Georgia State University ScholarWorks @ Georgia State University Physics and Astronomy Dissertations Department of Physics and Astronomy 8-13-2019 Detection And Characterization Of Hot Subdwarf Companions Of Massive Stars Luqian Wang Follow this and additional works at: https://scholarworks.gsu.edu/phy_astr_diss Recommended Citation Wang, Luqian, "Detection And Characterization Of Hot Subdwarf Companions Of Massive Stars." Dissertation, Georgia State University, 2019. https://scholarworks.gsu.edu/phy_astr_diss/119 This Dissertation is brought to you for free and open access by the Department of Physics and Astronomy at ScholarWorks @ Georgia State University. It has been accepted for inclusion in Physics and Astronomy Dissertations by an authorized administrator of ScholarWorks @ Georgia State University. For more information, please contact [email protected]. DETECTION AND CHARACTERIZATION OF HOT SUBDWARF COMPANIONS OF MASSIVE STARS by LUQIAN WANG Under the Direction of Douglas R. Gies, PhD ABSTRACT Massive stars are born in close binaries, and in the course of their evolution, the initially more massive star will grow and begin to transfer mass and angular momentum to the gainer star. The mass donor star will be stripped of its outer envelope, and it will end up as a faint, hot subdwarf star. Here I present a search for the subdwarf stars in Be binary systems using the International Ultraviolet Explorer. Through spectroscopic analysis, I detected the subdwarf star in HR 2142 and 60 Cyg. Further analysis led to the discovery of an additional 12 Be and subdwarf candidate systems. I also investigated the EL CVn binary system, which is the prototype of class of eclipsing binaries that consist of an A- or F-type main sequence star and a low mass subdwarf. From spectroscopic analysis using both the Hubble Space Telescope Cosmic Origins Spectrograph and the Apache Point Observatory ARC Echelle Spectrograph, I computed the system’s radial velocities and orbital parameters. I combined these with photometric measurements from the pt5m telescope to obtain the physical prop- erties of the system. The detection of these subdwarf stars only represents a small fraction of their population, and follow up spectroscopy will be needed to estimate their fundamen- tal parameters. Such information will be compared with models to trace their evolutionary history. INDEX WORDS: Stars: emission-line - Be, Stars: individual (HR 2142, HD 41335) - subdwarfs, Stars: individual (60 Cyg) - subdwarf, Stars: individual (EL CVn, HD 116608), Stars: binaries: spectroscopic, Stars: evolu- tion DETECTION AND CHARACTERIZATION OF HOT SUBDWARF COMPANIONS OF MASSIVE STARS by LUQIAN WANG A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the College of Arts and Sciences Georgia State University 2019 Copyright by Luqian Wang 2019 DETECTION AND CHARACTERIZATION OF HOT SUBDWARF COMPANIONS OF MASSIVE STARS by LUQIAN WANG Committee Chair: Douglas R. Gies Committee: Sébastien Lépine Geraldine J. Peters Mark Stockman Russel White Electronic Version Approved: Office of Graduate Studies College of Arts and Sciences Georgia State University August 2019 iv CHAPTER 0 DEDICATION To my grandmother Yaohui Cheng and mother Feng Cheng. This dissertation is also dedi- cated to the memory of my grandfather, Peihua Zhang, who passed away before my gradu- ation. v CHAPTER 0 ACKNOWLEDGMENTS It is my great pleasure to thank the many people who made this dissertation possible. First, I would like to express my deepest appreciation and gratitude to my PhD advisor, Dr. Dou- glas Gies. His deep enthusiasm and passion for astronomy research inspired and encouraged me. For the past six years Dr. Gies has given me instructive guidance, helpful advice, and positive encouragement. I would not have been able to finish the dissertation without his help. I would like to extend my sincere thanks to the project collaborators, Dr. Geraldine Peters, Kathryn Lester, Dr. Erika Grundstrom, Dr. Zhao Guo, Dr. Rachel Matson, Drew Cho- jnowski, and Dr. Ylva Götberg. I would like to thank the pt5m team members of Dr. Vik Dhillon, Dr. Tim Butterley, Dr. Stuart Littlefair, Dr. Richard Wilson, and the SMARTS team at GSU with members Dr. Todd Henry, Dr. Wei-Chun Jao, Leonardo Paredes, and Hodari James. I also would like to thank my committee members, Dr. Geraldine Peters, Dr. Russel White, Dr. Sébastien Lépine, and Dr. Mark Stockman. Support for Program number GO-14778 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, incorporated, under NASA contract NAS5-26555. I am grateful for additional support from NASA grant NNX10AD60G and National Science Foundation grant AST-1411654. Institutional support has been provided from the GSU College of Arts and Science and from the Research Program Enhancement fund of the Board of Regents of the vi University System of Georgia, administered through the GSU office of the Vice President for Research and Economic Development. Based in part upon observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium. pt5m is a collaborative effort between the Universities of Durham and Sheffield. The telescope is kindly hosted by the Issac Newton Group of Telescopes, La Palma. Last, I would like to express my deep appreciation to my grandmother and mother who cared for me during difficult time. Their guidance and support helped me through the darkest moments. vii CHAPTER 0 TABLE OF CONTENTS ACKNOWLEDGMENTS .............................. v LIST OF TABLES .................................. x LIST OF FIGURES ................................. xi LIST OF ABBREVIATIONS ............................xix 1 INTRODUCTION ................................ 1 1.1 Massive star evolution and the binary scenario ............. 1 1.2 Be stars and their formation scenarios .................. 2 1.3 Other approaches to search for sdO companions in Be binary systems 4 1.4 Population of Be stars in the local Galactic environment . 7 1.5 Motivation for searching sdO companions of rapidly rotating mas- sive stars ..................................... 8 1.6 Outline of this work .............................. 9 2 THE HOT COMPANION AND CIRCUMBINARY DISK OF HR 2142 10 2.1 HR 2142 binary system ............................ 10 2.2 Radial velocities and orbital solution ................... 10 2.3 Search for the hot subdwarf companion . 20 2.4 Shell line features ............................... 22 2.5 Disk models ................................... 25 3 DETECTION OF THE HOT SUBDWARF COMPANION OF 60 CYRGNI FROM A SURVEY ................................ 32 3.1 A survey of IUE spectra of 6 Be stars . 32 3.2 Search for the UV flux of hot companions . 34 viii 3.3 Implications of the detection of the companion of 60 Cyg . 40 4 DETECTION OF ADDITIONAL BE+SDO SYSTEMS FROM IUE SPECTROSCOPY ................................ 45 4.1 Search for additional subdwarf companion of Be stars from IUE . 45 4.2 Search for sdO companions ......................... 45 4.2.1 Target selection from CCFs .................... 45 4.2.2 Detection criteria for Be+sdO systems . 49 4.2.3 Detection of the Be+sdO systems . 51 4.3 Discussion of the candidate Be+sdO systems . 56 4.4 Population census of the identified Be+sdO candidate systems . 62 5 THE HOT SUBDWARF IN THE POST-MASS TRANSFER BINARY EL CVn ....................................... 67 5.1 Introduction ................................... 67 5.2 Observations and data reduction ...................... 69 5.2.1 FUV HST/COS spectroscopy ................... 70 5.2.2 Optical APO/ARCES spectroscopy . 71 5.2.3 Optical PT5M light curves ..................... 72 5.3 Radial velocity measurements ........................ 74 5.4 Light curve fit ................................. 80 5.5 Orbital elements, masses, and radii .................... 83 5.6 Component Reconstructed Spectra and Physical Properties . 86 5.7 Evolutionary State of EL CVn ....................... 96 6 CONCLUSIONS AND FUTURE WORK . 103 6.1 Implications of the candidate Be+sdO systems and follow-up ob- servations .................................... 103 6.1.1 Future FUV spectroscopy of Be+sdO candidate systems us- ing the HST/STIS . 103 ix 6.1.2 Future optical spectroscopy from the SMARTS/CHIRON and APO/ARCES . 106 6.1.3 Shell line variations in other Be+sdO systems . 108 6.1.4 Resolving the problem of missing late-type Be stars in Be+sdO candidate systems . 110 6.2 Implications of the EL CVn binary system . 112 6.3 Concluding remarks .............................. 114 REFERENCES ....................................116 APPENDIX ......................................127 A SHELL LINE VARIATIONS IN HR 2142 . 127 B SHELL EPISODE OF PLEIONE . 145 C NOTES ON INDIVIDUAL STARS IN THE IUE SUBDWARF SURVEY148 x CHAPTER 0 LIST OF TABLES Table 2.1 Spectroscopic Observations of HR 2142 . 11 Table 2.2 Be Star Radial Velocities ......................... 15 Table 2.3 Circular Orbital Elements ......................... 20 Table 3.1 IUE Observations of Be Binary Systems . 34 Table 3.2 Adopted Orbital Elements ........................ 36 Table 3.3 Co-moving Stars in the Vicinity of 59 Cyg and 60 Cyg . 43 Table 4.1 Stars with CCF S/N < 3 ......................... 47 Table 4.2 IUE Observations of sample stars .................... 53 Table 4.3 Radial Velocity Measurements of Candidate sdO Components . 60 Table 5.1 Radial Velocity Measurements