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Chromospheric and Coronal Activity in Solar-Like Stars

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Chromospheric and Coronal Activity in Solar-Like Stars Chromospheric and coronal activity in solar-like stars Dissertation zur Erlangung des Doktorgrades des Departments Physik der Universitat¨ Hamburg vorgelegt von Christian Schroder¨ geboren in Hamburg Hamburg 2008 ii Gutachter der Dissertation: Prof. J.H.M.M. Schmitt Dr. Ansgar Reiners Gutachter der Disputation: Prof. J.H.M.M. Schmitt Prof. P.H. Hauschildt Datum der Disputation: 18.07.2008 Vorsitzender des Pr¨ufungsausschusses: Prof. G. Wiedemann Vorsitzender des Promotionsausschusses: Prof. J. Bartels Dekan der Fakult¨at f¨ur Mathematik, Informatik und Naturwissenschaften: Prof. Dr. Arno Fr¨uhwald iv Zusammenfassung Der Begriff ”stellare Aktivit¨at” fasst diverse Ph¨anomene auf der Sternenoberfl¨ache und in der Stern- atmosph¨are zusammen. Die Ursache f¨ur diese Ph¨anomene sind Ver¨anderungen in den Magnetfeldern der Sterne. W¨ahrend es bei der Sonne m¨oglich ist, die verschiedenen Formen der Aktivit¨at direkt und detailliert zu beobachten, ist dies bei anderen Sternen nicht m¨oglich. Durch eine genaue Analyse der Spektren von Sternen lassen sich dennoch Informationen ¨uber deren Aktivit¨at gewinnen. Die folgende Arbeit befasst sich mit zwei Arten der stellaren Aktivit¨at: zum einen mit der R¨ontgenaktivit¨at von Sternen des Spektraltyps A und ihrer potentiellen Ursache, zum zweiten mit der chromosph¨arischen Aktivit¨at von Sternen im Bereich der sp¨aten A- bis sp¨aten K-Sterne. Da im Bereich der sp¨aten A- bis fr¨uhen F-Sterne der Dynamo, der f¨ur die Aktivit¨at in sonnen¨ahnlichen Sternen verant- wortlich ist, entsteht, ist dieser Spektralbereich f¨ur das Verst¨andnis der stellaren Aktivit¨at von besonderer Bedeutung. Zur Untersuchung der R¨ontgenaktivit¨at von A-Sternen wurde nach Ubereinstimmungen¨ zwischen den optischen Positionen der A-Sterne, die im Bright Star Catalogue aufgelistet sind, und den Positio- nen der in den ROSAT-Katalogen aufgef¨uhrten R¨ontgenquellen gesucht. Als den f¨ur eine Korrelation maximal zul¨assigen Abstand wurden 90 Bodensekunden f¨ur die Daten des ROSAT All-Sky Survey fest- gelegt, sowie 36 Bodensekunden f¨ur die gezielten PSPC- Beobachtungen und 18 Bogensekunden f¨ur die HRI-Daten. Die sich daraus ergebende Liste von Sternen wurde nach m¨oglichen Anzeichen f¨ur bisher unentdeckte, sp¨ate Begleitsterne untersucht. Dabei galten Schwankungen in der Radialgeschwindigkeit, der Eigenbewegung und der Lichtkurve der Sterne als Indizien f¨ur die Anwesenheit eines unentdeckten Begleiters. Die Korrelation der Kataloge ergab eine Liste von 312 A-Sternen, die mit einer R¨ontgenquelle assoziiert werden k¨onnen. 84 dieser Sterne sind Einzel- oder r¨aumlich aufgel¨oste Doppelsterne. In einem weiteren Schritt wurden 13 der r¨ontgenaktiven A-Sterne auf m¨ogliche Magnetfelder un- tersucht. Dazu wurden die Spektren dieser Sterne auf die Anwesenheit von Merkmalen zirkularer Po- larisation im Bereich der Balmerlinien des Wasserstoffs und in Kalziumlinien untersucht. Durch die Anwesenheit der nach Pieter Zeeman benannten Merkmale l¨asst sich die St¨arke des longitudinalen An- teils des ¨uber die Sternoberfl¨ache gemittelten Magnetfeldes messen. Die Beobachtungen ergaben f¨ur drei Sterne sichere Detektionen und bei neun weiteren Sternen m¨ogliche Hinweise auf schwache Mag- netfelder. Bei den Sternen mit sicherer Detektion wurde ¨uberpr¨uft, ob die R¨ontgenhelligkeit mit den Vorhersagen des magnetically confined wind shock models ¨ubereinstimmt. Ein Zusammenhang zwi- schen der beobachteten R¨ontgenhelligkeit und der St¨arke der Magnetfelder wurde nicht gefunden. Die chromosph¨arische Aktivit¨at von 481 sp¨aten A- bis zu sp¨aten K-Sternen wurde auf zwei Arten gemessen. Zum einen mit Hilfe der klassischen Mount-Wilson-Methode, die jedoch nur f¨ur langsam rotierende Sterne mit dem Farbindex B-V im Bereich von 0.44 bis 0.9 geeignet ist, zum anderen mit einer neu entwickelten Methode, die auch f¨ur schnell rotierende Sterne und f¨ur Sterne mit B-V < 0.44 geeignet ist. Bei dieser neuen Methode wird der zu vermessende Stern mit einem inaktiven Stern verglichen. Dazu muss dieser auf die Rotationsgeschwindigkeit des zu vermessenden Sterns und die damit verbundene Rotationsverbreiterung angepasst werden. Eine Analyse der Auswirkungen der Rotationsverbreiterung zeigt, dass bei schnell rotierenden, inaktiven Sternen die verbreiterten Linienfl¨ugel der Kalziumlinie die Messungen der klassischen Methode verf¨alschen, w¨ahrend die Ergebnisse der neuen Methode un- beeinflusst bleiben. F¨ur langsam rotierende Sterne sind die Ergebnisse der neuen Methode mit denen der klassischen Mount-Wilson-Methode vergleichbar und erlauben die Beobachtung des Einsetzens der chromosph¨arischen Aktivit¨at im Bereich der sp¨aten A- bis fr¨uhen F-Sterne. v Abstract The term “stellar activity” summarizes a number of phenomena on the stellar surface and in the stellar atmosphere. The origin of many of these phenomena are changes in the structure of the stellar magnetic field. While it is possible to directly observe different forms of activity on the Sun in great detail, this remains impossible for other stars. However, by analyzing the spectra of these stars, information about their activity can be obtained. This thesis addresses two aspects of activity: First, the X-ray emission from the positions of A-type stars and a possible mechanism to produce these X-rays and second, the chromospheric activity in the spectral range from late A- to late K-type stars. Since the dynamo, which is responsible for the activity of solar-like stars, emerges in the range of late A- to early F-type stars, this spectral range is of special interest for the understanding of the activity phenomena. To study the X-ray activity of A-type stars, their optical positions as given in the Bright Star Cata- logue were compared with the positions of the X-ray sources listed in the ROSAT catalogs. The matching criteria for the ROSAT All-Sky Survey data were 90 arcseconds, 36 arcseconds for the pointing observa- tions with the PSPC and 18 arcseconds for HRI data. Those stars which could be associated with X-ray sources were tested for indications of hidden late-type companions. Variations in the radial velocity, the proper motion, and the light curve were interpreted as signs for binarity. The correlation of the catalogs yielded 312 stars with an associated X-ray source, of which 84 are bona fide single or resolved binary stars. In a second step, 13 of the X-ray emitting A-type stars were observed in the optical to search for magnetic fields. To detect and quantify the magnetic fields, the spectra of the stars were searched for Zeeman features in the hydrogen Balmer and calcium lines, which are indicators for circular polarization. The analysis of the Zeeman features allows the determination of the strengths of the longitudinal field averaged over the whole stellar disk. In three out of 13 cases the observations revealed magnetic fields at a 3 σ significance level. Indications for weak magnetic fields were found in nine additional stars. For the three stars with certain detections the X-ray luminosity was compared to the predictions of the magnetically confined wind-shock model. A direct correlation between the magnetic field strength and the X-ray luminosity was not found. The chromospheric activity of 481 late A- to late K-type stars was measured with two methods. First, with the classical Mount Wilson method, which can be applied to slowly rotating main-sequence stars in the range from 0.44 <B −V < 0.9 and second with a new template method. This new method allows to measure the chromospheric activity in rapidly rotating stars and stars with B − V < 0.44. To determine the activity level of a star applying the new technique, one compares the spectrum of the target star with the spectrum of a slowly rotating inactive star. To fit the template spectrum to the spectrum of the stars, it has to be artificially broadened according to the rotational velocity of the target star. A study of the effects of the rotational broadening showed a strong influence on the classical measurement for rapidly rotating inactive stars, while the new template method is unaffected by rotational broadening. The results of the new method are consistent with those of the classical method and allow to observe the onset of the chromospheric activity in the late A- to early F-type stars. vi Contents 1 Introduction 1 1.1 Solar and stellar activity . ......... 2 1.1.1 Photospheric activity . ...... 2 1.1.2 Chromospheric activity . ...... 3 1.1.3 X-ray activity and the corona . ....... 7 1.1.4 Magnetic heating and stellar dynamo . ........ 7 1.1.5 On the distinctiveness of A-type stars . .......... 10 1.1.6 Windsandshocks................................ 14 1.2 Observational methods . ....... 17 1.2.1 X-rayastronomy ................................ 17 1.2.2 ROSAT....................................... 17 1.2.3 Magneticfields ................................. 19 1.2.4 CaIIH&Kmeasurements ............................. 20 1.3 Outline ......................................... 23 2 X-ray emission from A-type stars 29 3 Magnetic fields in A-type stars associated with X-ray emission 45 4 Ca II HK emission in rapidly rotating stars 57 5 Summary and outlook 77 5.1 Summary ......................................... 77 5.2 Outlook ......................................... 78 Acknowledgements 81 viii CONTENTS Chapter 1 Introduction Stellar activity describes a large variety of at- mospheric phenomena in the atmosphere of stars. Signs of activity can be found on the Sun and al- most all stars with a surface temperature of less than 7500 K down to the borderline between stars and brown dwarfs at around 2000 K. Indicators for activity are for example spots, prominences, X-ray flares, mottles and spiculae. Spatially resolved ob- servations of many of these activity phenomena re- quire special observational efforts or are only pos- sible for the Sun. Therefore, the knowledge about the existence of activity in other stars than the Sun is based on detailed studies of the light emitted by the whole star.
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