A PHOTOMETRIC AND SPECTROSCOPIC STUDY OF SELECTED SOUTHERN, CHROMOSPHERICALLY ACTIVE STARS A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ASTRONOMY IN THE UNIVERSITY OF CANTERBURY by LYNDON WATSON· University of Canterbury 1999 ii YSICAL IENCES 3RARY Q5 <6;23 ,W333 1999 Abstract The results of a combined photometric and high-resolution spectroscopic study of south­ ern active-chromosphere stars selected from the ROSAT Bright Source Catalogue are presented. The star CS Ceti (HD 6628) is shown to be a single-lined spectroscopic binary and an orbital solution derived from' radial-velocity measurements is presented. Evidence that one of the stars is an F-type dwarf and the other a highly active G-type subgiant is presented, together with information on the apparently asynchronous rotation of the latter and the spatial origin of its Ha emission. The system BB Sculptoris (HD 9770) is shown to include a chromospherically active eclipsing binary of the BY Dra class and evidence that this is the star previously known as HD 9770B is presented. A precise orbital period is presented. Further evidence is presented to show that the star known as HD 9770A is also a binary system, one member of which dominates the absorption spectrum of BB ScI. Two further stars selected from the Bright Source Catalogue are recommended for further study: HD 147633 in which a known binary system is found to contain a further chromospherically active, short-period, double-lined spectroscopic binary; and HD 222259, a known chromospherically active binary in the light curve of which a spot wave which persists on a time scale of years has been found and for which a precise period is presented. In addition, the results of a continuing program of observation of the chromospherically active system GT Muscae (HR4492) are presented. Evidence is presented that the known eclipsing binary system HD 101380 consists of AO V and AO/2 V stars, and the period of the system is further refined. A combined photometric and high-resolution spectroscopic study of the active binary HD 101379 provides evidence that its active star is a late G-type bright giant rather than a K4 star as suggested in a previous study. Evidence that the source of its Ha emission lies in the space between the stars is discussed. High-resolution spectra of two chromospherically active stars - CF Tucanae (HD 5303) and HD 219025 - observed for other programs are also presented and briefly discussed. III 1 5 MAR 7nnn IV ABSTRACT Preface The program of research described in this thesis had its origin in a proposal by Dr D. A. H. Buckley of the South African Astronomical Observatory (hereinafter "SAAO") for a collaborative program of photometric and high-resolution spectroscopic observations of chromospherically active stars with ultraviolet excess. Medium-resolution spectra of southern stars chosen from the ROSAT Bright Source Catalogue were obtained at SAAO, and stars with excess emission in the Hand K lines of calcium indicating chromospheric activity were selected for photometric observation at SAAO and the Mount John Uni­ versity Observatory (hereinafter "MJUO"), and high-resolution spectroscopy using the MJUO echelle spectrograph. U BV(RI)c photometry was obtained at SAAO in 1992 and 1993, and is included in the data analysed in this thesis. The balance of the photometric data which has been used was obtained by service observers at MJUO who also began U BV(RI)c photometry in 1993 and continued to obtain data until the observational program ended in 1997. The service observers have been entirely responsible for the reduction of the photometric data to standard systems. Some high-resolution spectra were obtained by several observers during 1993. All other spectra were obtained by the writer from 1994 to 1997. Chapters 3 and 4 relating to the binary systems CS Ceti (HD 6628) and BB Sculptoris (HD 9770) are expanded versions of papers submitted for publication in the Monthly Notes of the Royal Astronomical Society. Both papers were written by the present author, with the exception of the section of Chapter 4 relating to the use of a Wilson-Devinney program to fit physical stellar parameters to the photometry, and its results. The program which was used was a modification of the original \V"ilson-Devinney code by Dr J. D. Pritchard and the execution of the program, based on the present author's initial values of the physical parameters, was carried out entirely by Dr Pritchard who also contributed the relevant part of the paper which was submitted for publication. In addition to the stars selected for study at SAAO, the program of research also included the active system GT Muscae (HR4492) which has been the subject of photo­ metric observations at MJUO since 1981. The present author made use of the BV(RI)c (and a small amount of U) photometry which has accumulated since 1981, and addition­ ally obtained spectroscopic data on the system. Chapter 5, relating to this system, is intended for submission for publication in due course. v VI PREFACE Contents ... Abstract 111 Preface v 1 Introduction and background 1 1.1 Definition of the class of chromospherically active stars 1 1.2 Evolutionary status of active stars ......... 2 1.3 Rotation of active stars ............... 3 1.3.1 Chromospheric activity and Rossby number 4 1.3.2 Asynchronous rotation ........... 6 1.3.3 Transfer of angular momentum ...... 7 1.3.4 Co-rotation and coplanarity of orbit and rotation 8 1.4 Mechanisms of chromospheric excitation 9 1.4.1 Acoustic mechanisms ...... 9 1.4.2 Magnetic mechanisms ..... 11 1.5 Excess fluxes at non-optical wavelengths 13 1.5.1 Excess x-ray emission. 13 1.5.2 Excess ultraviolet emission 13 1.5.3 Excess infrared emission . 14 1.5.4 Excess radio emission ... 14 1.6 Differential rotation and spot size, lifetime and location 15 1.6.1 Size of spots on active stars .... 15 1.6.2 Differential rotation of active stars 16 1.6.3 Lifetime of spots on active stars . 16 1.6.4 Location of spots on active stars. 16 1. 7 Modelling star spots '" 17 1. 7 .1 High-latitude sp ots 17 1.8 Lithium excess 18 1. 9 Ha emission . 19 2 Observations and reduction of data 23 2.1 Selection of target stars. 24 2.2 Photometric observations ..... 25 2.3 High-resolution echelle spectroscopy. 27 2.3.1 Measurement of radial velocities 30 vii Vlll CONTENTS 2.3.2 Measurement of rotational broadening 31 2.4 Medium-resolution spectra 32 2.5 Some general results 34 2.5.1 HD 147633 . 34 2.5.2 HD 222259 . 36 3 CS Ceti 39 3.1 MJUO UBV RI photometry and spot-modulation 40 3.2 High-resolution spectroscopy ......... 41 3.2.1 Orbital parameters .......... 41 3.2.2 Derived stellar and orbital parameters 42 3.2.3 Rotation of the active star 47 3.2.4 Lithium abundance 49 3.2.5 Ha: emission 50 3.3 Conclusions . 4 BB Sculptoris 53 4.1 Identification as a chromospherically active star 4.2 The light curves ................. 56 4.2.1 Variability due to spots and tidal distortion 58 4.3 Physical parameters of the stars . 58 4.3.1 Wilson-Devinney analysis 60 4.3.2 The A star of BB Scl . 64 4.4 High-resolution spectroscopy .. 65 4.4.1 The metal-line spectrum 66 4.4.2 Radial velocities. 67 4.4.3 Ha: line ....... 67 4.4.4 Rotational broadening 68 4.4.5 Lithium lines 69 4.5 Conclusion . 69 5 GT Muscae 71 5.0.1 Hipparcos data . 72 5.1 The eclipsing binary system HD 101380 73 5.1.1 Colours ............ 75 5.1.2 Magnitudes and colours of the individual stars. 76 5.2 Photometry of HD 101379 . 78 5.2.1 Long-period variability ... 80 5.2.2 Spot waves ......... 80 5.2.3 Spot temperatures and extent 82 5.2.4 Colour excess . 87 5.3 High-resolution spectroscopy ..... 89 5.3.1 Rotation and radius of the active star. 89 5.3.2 The Ha: absorption line. 92 5.3.3 Ha: emission . 92 CONTENTS IX 5.4 Conclusion.................................... 93 6 Observations outside the main program 95 6.1 Observations of CF Tucanae ...... 95 6.1.1 Ha spectroscopy and interpretation 96 6.2 Observations of HD 219025 ........ · 100 7 Summary of conclusions 103 7.1 CS Ceti ... · 103 7.2 BB Sculptoris · 103 7.3 GT Muscae · 105 References 107 Appendix A 115 Appendix B 119 Appendix C 123 Appendix D 129 Acknowledgements 139 x OONTENTS List of Tables 1.1 Fraction of chromospherically active binaries which rotate asynchronously. 6 1.2 Orbital and rotational periods and inelinations for synchronous RS CVn systems ..................................... 8 1.3 Orbital and rotational periods and inelinations for asynchronous RS CVn systems ..................................... 9 2.1 Initial list of chromospherically active stars selected from the ROSAT Bright Source Catalogue . 24 2.2 Final selection of target stars ......................... 25 2.3 Target, comparison and check stars. .. 26 2.4 Spectral lines in orders 34 to 42 of the MJUO echelle spectrograph used, where available, for measuring radial velocities. ............... 28 2.5 Spectral lines in orders 43 to 45 of the MJUO echelle spectrograph used, where available, for measuring radial velocities. ............... 29 2.6 Spectral lines in orders 46 and 47 of the MJUO echelle spectrograph used, where available, for measuring radial velocities. 30 3.1 ROSAT EUV count rates for CS Cet . 40 3.2 IRAS fluxes, magnitudes and colours of CS Cet 40 3.3 MJUO U BV RI photometry of CS Cet .