Dense Cores Chemical Modeling of Interstellar Molecules in Dense Cores Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Donghui Quan, M.S. Graduate Program in Chemical Physics The Ohio State University 2009 Dissertation Committee: Professor Eric Herbst, Advisor Professor Frank C. De Lucia Professor Anil K. Pradhan Copyright by Donghui Quan 2009 ii Abstract There are billions of stars in our galaxy, the Milky Way Galaxy. In between the stars is where the so-called “interstellar medium” locates. The majority of the mass of interstellar medium is clumped into interstellar clouds, in which cold and hot dense cores exist. Despite of the extremely low densities and low temperatures of the dense cores, over one hundred molecules have been found in these sources. This makes the field of astrochemistry vivid. Chemical modeling plays very important roles to understand the mechanism of formation and destruction of interstellar molecules. In this thesis, chemical kinetics models of different types were applied: in Chapter 4, pure gas phase models were used for seven newly detected or confirmed molecules by the Green Bank Telescope; in Chapter 5, the potential reason of non-detection of O2 was explored; in Chapter 6, the mysterious behavior of CHNO and CHNS isomers were studied by gas-grain models. In addition, effects of varying rate coefficients to the models are also discussed in Chapter 3 and 7. ii Dedication Dedicated to my parents Quan He (全和), Li Lianxiang (李廉祥) and my wife Wang Jing (王璟) iii Acknowledgements First of all, I would like to thank my advisor, Professor Eric Herbst, who has always been supportive in all means: a mentor in my graduate study, a guide in astrochemical research, and a kind “father” in the life. My most appreciation also goes to my thesis committee: Professor Frank C. De Lucia and Professor Anile K. Pradhan, who give precious opinions and helpful comments on the research and thesis writing. I would also like to thank my collaborators: Professor Ian Smith, for evaluating new rates of C + Cn reactions; Professor Dahbia Talbi, for estimating possible formation channels of c-C3H2O; Professor Murray McEwan, for suggestions of + + maximum branching ratio of C2H3 + CO to form C3H3O ; Professor Hua Guo, for theoretical calculations of O + OH reaction rate coefficient; Dr. David Woon, for estimating useful data for CHNO and CHNS isomers; Dr. Sandra, Brünken, for detailed information of Sgr B2 physical conditions and CHNO isomers detection; Professor Yoshihiro Osamura, for evaluating possible neutral-neutral destruction channels of CHNO isomers. Without their beneficial discussion and numerous helps, works in this thesis would not be possible. My thankfulness is also to former and current group members: Dr. Oscar Osamara, Dr. Herma Cuppen, Dr. Robin Garrod, Dr. Valentine Wakelam, Dr. Qiang Chang, Dr. George Hassle, Paul Rimmer, Nanase Harade, Yezhe Pei. With everybody’s efforts, the group is like a big warm family. I want to thank my parents, who raised me, educated me, and supported me in all occasions. I want to thank my beloved wife, Jing, who always has confidence in me and accompanies me through all the difficulties and happy times. iv Vita Nov. 1978 .............................................. Born - Taizhou, Jiangsu, China P. R. 1998............................................................B.S. - Chemical Physics, University of Science and Technology of China, Anhui, China 2003............................................................M.S. - Chemical Physics, University of Science and Technology of China, Anhui, China 2003 to present ........................................Graduate Research Assistant Graduate Program of Chemical Physics, The Ohio State University, Columbus, Ohio, USA Publications (1) D. Quan, E. Herbst, and Y. Osamura ”Gas-grain Modeling of Cyanic Acid, Isocyanic Acid, Fulminic Acid, and Isofulminic Acid in Hot, Lukewarm, and Cold Cores” (in preparation ) v (2) N. Marcelino, S. Brünken, J. Cernicharo, D. Quan, E. Roueff, E. Herbst, and P. Thaddeus, “The Puzzling Behavior of HNCO Isomers in Molecular Clouds”, (submitted to A&A) (3) V. Wakelam, J.C. Loison, D. Talbi, D. Quan, and F. Caralp ”A Sensitivity study of the neutral-neutral reactions C + C3 and C + C5 in Cold Dense Interstellar Cores” Astronomy & Astrophysics 495, 513 (2009) (4) D. Quan, E. Herbst, T. J. Millar, G. E. Hassel, S. Lin, H. Guo, P. Honvault, and D. Xie ”New Theoretical Results Concerning the Interstellar Abundance of Molecular Oxygen” The Astrophysical Journal, 681, 1318 (2008) (5) D. Quan and E. Herbst ”Possible gas-phase syntheses for seven neutral molecules studied recently with the Green Bank Telescope” Astronomy & Astrophysics 474, 521 (2007) (6) I. W. M. Smith, A. M. Sage, N. M. Donahue, E. Herbst and D. Quan ”The temperature-dependence of rapid low temperature reactions: experiment, understanding and prediction” Faraday Discussions 133, 137 (2006) Fields of Study Major Field: Astrochemistry and Astrophysics vi Table of Contents Abstract ................................................................................................................ ii Dedication ............................................................................................................ iii Acknowledgements ..............................................................................................iv Vita ....................................................................................................................... v List of Tables ....................................................................................................... xii List of Figures ..................................................................................................... xiv Chapter 1: Introduction ......................................................................................... 1 1.1 Interstellar medium and interstellar clouds .............................................. 1 1.2 Interstellar molecules ............................................................................... 4 1.3 Astrochemistry ......................................................................................... 8 1.4 References ............................................................................................ 15 Chapter 2: Chemical Models .............................................................................. 18 2.1 Modeling Method ................................................................................... 18 2.2 Gas-Phase Model .................................................................................. 20 2.2.1 Physical conditions of the gas model .............................................. 21 2.2.2 Species in the gas model ................................................................ 23 2.2.3 Chemical reactions and the rate coefficients .................................. 23 vii 2.3 Gas-Grain Model ................................................................................... 31 2.3.1 Physical parameters of dust particles ............................................. 32 2.3.2 Surface species .............................................................................. 33 2.3.3 Surface reactions and gas-grain interactions .................................. 34 2.4 Application of the models ...................................................................... 38 2.5 References ............................................................................................ 38 Chapter 3: Inclusion of New Rapid Low Temperature Reaction Rate Coefficients ........................................................................................................................... 40 3.1 New Rates ............................................................................................. 40 3.2 Gas phase model settings ..................................................................... 41 3.3 Results and comparison to former models and observances ................ 42 3.4 References ............................................................................................ 47 Chapter 4: Gas-phase modeling of GBT Molecules in TMC-1 and Sgr B2.......... 49 4.1 Observational results ............................................................................. 49 4.1.1 Molecules in TMC-1 ........................................................................ 50 4.1.2 Molecules towards Sgr B2 .............................................................. 51 4.2 Synthesis of detected molecules ........................................................... 52 4.2.1 Synthesis of observed molecules in TMC-1 .................................... 60 viii 4.2.2 Synthesis of observed molecules towards Sgr B2(N) ..................... 64 4.3 Modeling results and comparison with observances ............................. 67 4.3.1 TMC-1 ............................................................................................. 67 4.3.2 Halo of Sgr B2(N) ............................................................................ 73 4.4 Conclusion ............................................................................................. 75 4.5 References ............................................................................................ 77 Chapter 5: Interstellar Abundance of Molecular Oxygen .................................... 79 5.1 Background of O2 problem in cold cores ............................................... 79
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