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The Origins of Hot Subdwarf Stars As Illuminated by Composite-Spectrum Binaries

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The Origins of Hot Subdwarf Stars As Illuminated by Composite-Spectrum Binaries The Pennsylvania State University The Graduate School Department of Astronomy and Astrophysics THE ORIGINS OF HOT SUBDWARF STARS AS ILLUMINATED BY COMPOSITE-SPECTRUM BINARIES A Thesis in Astronomy and Astrophysics by Michele A. Stark °c 2005 Michele A. Stark Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2005 The thesis of Michele A. Stark was read and approved1 by the following: Richard A. Wade Associate Professor of Astronomy and Astrophysics Thesis Adviser Chair of Committee Robin Ciardullo Professor of Astronomy and Astrophysics Lawrence W. Ramsey Professor of Astronomy and Astrophysics Head of the Department of Astronomy and Astrophysics Mercedes T. Richards Professor of Astronomy and Astrophysics Richard W. Robinett Professor of Physics 1Signatures on file in the Graduate School. iii Abstract For this investigation I studied hot subdwarf stars listed in the Catalogue of Spec- troscopically Identified Hot Subdwarfs (Kilkenny, Heber, & Drilling 1988, KHD). While the KHD catalog contains all varieties of hot subdwarfs, I primarily focused on the more numerous and homogeneous sdB stars. I used improved coordinates to collect near-IR flux measurements of KHD hot subdwarfs from the Two-Micron All Sky Survey (2MASS) All-Sky Data Release Catalog. I used these 2MASS magnitudes with visual photometry from the literature to identify those hot subdwarfs whose colors indicated the presence of a late type companion. I determined that »40% of sdB stars are composite in a magnitude limited sample (»25% in a volume limited sample). I compared the distributions of hot subdwarfs in 2MASS colors and found a bi- modally distributed population. The two peaks of the bimodal distribution can be under- stood as single hot subdwarf stars and composite systems. Based on these distributions is 2MASS colors, there are no (or very few) F or dM companions of the hot subdwarfs in the KHD catalog. These observed distributions of hot subdwarfs in 2MASS colors can be reproduced equally well by assuming either main sequence or subgiant companions — photometric data alone cannot distinguish between these two possibilities. I obtained spectroscopy of some of the 2MASS composite-colored hot subdwarfs to break the degeneracy between main sequence and subgiant companions present in photometry alone. My analysis focused on Mg i b, Na i D, and Ca ii infrared triplet equivalent widths (EWs) from the late-type companion. The observations (2MASS and visual photometry combined with EWs) for each composite hot subdwarf were compared with diluted models based on Hipparcos standard star observations, models of extended horizontal branch stars (Caloi 1972), and Kurucz (1998) spectral energy distributions, in order to determine the combination of sdB+late- type star that best explained all observations. With a few exceptions, I found that the late-type companions are best identified as main sequence (although some subgiant companions do exist). The majority of the well constrained main sequence companions < < have 0:5 » (B¡V )c » 1:1 (spectral types »F6–K5). Han et al. (2002, 2003) predict that all hot subdwarfs with main sequence com- < panions of »G-type or later are in short period (P » 40 days), post-common envelope > binaries (anything with a companion of »G-type or later and a long period, P » 40 days, has either a subgiant or giant companion). Yet, radial velocity studies including com- posite spectrum hot subdwarfs (i.e., Orosz et al. 1997; Saffer et al. 1994; Maxted et al. 2001), have said the periods for composite-spectrum binaries must be long (many months to years or more), which in the Han et al. scenario would imply that they contain the subgiant or giant companions. Yet, as I show, the majority of composite companions are consistent with main sequence stars. So, it appears something is incorrect or incomplete in the current Han et al. binary formation scenario. iv Table of Contents List of Tables :::::::::::::::::::::::::::::::::::::: viii List of Figures ::::::::::::::::::::::::::::::::::::: ix Acknowledgments ::::::::::::::::::::::::::::::::::: xi Chapter 1. Introduction :::::::::::::::::::::::::::::::: 1 1.1 What is a “Hot Subdwarf”? . 1 1.1.1 Comparison with Globular Clusters . 2 1.1.2 Physical Parameters . 2 1.1.3 Kinematics and Population Membership . 3 1.1.4 Ultraviolet Excess . 4 1.2 Sidetrack: Name-ology . 5 1.3 Pulsating sdBs . 5 1.3.1 V361 Hya (EC 14026, sdBV) Stars . 5 1.3.2 PG 1716+426 Stars . 6 1.4 Formation and Evolution of Hot Subdwarf Stars . 6 1.4.1 Binary Formation Models . 6 1.4.1.1 Known Binaries . 8 1.4.1.2 Radial Velocity Studies . 9 1.4.2 Single Star Evolution . 10 1.4.2.1 Delayed Helium Flashers . 10 1.4.2.2 Hot-Flashers . 11 1.5 The Mission . 12 1.6 Outline . 12 Chapter 2. The Kilkenny, Heber, & Drilling Hot Subdwarf Catalog ::::::: 16 2.1 The Sample . 16 2.2 Coordinates . 16 2.3 Classification and Identification . 17 Chapter 3. 2MASS All-Sky Data Release :::::::::::::::::::::: 19 3.1 Introduction . 19 3.2 Visual and IR Photometry . 19 3.2.1 Visual Photometry . 19 3.2.2 Infrared Photometry . 20 3.3 Hot Subdwarfs in Color Space . 21 3.3.1 Optical-IR Color-Color Plots . 21 3.3.2 Modelling the Distribution in Optical-IR Color-Color Space . 23 3.4 A “Volume Limited” Hot Subdwarf Sample . 25 3.5 2MASS Only Color Parameters . 26 v 3.6 Distribution in J–KS ........................... 26 3.6.1 Describing the sdB J–KS Distribution . 27 3.6.2 Composites in (J–H, J–KS) Color-Color Space . 28 3.7 Summary . 28 Chapter 4. Spectroscopic Observations of Hot Subdwarfs ::::::::::::: 46 4.1 Introduction . 46 4.2 KPNO GoldCam . 46 4.2.1 Instrumental Set-up . 46 4.2.2 Observing Procedures . 47 4.3 McDonald LCS . 47 4.3.1 Instrumental Set-up . 47 4.3.2 Observing Procedures . 48 4.4 Target Selection and Lists . 49 4.4.1 Hot Subdwarfs . 49 4.4.2 “Standards” . 50 4.5 KPNO GoldCam Data Reduction . 50 4.6 Notes on Individual Objects in the Spectroscopic Sample . 51 4.6.1 Reddened sdBs . 51 4.6.2 Observed Subdwarf O Stars . 52 4.6.3 Visual Doubles . 53 4.6.3.1 Single in 2MASS Colors . 53 4.6.3.2 Composite in 2MASS Colors . 54 4.6.4 Objects with Emission Lines . 55 4.6.5 Other Spectral Anomalies . 56 4.6.5.1 Residual Fringing . 56 4.6.5.2 The Blue Bump . 56 4.6.5.3 Telluric Water Vapor . 57 Chapter 5. Analysis of Spectroscopy of Hot Subdwarfs :::::::::::::: 97 5.1 Introduction . 97 5.2 Measuring Individual Line EWs Automatically . 97 5.3 Properties of the HIP Standard Stars . 98 5.3.1 Loess Parameters for EW Relations . 98 5.3.2 Loess Parameters for Color and Magnitude Relations . 99 5.3.3 Description of the EW Fits . 100 5.4 Models of Diluted EWs based on HIP Fits . 101 5.5 Walk-through of a Model Calculation . 102 5.5.1 Choosing the Hot Subdwarf . 102 5.5.2 Choosing a Late-Type Companion . 103 5.5.3 Calculating the Diluted Parameters . 104 vi Chapter 6. Classification of Composite Spectrum Hot Subdwarfs ::::::::: 118 6.1 Introduction . 118 6.2 Overall Comparison . 118 6.3 Matching Observations with Models . 119 6.4 Walk-through of an Individual Case . 119 6.5 Summary of Companion Classifications . 122 6.6 Interesting Cases . 123 6.6.1 Grid Edges . 123 6.6.2 Subgiant Companions . 125 6.6.3 Some Unusual Objects . 126 Chapter 7. Discussion and Summary :::::::::::::::::::::::: 158 7.1 Defining the Sample . 158 7.2 2MASS Results . 158 7.3 Spectroscopy of Composite Hot Subdwarfs . 159 7.4 Limitations and Directions for Future Work . 160 7.5 Implications . 160 Appendix A. Coordinates and Object Notes :::::::::::::::::::: 162 Appendix B. 2MASS All-Sky Data Release Photometry :::::::::::::: 209 Appendix C. Tricks with Magnitudes and Colors :::::::::::::::::: 246 C.1 Converting Bessel & Brett JHK to 2MASS JHKS . 246 C.2 Determining Extinction Vectors . 246 C.2.1 Extinction Vectors . 246 C.2.2 Reddening Correction . 247 C.3 Creating Composite Colors and Magnitudes . 248 C.3.1 Combining Two Colors . 248 C.3.2 Combining Two Magnitudes . 251 Appendix D. Confidence Intervals for a Binomial Distribution :::::::::: 255 Appendix E. HIP Standard MV Calculation :::::::::::::::::::: 256 Appendix F. Calculating Chi Squared Values and Errors ::::::::::::: 261 F.1 Chi Square Goodness-of-Fit Test . 261 F.2 Confidence Intervals for a Gaussian Fit Based on Chi Squared . 261 Appendix G. Defining a “Q” Parameter for (J–H, J–KS) Color-Color Space ::: 262 G.1 Finding the Least-Squares Linear Fit . 262 G.2 Defining the Color Parameter Q . 263 Appendix H. All About Equivalent Widths ::::::::::::::::::::: 265 H.1 Basics of Calculating Equivalent Widths . 265 H.1.1 EW and Poisson Error . 265 H.1.2 Continuum Fitting Errors . 266 vii H.2 Diluting Equivalent Widths by Combining Two Spectra . 267 H.2.1 Introduction . 267 H.2.2 Hot Subdwarf Gravity Effects . 267 H.2.3 Calculating a Diluted EW . 268 H.2.4 Predicting Diluted EWs using Stellar Flux Distributions . 269 Appendix I . Calculating Cubic Spline Approximations :::::::::::::: 279 I.1 Formalism of the Cubic Spline . 279 I.2 Algorithm for Coefficient Calculation . 280 Appendix J . Second Order Lagrange Polynomial Interpolation :::::::::: 281 Appendix K. Measurements of Equivalent Widths for Program Stars :::::: 282 Appendix L. Summary of Acronyms and Abbreviations :::::::::::::: 296 Bibliography :::::::::::::::::::::::::::::::::::::: 300 viii List of Tables 1.1 Hot Subdwarfs with dM or Brown Dwarf Companions. 15 3.1 Breakdown of the Hot Subdwarf sample discussed in x3.2. 43 3.2 Photometry of Hot Subdwarfs with dM or Brown Dwarf Companions. 44 3.3 Binomial fractions of single and composite-color subdwarfs observed by 2MASS.
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