Ca Ii H and K Emission from Late-Type Stars
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CA II H AND K EMISSION FROM LATE-TYPE STARS PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAS VAN DOCTOR IN DE WISKUNDE EN NATUURWETENSCHAPPEN AAN DE RIJKSUNIVERSITEIT TE UTRECHT, OP GEZAG VAN DE RECTOR MAGNIFICUS PROF. DR. M.A. SOUMAN, VOLGENS BESLUIT VAN HET COLLEGE VAN DECANEN IN HET OPENBAAR TE VE^EDIGBN OP MAANDAG 5 JULI 1982 DES NAMIDDAGS TE 4.IS UUR DOOR FRANS MIDDELKOOP GEBOREN OP 30 JANUARI 1953 TE ROTTERDAM •\ PROMOTOR: PROF.DR.C.ZWAAN opgedragen aan: Ans de Jong (mijn vrouw), Adrianus Middelkoop {mijn vader), Alie Twigt (mijn moeder). De hemelen vertellen Gods eer en het uitspansel verkondigt het werk zijner handen Psalm V9:2 Dankbetuiging Graag wil Ik alle personen en Instellingen die bijgedragen hebben tot de totstandkoming van dit proefschrift van harte bedanken. In de eerste plaats dank Ik mijn promotor, Prof.Dr. Kees Zwaan. Zijn steun en begeleiding zijn, vooral in het eerste jaar van dit onderzoek, van groot belang geweest. Zijn enthousiasme en toewijding hebben mij altijd zeer gestimuleerd. Ook ben ik veel dank verschuldigd aan mijn vriend en collega Barto Oranje. De vele gesprekken die ik met hem heb gevoerd hebben duidelijk invloed op dit proefschrift gehad. Dr. Rene' van Helden heeft mij, op bijzonder prettige en leerzame wijze, de eerste beginselen van de sterrekunde bijgebracht en mij in contact gebracht met mijn promotor. Dr. Tony Hearn stond altijd voor mij klaar als de Interne referee van mijn publikaties. I am also indebted to Dr. Arthur Vaughan who has instructed me on how to use his excellent Ca II H and K photometer. Be and his wife Ann never failed to make me feel at home in California. Among the many pleasant people I met in the U.S.A. I mention as well James Frazer, who often helped me as night assistent. I will always remember the many hours we did our job singing loudly (and probably out of tune). De adviezen en programma's van Ed van der Zalm hebben mij veel tijd bespaard. Ik dank hem voor de prettige wijze waarop hij mij altijd heeft bijgestaan. Ook gaat mijn dank uit naar Jan Odljk die al mijn typewerk heeft verricht, Evert Landre' dia alle figuren heeft verzorgd en de heer Repelaer van Driel die veel ponswerk voor mij heeft gedaan. Een groot deel van dit onderzoek is gesteund door de Stichting Astronomisch Onderzoek in Nederland (ASTRON) met financiële steun van de Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek. Ik ben dankbaar voor de soepele wijze waarop deze steun is verleend. I am also grateful for the hospitality of Mt.Wilson Observatory; the measurements with the Ca II H and K photometer at the 60 inch telescope axe essential for this thesis. Tenslotte dank ik mijn vrouw Ans voor alles wat zij voor de totstandkoming van dit proefschrift heeft overgehad. CONTENTS page CHAPTER SURVEY 9 CHAPTER II ROTATION MODULATION 11.1 Indications for Rotation Modulation and Short- 19 Term Variations in the Ca II H and K Emission from Cool Main-Sequence Stars 11.2 Stellar Rotation in Lower Main-Sequence Stars 27 Measured from Time Variations in H and K Emission-Line Fluxes I: Initial Results CHAPTER III MAIN-SEQUENCE STARS 111.1 Magnetic Structure in Cool Stars III: Ca II H and K Emission and Rotation of Main-Sequence Stars 111.2 Magnetic Structure in Cool Stars 49 IV : Rotation and Ca II H and K Emission of Main-Sequence Stars CHAPTER IV EVOLVED STARS IV.1 Magnetic Structure in Cool Stars 59 I : The Ca II H and K Emission from Giants IV.2 Magnetic Structure in Cool Stars 73 VI: C.a II H and K Fluxes from Evolved Stars SAMENVATTING 87 I CHAPTER I: SURVEY 1. INTRODUCTION The Ca II H and K resonance lines at 3934 A and 3968 & are an important diagnostic for the study of stellar chromospheres. In a number of main-sequence stars of spectral type later than 9a F5, and in nearly all giants of spectral type later than GO, emission features are visible in the cores of both of these lines (Fig. 1 and 2). This thesis is based on a study of these emission features. The Sun is among the late-type stars showing Ca II H and K emission. A picture of the Sun in one of the Ca II H and K line-cores clearly shows (Fig. 3) that the emission originates predominantly in discrete regions. These regions are cospatial to within 1" with discrete magnetic features (see Zwaan 1978) (Fig. 4). Hence we assume that the stellar Ca II H and K flux increases with the magnetic flux passing through the stellar surface. Except for the Sun, magnetic fields on stars can only be measured directly for bright and magnetically active stars (see Sect. 2). Hence,indirect methods to measure magnetic fields on stars have to be used to study a large sample of stars with a strongly differing magnetic activity. The measurement of the Ca II H and K emission of stars is one way to do just that. In this investigation the Sun is used as an example and guide for the study of stellar Ca II H and R emission. The amount of information contained in the Ca II H and K emission cores is enormous. It ranges in time from information about flare-like behavior on time-scales of minutes through information about stellar rotation periods on time-scales of days (weeks, months) and stellar activity cycles on time-scales of years to information about stellar evolution on time-scales of 1010 years. The width of the emission cores is a measure of the absolute magnitude of the star (Wilson and Bappu, 1957, Wilson 1976) while the intensity.in combination with the width contains information about the mass of stars (Sect. 3). Although this. Investigation is based on a tiny fraction of the electromagnetic spectrum it addresses fundamental characteristics of stars such as: mass, rotation period as a function of age, evolutionary phase and last but not least magnetic activity as a function of age. 2. DIRECT MEASUREMENTS OF MAGNETIC FIELDS On the Sun magnetic fields can be measured through the polarization effect in the magnetic Zeeman splitting of absorption lines. However, this method does not apply to cool stars, probably because opposite polarities produce opposite polarization effects, Like on the Sun, magnetic fields of cool stars are expected to appear 1B bipolar regions occupying a fraction of the stellar disk. The net polarization effect of the visible part of the star's surface will therefore be very small and undetectable for most stars (for example: the average magnetic field measured for the sun in this way would be f>a-\ Gauss (Scherrer et al. 1977). A method of measuring magnetic fields of late-type stars and the fractional area they cover has been applied by Robinson et al. (1980). It relies on the fact that magnetic Zeeman splitting (for a simple Zeeman triplet) produces two a components separated from the unshifted n component by an amount depending only on field strength (Unno (1956)). The method results in the average absolute value of the 10 It Fig. 2: Ca II H and K emission cores in speetva (Wilson and Bappu (195?)} CalK line core index CaUK emission index (indices defined relative to some continuum 1iux) Fig.2: Ca II H and K emission aores; definitions \ 11 Fig.3: Filtergram of the Sun in the core of the Ca II K line (courtesy B.J. Oranje) Fig.4: A mature active region: a) Filtergram in the core of the Ca II K Vine: b) magnetogram; the comparison between (a) and (b) demonstrates the dose spatial correspondence between faculae (as observed in Ca II K emission) and magnetic flux outside sunspots (Big Bear Solar Observatory, courtesy H. Zirin) L 12 magnetic field strength present and the area covered by magnetic fields. It requires a high spectral resolution and a large signal-to-noise ratio. For a signal-to-noise ratio of 100:1, field strengths of 1000 Gauss or more can be measured if they cover at least 3-4X of the stellar surface. Hence this method is limited to bright and magnetically active stars. Promising results on two stars have been published (Robinson et al. 1980) and the technique has been applied to a number of stars by Marcy (1981) who reports magnetic fields in at least 50% of the observed stars. In most cases the fraction of the surface covered by magnetic fields is small. This and the direct observation of magnetic fields in stars supports the idea of the presence of magnetic field in discrete magnetic structures on late-type stars and it adds to our confidence in using the Sun as an example and guide for the observation of magnetic activity In other late-type stars. 3. THE CA II H AND K PHOTOMETER AT MT. WILSON All the observations presented in this thesis were obtained wltf the Ca II H and K photometer at the 60 Inch telescope of Ht. Wilson. A short description of this instrument is given here; for a more detailed description I refer to Vaughan et al. (1978). The Ca II H and K photometer (Fig. 5) is a'four-channel photon-counting spectrophotometer consisting of an off-piane flat—field Ebert spectrometer equipped with a multi-exit slit and a chopper for measuring the fluxes in four spectral passbands sequentially at a chopping frequency of about 30 Hz, with a single detector. The spectrum can be shifted parallel to the dispersion to compensate for Doppler shifts.