Realistic models of star cluster evolution: Bridging the gap between theory and observations Anna Catharina Sippel Presented in fulfillment of the requirements of the degree of Doctor of Philosophy August 21, 2014 Faculty of Science, Engeneering and Technology Swinburne University i Abstract This thesis focuses on the evolution of star clusters within the global picture of galaxy evolution, in particular the dynamical evolution of globular clusters and how this affects the way they are observed. The high density of globular clusters allows us to use them as laboratories of stellar evolution and stellar interactions: even though they are old, they are still dynamically active. As they are in general over 10 billion years old, they witnessed the formation and evolution of their host galaxy, and with observations we can only study them in their current, evolved state. In this thesis, such observations are complemented by direct, star-by-star (so-called direct N-body) simulations of cluster evolution and new methods for the analysis are presented to facilitate the comparison between theory and observations. I introduce star clusters in general, key information about star cluster and stellar evolution, as well as the approach of N-body modelling. The bulk of the thesis is about the impact of metallicity on cluster evolution: the chemical composition affects stellar evolution and hence mass-loss rates and remnant masses of cluster members. Globular cluster systems emerge in two sub-populations: metal-poor (appearing blue) and metal- rich (appearing red), with the red clusters more centrally concentrated within their host galaxy. In addition, the blue globular clusters are on average ∼ 20% larger in size than the red ones. I show that while clusters of different metallicity are structurally identical, an apparent size difference is found owing to the combined effects of metallicity and mass segregation. These findings are followed up with a study on the effects of metallicity on the evolution of cluster colour over time, with a particular focus to disentangle internal and external effects. An example is the impact of the preferential removal of low-mass stars in the cluster outskirts on cluster colour. I show that the cluster colour is driven by the giants in the cluster, and that in terms of colour, the cluster is rather unaware of the removal of low-mass stars. Before concluding the thesis, I show that two recently discovered black hole candidates in the Milky Way cluster M22, the first black holes detected in a Milky Way globular cluster, are no surprise from a dynamical point of view. While these findings can only be applied to other clusters with exceptionally large core radii like M22, I predict that the evaporation of remnant black holes is happening on a slower rate than previously thought. The model created and used for this study includes more than 250 000 stars and is currently the largest (and ultimately one of the most realistic) direct N-body model of a globular cluster. ii iii Acknowledgments My thank goes to my fantastic supervisor Jarrod Hurley, without whom this project would not have been possible. Not only did the idea for this thesis come from him, but also he was very patient with me and my crazy ideas. He gave me the freedom to explore globular clusters from my point of view, travel, and carry out many outreach projects for which I am very grateful. I thank my other supervisors for their support, in particular Marie Martig, who man- ages to stay calm no matter how crazy life gets, to an extent that even rubs off on me, and George Hau, who welcomed me in Santiago. This thesis would not have been possible without the fantastic supercomputers at Swinburne. I am truly grateful to Gin Tan and Simon Forsayeth for their effort in keeping them up and running. Many thanks also go to my PhD brother Juan for endless discussions and group meetings at the bakery, which Guido and Luca later joined as well. Also thanks to Bil and Max for being the best office mates. With travels all around the world I am thankful to my amazing friends who have opened their homes to me: Feli and Jenny, Marie and Vincent, Christina and Michael, Felipe and Sheila with Rafa in Melbourne as well as Caro and Frick and Maria in Santiago and Anna back home. I am super happy to have met my chicas during the last four years: Becky, Feli, Christina, Marie, Julija, Maria, Rebekka, Amy, Cathy, Liz, Rebeca and of course Joanne and Anna, who has been there all along. The hours spent drinking coffee with you can't be replaced by anything in the world! I couldn't imagine my PhD without countless outreach projects, and I am happy I could show so many AstroTours at Swinburne. Mostly however I thank Joanne and Bill at the School of the Air in Alice Springs for \adopting" me as their Astronomer. I have joined many classes over web cam and am grateful that Scientists in Schools made a visit possible. To work with many students from all over the Northern Territory has been a truly enlightening experience. My thanks also go out to all the kids and students around the world that keep asking me amazing questions through various channels, you are truly inspiring! Finally, I want to thank my family for all their support in my life all around the globe and of course Javier for his endless encouragement, patience, help and simply being there. iv v Declaration The work presented in this thesis has been carried out at the Centre for Astrophysics & Supercomputing at Swinburne University of Technology in Australia between 2010 and 2014. During this time, I spent the majority of 2013 as a research student at the European Southern Observatory in Santiago, Chile. This thesis contains no material that has been accepted for the award of any other degree or diploma and to the best of my knowledge, it contains no material previously published or written by another author, except where due reference is made in the text. The content of the chapters listed below has appeared in refereed journals or is in prepa- ration for submission. Minor alterations have been made to the published papers in order to maintain argument continuity and consistency of spelling and style. • Chapter 2 has been published as N-body models of globular clusters: metallicities, half-light radii and mass-to-light ratios, Sippel A. C., Hurley J. R., Madrid, J. P. and Harris, W. E., MNRAS, Vol. 427, Issue 1 p. 167-179 and can be retrieved under the following link: http://mnras.oxfordjournals.org/content/427/1/167 • Chapter 3 is submitted to MNRAS with the title Slicing and dicing globular clusters: dynamically evolved single stellar populations, Sippel A. C. & Hurley, J. R. • Chapter 4 has been published as Multiple stellar-mass black holes in globular clusters: theoretical confirmation, Sippel A. C & Hurley J. R., MNRAS Letters, Vol. 430, Issue 1, p. 30-34 and can be retrieved under the following link: http://mnrasl. oxfordjournals.org/content/430/1/L30 My contribution to these papers was as follows: I evolved and analyzed all N-models and wrote the manuscripts. My supervisor, Jarrod Hurley, provided the idea for Chapter 2 and Chapter 3 in particular, as well as continuous help, ideas and guidance. Juan Madrid and Bill Harris contributed with observational advice and discussions. Anna Sippel Melbourne, Victoria, Australia 2014 Contents Abstract i Acknowledgments ii Declaration iv List of Figures viii List of Tables x 1 Introduction 1 1.1 Globular Clusters . 1 1.1.1 Evolution of globular clusters . 5 1.1.2 Stellar evolution 101 . 11 1.1.3 Combining stellar and dynamical evolution in globular clusters . 13 1.1.4 Stellar populations in globular clusters . 14 1.2 Open Clusters . 16 1.3 Star cluster models with NBODY6 ........................ 20 1.3.1 Development: Five decades from NBODY1 to NBODY7 . 20 1.3.2 Special Purpose Computers and Graphics Processing Units . 23 1.3.3 Initializing NBODY6 ............................ 26 1.3.4 Alternatives to NBODY6 .......................... 26 1.4 Comparing models and observations . 31 1.5 Outline . 32 2 Metallicity Effects on Globular Cluster Evolution 33 2.1 Globular cluster sizes . 33 2.2 Metallicity effects on stellar and star cluster evolution . 35 2.2.1 Stellar evolution of an entire population . 39 2.2.2 Size: projection effects vs. internal dynamics . 41 2.3 Simulation method & choice of parameters . 42 2.3.1 Binary fraction . 47 2.3.2 Treatment of remnants . 47 2.3.3 Models . 48 2.4 Evolution . 50 2.4.1 Binary systems . 51 vii viii Contents 2.4.2 Cluster size . 53 2.4.3 Surface brightness and half light radii . 57 2.4.4 Origin of the size difference and influence of remnants . 60 2.4.5 Mass-to-light ratio . 64 2.5 Discussion and conclusion . 64 3 Dynamically Evolved Single Stellar Populations 69 3.1 Introduction . 69 3.2 Simulation method & choice of parameters . 71 3.2.1 Colour . 72 3.3 Evolution . 73 3.3.1 General . 77 3.3.2 Integrated and dynamically evolved SSPs . 78 3.3.3 Resolved single stellar populations . 85 3.4 Conclusions . 95 4 Black Holes in Globular Clusters 99 4.1 Stellar mass black holes in globular clusters . 99 4.2 Simulation method & choice of parameters . 102 4.2.1 Velocity kicks . 102 4.2.2 Size and time scales: measurements . 103 4.3 Evolution . 104 4.3.1 Binary systems with black holes . 107 4.3.2 Comparison . 108 4.4 Conclusions .