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A STUDY of INTRINSIC POLARIZATION in Be STARS

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A STUDY of INTRINSIC POLARIZATION in Be STARS t 70-26,253 BOTTEMILLER, Robert Leland, 1942- A STUDY OF INTRINSIC POLARIZATION IN Be STARS. The Ohio State University, Ph.D., 1970 Astronomy University Microfilms, A XEROX Company, Ann Arbor, Michigani TWTR nTRRFBTATTnW HAS PFFN MTrRnTTT.MFn FVAfTT-Y AS RFTIFTVFT1 A STUDY OF INTRINSIC POLARIZATION IN Be STARS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Robert Leland Bottemiller, B.S., B.S. A A * A A A The Ohio State University 1970 Approved by lfff.tr *a 'f K ' — I Adviser Department of Astronomy ACKNOWLEDGMENTS Gratitude is extended to Dr. George W. Collins, II, who suggested this research problem and who contributed many beneficial ideas throughout the pursuit of this work. Thanks also go to Drs. Robert F. Wing and Terry P. Roark who made a number of suggestions which increased the clar­ ity and accuracy of this dissertation. All observations were made using the Perkins reflector of the Ohio Wesleyan and Ohio State Universities at Lowell Observatory. The author is indebted to these institutions for the use of their facilities, and special thanks go to Dr. John S. Hall, Director of Lowell Observatory, for the use of his polarimeter and for hospitality extended to the author during his visits to Lowell Observatory. The Com­ puter Center of The Ohio State University is also to be thanked for the allotment of machine time necessary for data reduction. This work was initiated while the author held a Traineeship from the National Science Foundation whose support is appreciated. Last mentioned, but foremost, is the love and grati­ tude felt for the author's wife for her encouragement through several years of study and for her perseverance through several months of thesis preparation and typing. i i VITA August 14, 1942 . Born - Portland, Oregon 1964 ................. B.S. (Mathematics), University of Portland, Portland, Oregon 1965 ................. B.S. (Physics), University of Portland, Portland, Oregon 1966-1967 ............ Teaching Assistant, Department of Astronomy, The Ohio State University, Columbus, Ohio 1967-1968 ............ Research Assistant, Department of Astronomy, The Ohio State University, Columbus, Ohio 1968-1969 ............ National Science Foundation Trainee PUBLICATIONS "HD 191980 A Peculiar B-Type Star," P. C. Keenan, A. Slettebak, and R. L. Bottemiller, Astrophysical Letters, Vol. 3, page 55, 1969. FIELDS OF STUDY Major Field: Astronomy Studies in Late Type Stars. Professor Philip C. Keenan Studies in Radiative Transfer. Associate Professors George W. Collins, II, and Eugene R. Capriotti Studies in Molecular Dissociation Equilibria. Assistant Professor Robert F. Wing TABLE OF CONTENTS Page ACKNOWLEDGMENTS .......................................... ii VITA ....................................................... i ii LIST OF T A B L E S .......................................... v LIST OF F I G U R E S .......................................... vii Chapt er I. INTRODUCTION ...................................... 1 II. OBSERVATIONAL PRELIMINARIES .................... 9 Program Systems Single and Double Standard Stars Equipment III. DATA REDUCTION AND ERROR ANALYSIS OF OBSERVATIONS ..................................... 22 Data Reduction Error Analysis Intrinsic Polarization IV. OBSERVATIONAL RESULTS: STANDARD STARS . 38 V. OBSERVATIONAL RESULTS: PROGRAM STARS .... 60 VI. INTERPRETATION OF RESULTS ....................... 90 APPENDIX A ......................................................... 101 B ......................................................... 109 BIBLIOGRAPHY ............................................. 115 iv LIST OF TABLES Double Star Systems ........................... 12 Standard Single Stars ......................... 15 Standard Double Star Systems ............... 15 Comparison with Other Observers ............ 59 Standard Stars with Published Stokes Parameters ........................... 47 Estimate of Instrumental Polarization . 48 Polarization Data: Standard Single Stars . 56 Calculated Intrinsic Polarization: Standard Double Stars ...................... 58 23 Orionis: U ................................ 61 23 Orionis: B ................................ 61 23 Orionis: V ......................... * . 62 HD 4 599 5: U ................................... 66 HD 45995: B..................................... 66 HD 45995: V ................................... 67 59 Cygni ........................................ 68 e Capricorni ................................... 69 8 Lacertae: U ................................ 71 8 Lacertae: B ................................ 71 8 Lacertae: V.................................. 72 v LIST OF TABLES--Continued Table Page 20. 8 M o n ......................................................74 21. 8 Mon: Intrinsic Polarizations.................... 76 22. Summary of Polarimetric D a t a ........................78 23. Fractional Breakup Velocities for Program Stars ..................................... 85 vi LIST OF ILLUSTRATIONS Figure Page 1 . Idealized Scan Records of a Typical Program Double System and g M o n .................. 24 2 . Summary of Observational Data for Standard HD 154445 43 3. The Deviation of Individual Polarization Observations from the Mean Quantity Versus Night of Observation .................... 51 4 . Observed Stokes Parameters (U Filter) of Low Polarization Objects ...................... 52 5. Observed Stokes Parameters (B Filter) of Low Polarization Objects ...................... 53 6 . Observed Stokes Parameters (V Filter) of Low Polarization Objects ...................... 54 7. Summary of Results for Single Star Standards............................................. 57 8 . Results for e Cap A and Comparison with Other Observers...................................... 70 9. Derived Intrinsic Polarizations for the Seven Program S t a r s .................................79 10. Derived Intrinsic Polarization Against the Ratio of V sin (i) to Theoretical Breakup Velocity for U Filter Observations .......... 86 11. Derived Intrinsic Polarization Against the Ratio of V sin (i) to Theoretical Breakup Velocity for B Filter Observations .......... 87 12. Derived Intrinsic Polarization Against the Ratio of V sin (i) to Theoretical Breakup Velocity for V Filter Observations .......... 88 vi i LIST OF ILLUSTRATIONS--Continued Figure Page 13. Average Normalized Polarization from Coyne and Kruszewski (1969) and the Same for Derived Intrinsic Polarization from This Research Versus Inverse Wavelength........................................ 94 vi i i I. INTRODUCTION The possibility that early type stars may exhibit intrinsic polarization is an idea that, for much of its 24-year history, has been neglected in favor of the pheno­ menon of interstellar polarization discovered so serendi- pitously by researchers looking for intrinsic polarization. Chandrasekhar (1946) first demonstrated the possible existence of the effect. He calculated the transport of radiation for a semi - infinite, p 1ane-paral1 el atmosphere where such transport is governed solely by the scattering due to free electrons. By allowing for the polarizing effect of the scattering process on the radiation field, he was able to derive a polarization for the emergent light at the stellar limb of some 11 per cent. Since electron scattering plays an important role in the opacity of early type stars, Chandrasekhar suggested the observation of eclipsing binaries with such primaries near the time of deepest minimum as a test of his results. The first subsequent attempts at this were made photo­ graphically with ambiguous results of low accuracy (Janssen, 1946; Hiltner, 1947). Much better accuracy was deemed necessary since masking effects by the eclipsing star 2 reduced the predicted polarization, in the example of RY Persei, from 11 per cent to 1.2 per cent (Hiltner, 1949). During this period, Hall (1949) had been building a photoelectric polarimeter at Amherst College and made contact with Hiltner which led to initial observations at the 82-inch telescope of the McDonald Observatory of the University of Texas in the summer of 1947. Although there were "no dependable results" (Hiltner, 1949), at least some evidence of polarization in the light of the Wolf-Rayet eclipsing binary CQ Cephei was obtained (Hall, 1949). Improvements in Hall's apparatus made polarization easily measurable in CQ Cephei the following summer at Amherst and no dependence on the light curve phase was discernible. He soon moved to the United States Naval Observatory, and observations made with the 40-inch telescope in Washington from November, 1948, to January, 1949, culminated in Hall's 1949 results for nearly thirty stars. By the summer of 1948, Hiltner's own photoelectric polarimeter was complete and observations were underway. Data obtained at the McDonald and Lick Observatories also showed measurable polarization for CQ Cephei, independent of phase, and for other stars as well. The unexpected conclusion was that the polarization was introduced not in the stellar atmosphere but during the photons' traversal of the interstellar medium 3 (Hiltner, 1949). Hall's report also showed a rough corre­ lation with color excess. This discovery raised many questions yet to be fully answered and at the same time opened an entirely new means of probing the characteristics of the interstellar medium. One question that was seemingly laid to rest was that of Chandrasekhar's original prediction. The total lack of time variation in
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