Circumstellar Disks Around Rapidly Rotating Be-Type Stars
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Georgia State University ScholarWorks @ Georgia State University Physics and Astronomy Dissertations Department of Physics and Astronomy Spring 3-20-2012 Circumstellar Disks Around Rapidly Rotating Be-Type Stars Yamina Touhami Georgia State University Follow this and additional works at: https://scholarworks.gsu.edu/phy_astr_diss Recommended Citation Touhami, Yamina, "Circumstellar Disks Around Rapidly Rotating Be-Type Stars." Dissertation, Georgia State University, 2012. https://scholarworks.gsu.edu/phy_astr_diss/53 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]. CIRCUMSTELLAR DISKS AROUND RAPIDLY ROTATING Be-TYPE STARS by YAMINA N. TOUHAMI Under the Direction of Douglas R. Gies ABSTRACT Be stars are rapidly rotating B-type stars that eject large amounts of material into a circum- stellar disk. Evidence of the presence of a disk is found through hydrogen emission lines in their spectra, IR excess flux, and linear intrinsic polarization. In this dissertation, we report the first simultaneous interferometric and spectroscopic observations of circumstellar disks around 24 bright Be stars made using the techniques of long baseline interferometry and moderate resolution spectroscopy in the near infrared. The goal of the project is to char- acterize the fundamental geometrical and physical properties of the emitting regions that are responsible for the IR flux excesses detected in the K-band in our sample stars. This observational work has been conducted with both the Center for High Angular Resolution Astronomy (CHARA) Array at Mount Wilson Observatory, and the Mimir spectrograph at Lowell Observatory. The visibility measurements were interpreted with different geometrical and physical disk models in order to determine the spatial extension of the disk, the inclination angle, the position angle, and the density profile of the disk. We find that the spatial extension of the circumstellar disk in the K-band is only about a few stellar radii, and that the density structure of the disk is consistent with a radially decreasing function with a density exponent that ranges between 2.5 and 3.5. The resulting disk densities are in a good agreement with those derived from the Infrared Astronomical Satellite (IRAS) measurements, and the resulting disk geometries are consistent with previous polarimetric measurements. We find that the K-band sizes of the emitting regions in the disk are smaller by a factor of two than the Hα sizes, and we show that this is due to the lower opacity of the continuum in the disk. By combining recent measurements of the projected rotational velocities with the disk inclination angles derived from interferometry, we were able to estimate the actual equato- rial linear rotational velocities of the Be stars in our sample. The obtained linear rotational velocities indicate that Be stars are rapid rotators with an equatorial velocity that is about 0.7 - 0.9 of their critical velocities. INDEX WORDS: Stars, Circumstellar disk, Be stars, Infrared, Stellar interferometry, Spectroscopy CIRCUMSTELLAR DISKS AROUND RAPIDLY ROTATING Be-TYPE STARS by YAMINA N. TOUHAMI A Dissertation Submitted in Partial Fulfillment of Requirements for the Degree of Doctor of Philosophy in the College of Arts and Sciences Georgia State University 2012 Copyright by Yamina N. Touhami 2012 CIRCUMSTELLAR DISKS AROUND RAPIDLY ROTATING Be-TYPE STARS by YAMINA N. TOUHAMI Committee Chair: Douglas R. Gies Committee: Harold A. McAlister Russel J. White Vincent Coud´edu Foresto Carol E. Jones D. Michael Crenshaw Unil Perera Electronic Version Approved: Office of Graduate Studies College of Arts & Sciences Georgia State University May 2012 iv DEDICATION{ 0 { To my parents v ACKNOWLEDGMENTS{ 0 { I want to begin by thanking my parents who allowed the little girl's dreams to become a reality. I owe them the greatest debt of gratitude. It has been a great privilege to work under the supervision of Professor Douglas Gies, whose professional dedication and love for astronomy are a continual inspiration. I have im- mensely enjoyed working with him, and I would like to thank him for providing a supportive research environment. I also would like to thank Dr. Stephen Ridgway for the excellent talk about stellar interferometry he gave at the Department of Physics of the Ecole Normale Superieure in the fall of 2003, which triggered my interest in joining this research field. I also would like to thank him for supervising my undergraduate research project in spring/summer 2004, which put me in the way of my current research specialization. I also appreciate the tremendous support I have received from Theo ten Brummelaar, Gail Schaefer, and the rest of the CHARA crew who kept the Array working very efficiently and allowed me to collect as much data as possible. I wish to thank the members of my committee, Harold A. McAlister, Russel J. White, Vincent Coud´edu Foresto, Carol E. Jones, Michael Crenshaw, and Unil Perera for overseeing the progress of this thesis. Special thanks is extended to Rafael Millan-Gabet and NASA/NExScI for sponsoring my six-month visit to Caltech, and to John D. Monnier for the helpful conversations and good tips all along the progress of this dissertation. Many thanks to Brian Mason (USNO), Bill Hartkopf (USNO), and Lewis Roberts (Cal- tech) for their expert advice on binaries among the Be stars detected by speckle and adaptive optics methods. I am also grateful to Christopher Tycner who sent us unpublished Hα di- ameter measures for some of my sample stars. Lastly, this thesis research could not have been completed without the continuous support from my dearest friends Sandy Guerrier, Ramata Cisse, and Smail Beghdadi. I thank them vi for their tremendous love and support. I also want to express my gratitude to my extended family members and friends in France, Algeria, and here in the US. This material is based upon work supported by the National Science Foundation under Grants AST-0606861 and AST-1009080. Institutional support has been provided from the GSU College of Arts and Sciences and from the Research Program Enhancement fund of the Board of Regents of the University System of Georgia, administered through the GSU Office of the Vice President for Research. I am also grateful for the financial support of a Georgia Space Grant Consortium fellowship. vii TABLE OF{ 0 CONTENTS { ACKNOWLEDGMENTS . v LIST OF TABLES . xiv LIST OF FIGURES . xvi LIST OF ABBREVIATIONS . xxi 1 Introduction . 2 1.1 Long Baseline Interferometry . 2 1.1.1 Basic Concepts . 2 1.1.2 The Zernike-van Cittert Theorem . 3 1.1.3 The Observed Visibility . 4 1.1.4 The CHARA Array . 6 1.2 Be stars . 8 1.2.1 General Characteristics of Be Stars . 9 1.2.2 Interferometry of Be stars . 11 1.3 Outline of the presentation . 13 2 Disk Models . 16 2.1 Introduction . 16 2.2 Geometrical Models . 19 2.2.1 The Effective Baseline . 19 2.2.2 The Uniform Disk Model . 20 viii 2.2.3 Gaussian Elliptical Model . 20 2.3 The Thick Disk Model of Be Star Disks . 22 2.3.1 Description of the Model . 22 2.3.2 Disk Emissivity . 23 2.3.3 Continuum Images and Interferometric Visibility . 26 2.3.4 The Interferometric Visibility . 27 2.3.5 Inner Gaps . 28 2.4 Conclusions . 31 3 The Near-IR Spectral Energy Distributions of Be Stars . 33 3.1 Introduction . 33 3.2 Observations and Reductions . 35 3.3 Optical to Near-IR Spectral Energy Distributions . 41 3.4 Emission Line Equivalent-Widths . 50 3.5 Discussion . 57 3.6 Conclusions . 65 4 The Infrared Continuum Sizes of Be Star Disks . 68 4.1 Introduction . 68 4.2 Be Star Circumstellar Disk Models . 71 4.3 AKARI IR Fluxes of Be Stars . 81 4.4 Angular Size of the Disk of ζ Tau . 90 ix 5 The Circumstellar Environment of γ Cassiopeia . 95 5.1 Introduction . 95 5.2 Observations and data analysis . 98 5.2.1 Classic Observations . 98 5.2.2 FLUOR Observations . 103 5.3 Gaussian Elliptical Model Fits . 106 5.3.1 Method . 106 5.3.2 The Angular Size of the γ Cas Disk . 109 5.3.3 Time Variability of the Disk of γ Cas . 113 5.3.4 Comparison with Previous Studies . 119 5.4 Thick Disk Models of γ Cas . 123 5.4.1 Thick Disk Models . 123 5.4.2 The Density Profile of γ Cas . 124 5.4.3 The Degeneracy of the Thick Disk Model . 128 5.4.4 The Inner Radius of γ Cas . 131 5.4.5 Discussion . 136 5.5 Conclusions . 140 6 A K-band Interferometric Survey of Be Stars . 142 6.1 Introduction . 143 6.2 Observations and Data Reduction . 145 6.2.1 Description of the Sample . 145 x 6.2.2 Properties of the Calibrator Stars . 148 6.2.3 CHARA Classic Observations and Data Calibration . 155 6.3 Correction for the Flux of Nearby Companions . 174 6.4 SEDs of Be Stars . 177 6.5 Gaussian Elliptical Fits . 182 6.5.1 Method . 182 6.5.2 Fitting Results . 186 6.5.3 Notes on Individual Targets . 193 6.5.4 Corrections to the Gaussian Fit Model . 199 6.6 Discussion . 207 6.6.1 Detection Limits . 207 6.6.2 K-band and Hα Disk Sizes . 212 6.6.3 Emission Lines and Disk Sizes . 216 6.6.4 Distributions of Be Disk Sizes and Inclinations . 220 6.6.5 Be Star Linear Rotational Velocities . 222 6.7 Conclusions . 226 7 Determining the Fundamental Stellar Properties of the Yellow Hyper- giant ρ Cassiopeia .