Feedback from Winds and Supernovae in Massive Stellar Clusters
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Feedback from Winds and Supernovae in Massive Stellar Clusters Hazel Claire Rogers Department of Physics and Astronomy University of Leeds Submitted in accordance with the requirements for the degree of Doctor of Philosophy September 2013 ii The candidate confirms that the work submitted is her 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 2013 The University of Leeds and Hazel Rogers. For my parents, Phil and Annette Rogers. Preface Within this thesis, some chapters have been based on work presented in the following jointly authored publications: I. “Feedback from Winds and Supernovae in Massive Stellar Clus- ters. I: Hydrodynamics”, H.Rogers and J.M.Pittard, 2013, MNRAS, 413,1337. II. “Feedback from Winds and Supernovae in Massive Stellar Clus- ters. II: X-Ray Emission”, H.Rogers and J.M.Pittard, 2013, in prep. III. “Feedback from Winds and Supernovae in Massive Stellar Clus- ters. III: Radio Emission”, H.Rogers and J.M.Pittard, 2013, in prep. Paper I forms the basis of Chapter 2. The hydrodynamical code used to perform the simulations in this Chapter was written and developed by J.M.Pittard, with the input background conditions for the GMC provided by E.Vasquez-Semadeni. The primary author (H.Rogers) was responsible for small modifications to the code to set up a cluster of stars contained within a GMC clump surrounded by homogeneous material. The primary author wrote the initial draft of the publica- tion, and then incorporated comments from co-authors in the final draft. Paper II forms the basis of Chapter 3. The simulations and analysis were carried out by the primary author using the radiative transfer X- ray tracing code provided by J.M.Pittard.The primary author wrote the initial draft of the publication, and then incorporated comments from co-authors in the final draft. Paper III forms the basis of Chapter 4. The simulations and analysis were carried out by the primary author using the radiative transfer radio tracing code provided by J.M.Pittard.The primary author wrote the initial draft of the publication, and then incorporated comments from co-authors in the final draft. Acknowledgements I would like to thank Julian Pittard for his supervision and guidance over the past few years. His constant patience and encouragement have been instrumental in this thesis coming together. I have really enjoyed working with you. I would also like to thank Tom Hartquist for his input and wisdom over the years. My thanks also go to the other PhD students, who over the past four years have put up with me bossing them around. Special thanks must go to John and Mo who had to put up with me in a small confined space for a year. More recently, thank you to Jonny, Luke and Robbie for putting up with my constant monologuing and various states of panic. I would also like to give special thanks to Jim and Jo. You are two of the most generous people I know, and I’ll always be grateful for your help when I needed it the most. I would like to say a huge thank you to Kate, Jess, Josie, Faith and Ingrid. Not only have you helped me out and kept me sane (sort of), you have been a constant source of inspiration and laughter. Sarah, you have been an amazing friend for over four years, and I will always appreciate your generosity, patience and bubbly northern humour. I probably couldn’t have done this without you. I would also like to thank my family for all their support over these years. Sam, Ben, Aunty Lesley, Aunty Helen and Uncle Pete, thank you all so much for inspiring me and keeping me going. You’ve helped me more than you know. Finally, I’d like to thank my parents for their unequivocal support and love. Your constant encouragement and belief in me has got me to where I am today. Abstract This thesis contains a study of the mechanical feedback from winds and supernovae on inhomogeneous molecular material left over from the formation of a massive stellar cluster. Firstly, the mechanical input from a cluster with three massive O-stars into a giant molec- ular cloud (GMC) clump containing 3240 M⊙ of molecular material within a 4 pc radius is investigated using a 3D hydrodynamcial model. The cluster wind blows out of the molecular clump along low-density channels, into which denser clump material is entrained. The dens- est regions are surprisingly resistant to ablation by the cluster wind, in part due to shielding by other dense regions close to the cluster. Nonetheless, molecular material is gradually removed by the cluster wind during which mass-loading factors in excess of several hundred are obtained. Because the clump is very porous, 60-75 % of the in- jected wind energy escapes the simulation domain. After 4.4 Myrs the massive stars in the simulation start to explode as supernovae. The highly structured environment into which the SN energy is re- leased allows even weaker coupling to the remaining dense material and practically all of the SN energy reaches the wider environment. Secondly, the X-ray emission from the simulated stellar cluster is pre- sented. The GMC clump causes short–lived attenuation effects on the X-ray emission of the cluster. However, once most of the mate- rial has been ablated away by the winds the remaining dense clumps do not have a noticable effect on the attenuation compared with the assumed interstellar medium (ISM) column. The evolution of the X-ray luminosity and spectra are presented, and synthetic images of the emission are generated. The X-ray luminosity is initially high whilst the winds are “bottled up”, but reduce to a near constant value once the GMC clump has been mostly destroyed. The lumi- nosity decreases slighly during the red supergiant phase of the stars due to the depressurization of the hot gas. However, the luminos- ity dramatically increases during the Wolf-Rayet stage of each star. The X-ray luminosity is enhanced by 2-3 orders of magnitude for at least 466 yrs after each supernova explosion, at which time the blast wave leaves the grid. Comparisons between the simulated cluster and both theoretical models and observations of young stellar clusters are presented. Thirdly, the radio emission from the simulated cluster is presented. Similar to the X-ray emission, the thermal radio emission is intially high when the winds are confined in the GMC clump and reduce as the material is ablated away. The evolution of the radio flux density and spectra are presented, and synthetic images of the emission are generated. The radio emission is compared with the X-ray results throughout the evolution of the cluster. The flux density increases during the RSG phase, and remains high during the WR phgase of the stars. The radio flux density is enhanced by three orders of magni- tude during the first supernova explosion. Comparisons between the simulated cluster and observations of young stellar clusters are made. Finally, a preliminary investigation of the interaction of stellar winds within a massive cluster are presented. The hydrodynamcial simula- tions examine the energy and mass input of a stellar cluster into the ISM. ix Abbreviations GMC Giant Molecular Cloud IMF Initial Mass Function ISM Interstellar Medium MS Main Sequence MSFR Massive Star Forming Region RSG Red Supergiant RT Rayleigh-Taylor SFR Star Formation Rate SN Supernova SSC Super-Star Cluster WR Wolf-Rayet x Contents 1 Introduction 1 1.1 StarFormationandGMCs. 3 1.1.1 MassiveStarFormation . 5 1.1.2 Initial Mass Function and Mass Segregation in Clusters . 8 1.2 StellarFeedback............................ 11 1.2.1 Winds ............................. 12 1.2.2 Photoionization . 26 1.2.3 Supernovae .......................... 28 1.2.4 Triggering of Star Formation . 30 1.3 Multi-Wavelength Analysis of Feedback in Stellar Clusters . 32 1.3.1 X-rayEmission ........................ 32 1.3.2 RadioEmission ........................ 35 1.3.3 Multi-Wavelength Observations of Feedback in Stellar Clus- ters............................... 38 1.4 AnIntroductiontothisThesis. 45 2 Mechanical Feedback from Winds and Supernovae in a Massive Young Stellar Cluster 47 2.1 Introduction.............................. 47 2.2 Simulations of Stellar Feedback . 50 2.2.1 NumericalSetup ....................... 50 2.2.2 StellarEvolution . 52 2.2.3 InitialConditions . 53 2.2.4 Neglected Processes and Simplifications . 55 2.3 Results................................. 57 xii CONTENTS 2.3.1 Initial Blowout . 57 2.3.2 Evolution during the star’s MS stage . 60 2.3.3 Later evolutionary stages . 64 2.3.4 WindVelocity......................... 71 2.3.5 Mass and energy fluxes into the wider environment . 72 2.3.6 Evolution of column densities . 77 2.3.7 Comparison of SimA and SimB and evolution of the molec- ularmass ........................... 83 2.4 Conclusion............................... 88 3 X-ray Emission from a Massive Young Stellar Cluster 91 3.1 Introduction.............................. 91 3.2 Simulations .............................. 94 3.2.1 TheNumericalModel . 94 3.2.2 Modelling the X-ray Emission and Absorption . 95 3.3 Results................................. 96 3.3.1 TheMainSequencePhase . 96 3.3.2 ISMAbsorptionEffects. 101 3.3.3 RSG and WR Phases for the 35 M⊙ Star .......... 102 3.3.4 TheFirstSupernova . 104 3.3.5 Further Evolutionary Stages .