ABSTRACT VARIABILITY OF HOT SUBDWARF STARS FROM THE PALOMAR-GREEN CATALOG OF ULTRAVIOLET EXCESS STELLAR OBJECTS This thesis presents a survey for variability in 985 objects identified as hot subdwarf stars by the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects. Catalina Real-Time Transient Survey (CRTS) light curves are used to find the variability. Landolt standard stars that are also hot subdwarf stars in the Palomar-Green Catalog are used to calibrate CRTS photometry in the visual (V) band, and show it is accurate to within ΔV = 0.1 magnitudes between V = 12.0 and V = 16.0. Eleven objects are found to have variability that exceeds four standard deviations above the means of their CRTS light curves. Two are objects already known to be variable. They are PG 1101+385 (Mrk 421), a BL Lac object, and PG 1419+081, a short-period, pulsating sdB star. The other nine are new discoveries of variability: PG 0901+309, PG 0923+329, PG 0934+554, PG 1201+258, PG 1025+244, PG 1411+219, PG 1640+645, PG 1700+315, and PG 2217+059. PG 0934+554 is a spectrophotometric standard of Massey et al. (1988): this discovery of its variability shows that it should not be used as a standard. PG 0934+554 is identified as a BL Lac object. More tentative identifications include PG 1411+219 as an AM CVn star, PG 1640+645 and PG 2217+059 as dwarf novae, and PG 1700+315 as an eclipsing binary star system. Detailed follow-up observations are needed for the other variables, although they may be pulsating hot subdwarf stars. Melissa Anne Blacketer December 2015 VARIABILITY OF HOT SUBDWARF STARS FROM THE PALOMAR-GREEN CATALOG OF ULTRAVIOLET EXCESS STELLAR OBJECTS by Melissa Anne Blacketer A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Physics in the College of Science and Mathematics California State University, Fresno December 2015 APPROVED For the Department of Physics: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Melissa Anne Blacketer Thesis Author Frederick A. Ringwald (Chair) Physics Gerardo Munoz Physics Douglas Singleton Physics For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS I would like to thank the Department of Physics faculty at California State University, Fresno for their endless support and encouragement I received since day one. I thank the College of Science and Mathematics of California State University, Fresno for a Faculty Sponsored Student Research Award, which allowed me to share my research with the top scientists in the field of astronomy at the American Astronomical Society meeting in Boston. Also much thanks to the Department of Physics of California State University, Fresno for support through a teaching assistantship. Many thanks to the Catalina Real-Time Transient Survey (CRTS) for their photometry used in this research. The Catalina Sky Survey (CSS) is funded by the National Aeronautics and Space Administration under Grant No. NNG05GF22G issued through the Science Mission Directorate Near-Earth Objects Observations Program. The CRTS survey is supported by the U. S. National Science Foundation under grants AST-0909182 and AST-1313422. This research has made use of data obtained from, and software provided by, the US Virtual Astronomical Observatory, which is sponsored by the National Science Foundation and the National Aeronautics and Space Administration (NASA). This research has made use of the SIMBAD database, operated by CDS, Strasbourg, France. This research has also made use of NASA's Astrophysics Data System Bibliographic Services. Last but certainly not least, I would like to thank my family for their endless love and support. To Danny Guice, thank you for believing in me even when I didn’t. I could not have done this without you. TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................. vii LIST OF FIGURES ............................................................................................... viii INTRODUCTION .................................................................................................... 1 What Are Hot Subdwarf Stars .......................................................................... 1 Formation Theories ........................................................................................... 6 Hot Subdwarf Research ................................................................................... 10 RESEARCH PROGRAM ...................................................................................... 14 Palomar-Green Catalog ................................................................................... 14 Catalina Real-Time Transient Survey ............................................................. 16 Types of Variability ........................................................................................ 17 Method ............................................................................................................ 18 OBSERVATIONS AND ANALYSIS ................................................................... 20 Analysis Part I ................................................................................................. 20 Analysis Part II ................................................................................................ 22 Calibration ....................................................................................................... 23 List of Standard Stars ...................................................................................... 26 List of PG Hot Subdwarf Stars Part I .............................................................. 29 List of PG Hot Subdwarf Stars Part II ............................................................ 49 RESULTS AND DISCUSSION ............................................................................ 73 PG 0934+554 .................................................................................................. 73 PG 1101+385 .................................................................................................. 75 PG 1025+244 .................................................................................................. 75 PG 0923+329 .................................................................................................. 76 vi vi Page PG 1201+258 .................................................................................................. 76 PG 0901+309 .................................................................................................. 78 PG 1640+645 .................................................................................................. 78 PG 1419+081 .................................................................................................. 79 PG 2217+059 .................................................................................................. 80 PG 1700+315 .................................................................................................. 81 PG 1411+219 .................................................................................................. 82 CONCLUSION ...................................................................................................... 84 REFERENCES ....................................................................................................... 87 LIST OF TABLES Page Table 1: PG highest stars on Figure 5. ................................................................... 21 Table 2: PG hot subdwarf stars and related objects significant variance ............... 21 Table 3: PG highest variable stars on Figure 6. ..................................................... 23 Table 4: PG hot subdwarf stars and related objects significant variance ............... 23 Table 5: PG Standard Stars from Figure 7. ............................................................ 26 Table 6: PG Standard Stars from Figure 5. ............................................................ 29 Table 7: PG Standard Stars from Figure 6. ............................................................ 49 Table 8: The candidates .......................................................................................... 86 LIST OF FIGURES Page Figure 1. Diagram of the stellar evolution of a typical 1 solar mass star. The evolution starts on the MS curve, and progresses to the red giant branch (RGB), to the horizontal branch, to the AGB, planetary nebula phase, and down to the last stage white dwarf stars. (Carrol, nd B., Ostlie, D., 2007, Introduction to Modern Astrophysics, 2 ed.) ........ 4 Figure 2. This Hertzsprung-Russell diagram shows the placement of the hot subdwarf stars sdB and sdO on the extreme horizontal branch (EHB). (Heber U. 2009, ARAA, 47, 211) ................................................ 5 Figure 3. The theoretical formation channels for hot subdwarf stars: Roche lobe overflow, Common Envelope phase,