Kinematics and physical properties of a highly filamentary Infrared Dark Cloud Jonathan David Henshaw School of Physics and Astronomy University of Leeds Submitted in accordance with the requirements for the degree of Doctor of Philosophy May 2014 ii The candidate confirms that the work submitted is his own, except where work which has formed part of jointly authored publications has been included. The contribution of the candidate and the other authors of this work has been explicitly indicated. The candidate confirms that appropriate credit has been given within this thesis where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. c 2014 The University of Leeds and Jonathan David Henshaw. For my parents Preface Within this thesis, chapters 2 and 3 have been based on work presented in the following jointly authored publications: I. Complex, quiescent kinematics in a filamentary infrared dark cloud, J. D. Henshaw, P. Caselli, F. Fontani, I. Jim´enez-Serra, J. C. Tan, A. K. Hernandez, MNRAS, 428, p.3425-3442. II. The dynamical properties of dense filaments in the Infrared Dark Cloud G035.39-00.33, J. D. Henshaw, P. Caselli, F. Fontani, I. Jim´enez-Serra, J. C. Tan, MNRAS, 440, p.2860-2881. Paper i forms the basis of chapter 2. The data were obtained with pro- posals written by P. Caselli. The data were collected by P. Caselli, F. Fontani, and I. Jim´enez-Serra. The primary author (J. D. Henshaw) was responsible for the data analysis and interpretation. The anal- ysis involved the development of a technique required to decompose and analyse the spectra. The information extracted from this analysis technique was then used to comprise the primary results of this pa- per. Further analysis incorporated the use of radiative transfer code, radex, written by F. F. S. Van der Tak. The primary author wrote the initial draft of the publication, and then incorporated comments from co-authors in the final draft. Paper ii forms the basis of chapter 3. The data were obtained with pro- posals written by the primary author (J. D. Henshaw) and P. Caselli. The primary author was responsible for the data reduction, analysis (including the development of the fitting routine used), and interpre- tation. The primary author wrote the initial draft of the publication, and then incorporated comments from co-authors in the final draft. Acknowledgements First and foremost, I would like to express my gratitude to my su- pervisor Paola Caselli, for being so generous with her time, patience, knowledge, and for regularly encouraging me to step out of my com- fort zone throughout this process. It has been (and continues to be) a pleasure to work with Paola, and a fantastic group of researchers whose knowledge and experience has enriched my research. I would particularly like to thank Izaskun Jim´enez-Serra, Francesco Fontani, and Jonathan Tan for their help and guidance, and for taking the time to read countless drafts of papers and proposals. I would also like to thank Jaime Pineda and Cormac Purcell for their assistance, particu- larly in the earliest stages of my PhD. I am grateful to my examiners, Gary Fuller and Tom Hartquist, for taking the time to read my thesis, and for making my viva an enjoyable experience. Having been a tutee of Tom during my undergraduate degree, I am extremely proud to have completed my PhD with Tom as my examiner. Thanks must also go to the staff at Leeds, Melvin Hoare, Stuart Lumsden, Ren´eOudmaijer, and Julian Pittard for sharing their knowledge over the years I have spent at Leeds. A warm thank you goes to my fellow PhD students and the postdoc- toral researchers at the University of Leeds. I thank in particular Mo Ali, Robbie Al¯uzas, Tom Douglas, John Fairlamb, Dan Parsons, Hazel Rogers, and Hugh Wheelwright for making Leeds such an enjoyable place to work. A special thank you goes to my good friends Nichol Cunningham, John Ilee and Luke Maud for always being there for a pint when things got tough (and for those other times, when things were not so tough and we went to the pub anyway). I am extremely fortunate to have to love and support of my wonderful family and friends. Thank you to my grandparents for always being interested in what I do, it is a constant source of encouragement to know that it makes you proud. To my brother Alex and sister-in-law Gemma, thank you for having been through this experience before me, understanding, and for offering advice whenever I needed it. To Grace, thank you for being so selfless, and for so often picking me up, dusting me off, and sending me back off to work in those last few months. Finally, I would like to thank my parents, Chris and Margaret, for always believing in me, even when I don’t always believe in myself. Abstract This thesis contains a detailed study of the kinematics and physical properties of a potential site of massive star formation; the IRDC G035.39-00.33. The gas kinematics are first of all investigated using high-spectral res- olution and high-sensitivity data from the IRAM 30 m telescope. The + primary focus of this work is the J = 1 → 0 transition of both N2H 18 + and C O, as well as N2H (3 − 2). Dense gas is found to be extended over ∼ 3 pc scales within G035.39-00.33. The C18O observations con- firm the presence of at least three morphologically distinct filamentary components. It is speculated that the merging of filaments may be re- sponsible for the formation of localised density enhancements at their interface; the potential sites for massive star and star-cluster formation. The kinematic properties of the dense gas are then probed at high- + angular resolution, using observations of N2H (1−0) from the Plateau de Bure Interferometer. It is revealed that the dense gas of G035.39- 00.33 is organised into a complex network of mildly supersonic filaments separated in velocity by < 1 km s−1. Whilst global velocity gradients throughout each filament are small, there is evidence for dynamic pro- cesses on local scales. This suggests that the kinematics are influenced by the dense (and in some cases, starless) cores. The physical properties of the embedded core population are derived in the final study of this thesis. A total of 14 continuum peaks are identified, representative of the pre- and protostellar core population covering two main clumps within G035.39-00.33. The derived core masses are found to be between 2.4-12.3 M⊙, with sizes and densities between 0.03-0.07 pc and 1.6×105-7.3×105, respectively. Some of the cores exhibit irregular boundaries, which may imply the presence of unresolved sub-structure. Although the dynamical state of each core is dependent on both its geometry and density profile (which are both sources of uncertainty) it is found that many of the identified cores are unstable to collapse. Cores which are well represented by mono- lithic, centrally condensed structures, exhibiting low virial parameters and many Jeans masses, are good candidates for the progenitors of intermediate-to-high-mass stars. Within the selected area of G035.39- 00.33, two of the identified cores meet this criteria. ix Abbreviations ALMA Atacama Large Millimeter Array arcsec Arc Second CMZ Central Molecular Zone EMIR Eight MIxer Receiver FWHM Full-Width at Half-Maximum GMC Giant Molecular Cloud HFS Hyperfine Structure HMSC High-Mass Starless Core HWHM Half-Width at Half-Maximum IDL Interactive Data Language IMF Initial Mass Function IRAM Institut de Radioastronomie Millim´etrique IRAS Infrared Astronomical Satellite IRDC Infrared Dark Cloud ISM Interstellar Medium ISO Infrared Space Observatory LTE Local Thermodynamic Equilibrium MIR Mid-Infrared MSC Massive Starless Core MSX Midcourse Space Experiment MYSO Massive Young Stellar Object NIR Near-Infrared NOEMA Northern Extended Millimetre Array PACS Photodetector Array Camera & Spectrometer pc Parsec x PdBI Plateau de Bure Interferometer PPV Position-Position-Velocity PV Position-Velocity RMS Red MSX Source SMA Submillimeter Array SPIRE Spectral and Photometric Imaging Receiver VESPA VErsatile SPectrometer Assembly Contents 1 Introduction 1 1.1 An introduction to molecular clouds . 3 1.1.1 The structure of molecular clouds . 4 1.1.2 Formation mechanisms of molecular clouds . 12 1.2 Anintroductiontostarformation . 19 1.2.1 Virialized vs. transient clouds . 20 1.2.2 The stellar initial mass function & low-mass star formation . 23 1.2.3 Massivestarformation . 25 1.3 TheroleofInfraredDarkClouds . 33 1.3.1 Physicalstructure. .. .. 33 1.3.2 Kinematics ........................... 36 1.3.3 Chemistry............................ 37 1.4 IRDCsummary&thesismotivation. 39 2 Complex kinematics in a filamentary IRDC 41 2.1 Introduction............................... 41 2.1.1 G035.39–00.33.......................... 41 2.1.2 Previouswork.......................... 43 2.1.3 Investigating the large-scale structure of G035.39–00.33 . 46 2.2 Observations&dataprocessing . 49 2.3 Observationalresults .. .. .. 53 2.3.1 Averagespectraandintegratedintensity . 53 2.3.2 Opticaldepth .......................... 62 2.4 Analysis: The physical properties of G035.39–00.33 . 67 xii 2.4.1 Columndensity ......................... 67 2.4.2 Number density and kinetic temperature . 70 2.5 Analysis: The kinematics of G035.39–00.33 . 81 2.5.1 Evidence for multiple velocity components . 81 2.5.2 Position-velocity analysis . 84 2.5.3 Centroid velocity and line-width . 87 + 18 2.5.4 The N2H –C Ovelocityshift................. 91 2.6 Discussion................................ 97 2.6.1 Are the filaments of G035.39–00.33 interacting? .