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Non-Thermal Emission from Jets of Massive Protostars

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Non-Thermal Emission from Jets of Massive Protostars Non-Thermal Emission From Jets of Massive Protostars Willice Odhiambo Obonyo Department of Physics and Astronomy University of Leeds A thesis submitted for the degree of Doctor of Philosophy March 2020 Acknowledgements First and foremost, my gratitude goes to God, the Almighty Father, for granting me this rare opportunity, and for the wisdom, peace of mind, good health and strength to conduct the research. Many thanks to Stuart, a great mentor, a patient and supportive ad- visor whose door was always opened. You allowed me to experiment, make mistakes and learn. I still wonder how you never got tired of reading my many drafts of write-ups, sometimes giving me feedback earlier than I expected. You are a blessing, Stuart. To Melvin, I cant thank you enough. Besides your many challenging questions during supervision meetings, your project, DARA (Devel- opment in Africa through Radio Astronomy) financed my studies and upkeep in the UK. Special thanks to you and the DARA project. A big thank you also goes to Trish for her support. Julian, without your hydrodynamics code, this thesis is incomplete. Your advice on the use and understanding of the code were invaluable. Thank you very much. To all my colleagues, past and present, as well as other staff members, thank you very much for the constant support and encouragements. Simon Purser, my unofficial fourth supervisor and football captain, accept my deepest thanks and appreciation. I know I asked you very many questions during my first year of PhD. Finally, to my family, you were always by my side, encouraging me and providing fun moments. It is difficult to express the deep love I have for you. Carol, I cannot thank you enough for taking good care of the family while I was away. Min Awil, your prayers and wise counsel kept me going. Wuon Awil, thank you for the support and constant encouragement. Lucas Obonyo Junior, Adushe Manuar, thank you for the friendship, humorous conversations, and prayers. Brent and Oliver, thanks for the exciting video conversations. Many thanks and God bless you all. Abstract This thesis details the outcome of a study of non-thermal radio emis- sion from a sample of massive protostellar jets. The jets are ionized and can interact with magnetic fields within their vicinity to generate non-thermal emission. A search for the emission was conducted on a sample of fifteen massive protostars, observed using the JVLA at 1.5 GHz in 2015. Emission from the objects was characterised based on their spectral indices and spectral index maps, generated from their 1.5 GHz (L-band) data, and previous observations at 6.0 GHz (C-band) and 44.0 GHz (Q-band). 40% of the jets have characteristic non-thermal lobes, associated with synchrotron emission, especially sources of higher bolometric luminosity. All the cores, on the other hand, were found to be thermal, driving out material at rates that lie −7 −6 −1 in the range 3 × 10 to 7 × 10 M yr . Spectral energy distributions (SEDs) and spectral index maps of some of the protostellar jets displayed evidence of variability. As a result, four protostellar jets, previously observed in 2012 at C-band were identified and re-observed in 2018, at the same frequency, to study their variability property. Two of the protostars, S255 NIRS3 and W3 IRS5, displayed significant flux and positional variabilities re- spectively. S255 NIRS3 was in outburst while W3 IRS5 manifested clear proper motions and precession. Lastly, some of the observable properties of the protostellar cores were used in simulating their features. Numerical calculations of their hy- drodynamic properties were performed, followed by computation of their spectra using a ray-tracing code. Examples of the properties that were used to initiate the hydrodynamic simulations include; mass-loss rates, velocities and opening angles. The radio emission of the cores were generally found to be stable unless the input parameters were varied. Indeed, a change in the velocities and mass-loss rates of the thermal jets led to a corresponding change in their fluxes. Abbreviations ARWEN Astrophysical Research With Enhanced Numerics CGS centimetre-gram-second system CASA Common Astronomy Software Application FWHM Full Width Half Maximum GMC Giant Molecular Cloud HH Herbig-Haro HMPO High Mass Protostellar Object HPBW Half Power Beamwidth IMF Initial Mass Function IRDC Infrared Dark Cloud JVLA Karl G. Jansky Very Large Array LOFAR Low Frequency ARray MHD Magneto-hydrodynamics MSX Midcourse Space eXperiment MYSO Massive Young Stellar Object NIR Near-Infrared NRAO National Radio Astronomy Observatory RMS Red MSX Source (survey) SED Spectral Energy Distribution SMH Smooth Particle Hydrodynamcis SNR Signal-to-Noise Ratio UCHII Ultra compact HII region UKIDSS UKIRT Infrared Deep Sky Survey UKIRT United Kingdom Infrared Telescope WFCAM Wide Field Camera ZAMS Zero Age Main Sequence Contents 1 Introduction1 1.1 Star Formation Sites . .2 1.1.1 Molecular Clouds . .2 1.1.2 Giant Molecular Clouds . .4 1.2 Star Formation . .6 1.2.1 Low Mass Star Formation . .9 1.2.2 Massive Star Formation . 12 1.2.2.1 Radiation Pressure Problem . 13 1.2.2.2 Fragmentation Problem . 14 1.2.3 Models of Massive Star Formation . 14 1.2.3.1 Core Accretion (Monolithic Collapse) Model . 14 1.2.3.2 Competitive Accretion . 16 1.2.3.3 Mergers and Collisions . 17 1.2.4 Evolutionary Stages in High Mass Star Formation . 18 1.3 Jets and Outflows . 19 1.3.1 Introduction . 19 1.3.2 Roles of Protostellar Jets . 22 1.3.3 Properties of Jets . 23 1.3.3.1 Size and Proper Motion . 23 1.3.3.2 Opening Angles . 23 1.3.3.3 Rotation and Precession . 24 1.3.3.4 Variability . 24 1.3.3.5 Radio Emission From Jets . 27 1.3.4 Models of Protostellar Jets . 28 i CONTENTS 1.3.4.1 Radiatively driven jets . 29 1.3.4.2 Magnetically Driven Jets . 30 1.4 Instruments and Techniques of Radio Astronomy . 31 1.5 Thesis Outline . 35 2 Non-Thermal Emission From Massive Protostars 37 2.1 Introduction . 37 2.2 Sample Selection . 40 2.3 Observations and Data Reduction . 41 2.3.1 Observation . 41 2.3.2 Calibration and Imaging . 43 2.4 Infrared Emission . 47 2.5 Results and Discussion . 48 2.5.1 L-Band Results . 48 2.5.2 Spectral Indices and Maps of the Sources . 55 2.5.2.1 Spectral Indices of the Cores . 57 2.5.2.2 Spectral Indices of Lobes . 57 2.5.2.3 Magnetic Field Strength of the Jet Lobes . 58 2.5.3 Nature of the Cores and Their Radio Luminosity at L-Band 68 2.5.3.1 Relationship Between Infrared Colour and Bolo- metric Luminosity . 68 2.5.3.2 Relationship Between L-Band Radio Luminosity and Bolometric Luminosity . 68 2.5.4 Derived Properties of the Radio Jets . 70 2.5.4.1 Mass Loss Rate . 70 2.5.4.2 Accretion Rate . 73 2.5.4.3 Ejection Radius . 74 2.6 Discussion of Individual Objects . 75 2.6.1 G083.7071 . 75 2.6.2 G094.2615 . 75 2.6.3 G094.4637 . 77 2.6.4 G094.6028 . 78 2.6.5 G103.8744 . 79 ii CONTENTS 2.6.6 G108.5955C . 80 2.6.7 G110.0931 . 80 2.6.8 G111.2552 . 82 2.6.9 G111.5671 . 82 2.6.10 G114.0835 . 83 2.6.11 G126.7114 . 85 2.6.12 G136.3833 . 85 2.6.13 G138.2957 . 85 2.6.14 G139.9091A . 87 2.6.15 G141.9996 . 87 2.7 Conclusions . 89 3 Multi-Epoch Study of Massive Protostars 91 3.1 Introduction . 91 3.2 Observation and Data . 92 3.2.1 Source Selection . 92 3.2.2 VLA Observation . 93 3.3 Results . 96 3.3.1 Continuum Emission . 96 3.3.2 Radio Sources . 97 3.3.2.1 G133.7150+01.215 . 97 3.3.2.2 G173.4839+02.4317 . 105 3.3.2.3 G192.6005-00.0479 . 109 3.3.2.4 G196.4542−01.6777 . 118 3.3.3 Mass Loss Rates . 121 3.4 Conclusion . 121 4 Radio Emission From Jets of Massive Protostars 125 4.1 Introduction . 125 4.2 Description of Jets . 126 4.3 Numerical Hydrodynamic Simulations . 128 4.3.1 Introduction . 128 4.3.2 Equations of Hydrodynamics . 129 4.3.2.1 Conservation of Mass . 131 iii CONTENTS 4.3.2.2 Conservation of Momentum . 131 4.3.2.3 Conservation of Energy . 132 4.3.2.4 Material Derivatives . 133 4.3.3 The Lagrangian Forms of the Hydrodynamics Equation . 135 4.3.3.1 Mass Conservation Equation . 135 4.3.3.2 The Momentum Conservation Equation . 135 4.3.3.3 The Equation of Conservation of Energy . 136 4.3.4 Numerical Methods . 136 4.3.4.1 The Finite Difference Method . 136 4.3.4.2 The Finite Volume Method . 137 4.4 The Numerical Simulation Code - ARWEN . 139 4.4.1 The Courant-Friedrichs-Lewy (CFL) Condition . 140 4.4.2 Boundary Conditions . 141 4.4.3 Ionization, Heating and Cooling . 141 4.4.4 Testing ARWEN . 143 4.4.4.1 Analytic Approximation . 143 4.4.4.2 Comparing Outputs From ARWEN and ZEUS- 2D Codes . 144 4.5 Initial Conditions: Numerical Input Parameters . 147 4.5.1 Mass-Loss Rates . 148 4.5.2 Lifespan . 148 4.5.3 Opening Angle . 149 4.5.4 Length . 149 4.5.5 Velocity . 150 4.6 Results: A Steady Jet . 150 4.7 Radio Emission From the Simulated Jets .
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