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The Impact of Protostellar Feedback on Astrochemistry University of Massachusetts Amherst ScholarWorks@UMass Amherst Doctoral Dissertations Dissertations and Theses October 2019 The Impact of Protostellar Feedback on Astrochemistry Brandt Gaches University of Massachusetts Amherst Follow this and additional works at: https://scholarworks.umass.edu/dissertations_2 Part of the Stars, Interstellar Medium and the Galaxy Commons Recommended Citation Gaches, Brandt, "The Impact of Protostellar Feedback on Astrochemistry" (2019). Doctoral Dissertations. 1721. https://doi.org/10.7275/14754153 https://scholarworks.umass.edu/dissertations_2/1721 This Open Access Dissertation is brought to you for free and open access by the Dissertations and Theses at ScholarWorks@UMass Amherst. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contact [email protected]. THE IMPACT OF PROTOSTELLAR FEEDBACK ON ASTROCHEMISTRY A Dissertation Presented by BRANDT A. L. GACHES Submitted to the Graduate School of the University of Massachusetts Amherst in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY September 2019 Astronomy c Copyright by Brandt Gaches 2019 All Rights Reserved THE IMPACT OF PROTOSTELLAR FEEDBACK ON ASTROCHEMISTRY A Dissertation Presented by BRANDT A. L. GACHES Approved as to style and content by: Ron Snell, Chair Stella Offner, Member Mark Heyer, Member Bret Jackson, Member Karin Oberg,¨ Member D. Calzetti, Department Chair Astronomy DEDICATION To my amazing family who have provided endless support over the years, and to my faithful companion of 9 years, through thick and thin, a true best friend, Mojo the beagle. EPIGRAPH \The value of ζ for any species is a function of depth into an interstellar cloud, although this dependence is most frequently ignored because it is difficult to calculate." - V. Wakelam 2013, Astrochemistry: Synthesis and Modelling ACKNOWLEDGMENTS This thesis would not have been possible without the incredible support of my dis- sertation adviser, Prof. Stella Offner. My most sincere gratitude goes to her for her patience, guidance, and advice throughout my time as her graduate student. She has provided a huge amount of time during my tenure as her student to support my re- search, both as a collaborator and a mentor, and in the preparation of papers which have been accepted for publication. Her mentoring has led to great improvement in every aspect of academic life. She has continuously encouraged attending inter- national conferences and applying for external funding sources, which through her mentoring, have funded the majority of my conference trips. I am eternally grateful to have such a fantastic adviser, and I am anticipating a long-lasting collaboration. I am extremely grateful to the many collaborators and colleagues who have assisted me throughout my graduate career. I acknowledge the support of my fellow graduate students, Dr. Kathryn Grasha, Dr. Andrew Battisti, Riwaj Pokhrel, Duo Xu, and Dr. Yao-Lun Yang, and collaborators Dr. Anna Rosen and Dr. Aaron Lee for assisting with everything from brainstorming to coding advice to career advice and personal support. I wish to thank my long time collaborator, Dr. Thomas Bisbas, for his continued support and help through many early-morning/late-night telecons. I am incredibly grateful to my undergraduate mentors Prof. Romeel Dav´e,Prof. Phil Pinto and Prof. Daniel Eisenstein for providing the opportunity to develop coding skills that were necessary for dissertation work. I wish to thank Prof. Neal Evans for his continued advice and help, particularly in guiding new observational proposals. The work presented in this dissertation was supported by the National Science Foundation (NSF) grant AST-1510021 and the Massachusetts Space Grant Consor- vi tium (MSGC). I acknowledge support from the American Astronomical Society and the University of Massachusetts Department of Astronomy Mary Dailey Irvine travel grant for support allowing me to attend numerous international and domestic con- ferences. The work presented in this thesis utilized the Massachusetts Green High Performance Computing Center (MGHPCC) in Holyoke, Massachusetts supported by the University of Massachusetts, Boston University, Harvard University, MIT, Northeastern University and the Commonwealth of Massachusetts. I also wish to acknowledge important software packages which were essential to this dissertation: Matplotlib, NumPy, SciPy, JupyterLab, Plotly, 3d-pdr, Radmc-3d and Orion2. vii ABSTRACT THE IMPACT OF PROTOSTELLAR FEEDBACK ON ASTROCHEMISTRY SEPTEMBER 2019 BRANDT A. L. GACHES B.S., UNIVERSITY OF ARIZONA Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Professor Ron Snell Star formation is the process by which gas within a galaxy is converted into stars. In the Milky Way, star formation occurs in molecular clouds, regions of the interstel- lar medium in which hydrogen exists in its molecular form, H2. However, molecular hydrogen is a perfectly symmetric molecule, rendering it largely invisible at the tem- peratures of molecular clouds. Therefore, studies of star formation rely on tracers of the gas mass. Molecular line emission at sub-millimeter and millimeter wavelengths from rotational transitions provides crucial information into the dynamical state of the gas through Doppler shifts in the line. There are currently over 200 molecules which have been detected in the interstellar medium, ranging from simple diatomic molecules, such as cyanide (CN) and carbon monoxide (CO), to complex organic molecules, such as methyl acetate (CH3COOCH3). Carbon monoxide is among the most important molecules for astronomers: it is the most abundant molecule after H2 making it ubiquitous in molecular clouds. Emission from CO is used to study viii the dynamics of molecular clouds, and thus star formation, from locally star forming regions in the Milky Way to galaxies in the early universe. As such, understanding the chemistry of the molecular phase of the interstellar medium is vital to interpreting physics form molecular line emission. In the first part of the dissertation, we present a statistical study quantifying how emission from different molecules changes the inferred properties of turbulence. We post-process a simulation of a molecular cloud with an astrochemical code providing the spatial distribution of over 200 molecules. We perform synthetic observations for a subset of these molecules, those of particular importance, to generate predic- tions for the molecular line emission. Having synthetic observations of many different molecules for the same simulated molecular cloud allows us to directly quantify how the appearance of turbulence changes with the tracer used. We find three differ- ent groupings of molecules which trace similar density regimes: diffuse, intermediate and dense densities. We show that the turbulence traced by neutral carbon emis- sion is statistically consistent with that traced by carbon monoxide. We conclude that turbulence appears quite different when traced by a variety of molecules. The physics derived through molecular emission is thus observed through a chemical lens. Understanding this lens allows a proper inference of physics through the chemistry. In the second part, we present our methodology for generating synthetic pro- toclusters using semi-analytic accretion models. We calculate the bolometric, far ultraviolet and ionizing luminosities of protoclusters using different accretion models. We find that the Tapered Turbulent Core model best explains the observed bolomet- ric luminosities of clusters in Milky Way star forming regions. We generate synthetic protoclusters using the Tapered Turbulent Core model for clusters with a wide range of constituent numbers, from 102 to 106 protostars and a range of star formation efficiencies. We use the average far ultraviolet luminosities of these clusters as input into astrochemical models of molecular clouds. We conclude that far ultraviolet radi- ix ation has a small effect on the total emission from carbon monoxide, but significantly changes thermo-chemistry of dense gas and the emission from optically thin species. The third part of the thesis focuses on the possibility of cosmic ray acceleration in protostellar accretion shocks. We use semi-analytic accretion models to calculate the acceleration of cosmic rays in accretion shocks on the surface of protostars. We find that protostars can accelerate cosmic ray protons to energies of 10 - 20 GeV in their accretion shocks. Our protostellar cosmic ray models explain the increased ionization found in the protocluster OMC-2 FIR 4. We calculate the attenuation of protostellar cosmic rays through their natal cores and the resulting ionization rate gradient. We generate synthetic protoclusters and conclude that the cosmic rays accelerated by embedded protostars can exceed that of external sources for clusters hosting more than a few hundred protostars. We extend a photo-dissociation region astrochemistry code to include cosmic ray attenuation in-situ. We use the modified code to quantify the impact of cosmic ray attenuation, different external spectra, and the inclusion of embedded sources on molecular cloud chemistry. We find that embedded sources significantly alter the chemistry of molecular clouds, although the H2 is left unaltered. Molecular ions, such as HCO+ are enhanced while neutral species, such as carbon monoxide and ammonia, are reduced by orders of magnitude. Furthermore, the abundance of atomic carbon
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