CHARACTERIZING OPTICAL COUNTERPARTS OF X-RAY SOURCES IN THE CORE OF OMEGA CENTAURI A thesis presented to the faculty of San Francisco State University In partial fulfilment of Z e \ * i The Requirements for The Degree fHY* M 8 ? Master of Science In Physics by Kyle Murphy San Francisco, California May 2019 Copyright by Kyle Murphy 2019 CERTIFICATION OF APPROVAL I certify that I have read Characterizing Optical Counterparts of X-Ray Sources in the Core of Omega Centauri by Kyle Murphy and that in my opinion this work meets the criteria for approving a thesis submitted in partial fulfillment of the requirements for the degree: Master of Science in Physics at San Francisco State University. Professor of Physics & Astronomy •IbsepK Barranco Associate Professor of Physics & Astronomy Huizhong Xu Associate Professor of Physics CHARACTERIZING OPTICAL COUNTERPARTS OF X-RAY SOURCES IN THE CORE OF OMEGA CENTAURI Kyle Murphy San Francisco State University 2019 X-ray imaging of globular clusters is a powerful tool to determine their overall X-ray emissivity as well as identifying their binary star populations that greatly influence cluster dynamics and evolution. One such cluster in which a great deal has yet to be discovered about its population of X-ray emitting binaries is Omega Centauri, the most massive (4x 106 Msun) globular cluster in the Milky Way. 67 X-ray sources have already been detected by the Chandra X-ray Observatory within its large core (rc = 3.9 pc = 155"). Identifying the optical counterparts of these X-ray sources is almost always necessary to properly classify the source of the X-ray emission. Utilizing the most comprehensive catalog of photometry and proper motions ever constructed for a globular cluster, based on over 650 exposures taken in 26 different filters by the Hubble Space Telescope’s Wide-Field Camera 3, we constructed numerous color- magnitude diagrams to search for and characterize cluster binary stars. We have identified ten new possible binary stars in the core of Omega Centauri. We also recovered seven previously known systems and provided independent confirmation that they have the properties expected of binary stars. These potential optical counterparts include five cataclysmic variables, four faint blue cataclysmic variables, five active binaries, and three red stragglers that may be associated with the metal- rich anomalous giant and subgiant branches of the cluster. I certify that the Abstract is a correct representation of the content of this thesis. ACKNOWLEDGMENTS I would first like to thank my incredible advisor, Dr. Adrienne Cool, whose expertise, mentorship, and encouragement made this thesis possi­ ble. Thank you so much for your unwavering support through each stage of the process and everything you have taught me. I would also like to thank the other members of my thesis committee, Dr. Joseph Barranco, and Dr. Huizhong Xu, for their insightful suggestions and questions. Fi­ nally, I wish to express my deep and sincere gratitude to my parents for their unconditional love and support. I can never thank you enough for everything you have done for me. v TABLE OF CONTENTS 1 Introduction............................................................................................................ 1 1.1 Binary Stars in Globular C lusters............................................................. 1 1.2 Omega Centauri............................................................................................ 4 1.3 Searching for Optical Counterparts of X-ray Sources.............................. 8 2 Observations Sz D a t a ................................................................................................11 2.1 Chandra X-ray Observations....................................................... 11 2.2 HST Observations......................................................................................... 14 3 A nalysis......................................................................................................................23 3.1 Color-magnitude Diagrams of Omega Centauri............................................23 3.2 Equivalent W id th ............................................................................................. 27 3.3 Classes of Optical Counterparts within Omega Centauri............................29 4 Results......................................................................................................................... 35 4.1 Cataclysmic Variables ....................................................................................35 4.1.1 1 2 a ...........................................................................................................35 4.1.2 1 3 a ...........................................................................................................36 4.1.3 1 3 c ........................................................................................................... 37 4.1.4 1 3 f ........................................................................................................... 37 4.1.5 2 3 b ........................................................................................................... 38 4.1.6 2 4 c ........................................................................................................... 39 vi 4.1.7 2 2 h ...........................................................................................................40 4.2 Faint Blue Cataclysmic V aria bles................................................................41 4.2.1 2 1 b ...........................................................................................................41 4.2.2 2 2 c ...........................................................................................................42 4.2.3 I l f ........................................................................................................... 43 4.2.4 2 2 i ...........................................................................................................44 4.3 Active Bin a ries................................................................................................. 45 4.3.1 1 4 a ........................................................................................................... 45 4.3.2 2 4 e ...........................................................................................................45 4.3.3 1 3 e ........................................................................................................... 46 4.3.4 1 4 f ........................................................................................................... 48 4.3.5 2 3 a ........................................................................................................... 48 4.3.6 2 4 h ........................................................................................................... 50 4.4 Red Stragglers or RGB/SGB-a S t a r s ..........................................................51 4.4.1 1 3 b ........................................................................................................... 52 4.4.2 2 2 e ........................................................................................................... 53 4.4.3 2 4 f ........................................................................................................... 53 4.4.4 l i e ........................................................................................................... 53 4.4.5 1 4 b ........................................................................................................... 54 4.4.6 2 2 b ........................................................................................................... 55 4.5 Blue-only and Ha-only Stars..........................................................................56 vii 4.5.1 2 4 a .......................................................................................................... 57 4.5.2 2 1 c .......................................................................................................... 58 4.5.3 2 1 d .......................................................................................................... 58 4.5.4 3 3 d .......................................................................................................... 58 4.6 Variable Star ...................................................................................................60 4.6.1 l i b ...........................................................................................................60 4.7 Foreground S t a r s .............................................................................................62 4.7.1 2 2 a .......................................................................................................... 62 4.7.2 2 2 d .......................................................................................................... 62 4.7.3 2 2 j .......................................................................................................... 63 4.7.4 1 4 c ...........................................................................................................64 4.8 Active Galactic N u c le i.................................................................................... 65 4.8.1 2 2 f .......................................................................................................... 66 4.8.2 2 3 c .......................................................................................................... 67 4.8.3 2 4 g ...........................................................................................................67