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X-Ray Properties and Activity of Cool Stars the View of XMM-Newton X-ray properties and activity of cool stars The view of XMM-Newton Dissertation zur Erlangung des Doktorgrades des Fachbereichs Physik der Universitat¨ Hamburg vorgelegt von Jan Robrade aus Krefeld Hamburg 2005 ii Gutachter der Dissertation: Prof. Dr. J.H.M.M. Schmitt Prof. Dr. S. Sciortino Gutachter der Disputation: Prof. Dr. J.H.M.M. Schmitt Prof. Dr. P.H. Hauschildt Datum der Disputation: 17. 02. 2006 Vorsitzender des Prufungsausschusses:¨ Dr. habil. F.-J. Zickgraf Vorsitzender des Promotionsausschusses: Prof. Dr. G. Huber Dekan des Fachbereichs Physik: Prof. Dr. G. Huber ii iii iv Zusammenfassung Die Entdeckung und Untersuchung von Sonnenflecken und der Korona der Sonne im sichtbaren Licht und spater¨ bei hoheren¨ Energien erof¨ fnete ein neues Feld der Astronomie. Die Entstehung dieser Strukturen ist eng mit der Existenz vom starken Magnetfeldern verknupft,¨ welche durch Dynamoprozesse im Inneren der Sonne erzeugt werden. Veranderliche¨ magnetischen Strukturen sind mit einer ganzen Reihe von Erschei- nungen verbunden, welche als solare bzw. stellare Aktivitat¨ bezeichnet werden. Die sehr hohen Tempera- turen der koronalen Plasmen ermoglichen¨ das Studium von stellarer Aktivitat¨ und der zugrunde liegenden Physik insbesondere durch Untersuchung ihrer Rontgenstrahlung.¨ Da kuhle¨ Sterne einen vergleichbaren inneren Aufbau wie die Sonne besitzen und ihre Rontgenemission¨ ebenfalls mit magnetischen Prozessen verknupft¨ ist, konnen¨ ihre Aktivitatsph¨ anomene¨ ahnlich¨ denen der Sonne beschrieben werden. Allerdings ubersteigt¨ das Ausmaß stellarer Aktivitat¨ das solare Niveau um das bis zu tausendfache. Da eine raumliche¨ Auflosung¨ von koronalen Strukturen bisher nur auf der Sonne moglich¨ ist, basieren die Erkenntnisse uber¨ stellare Aktivitat¨ hauptsachlich¨ auf der Untersuchung von Leuchtkraften,¨ Lichtkurven und Spektren. Mit den neuen Rontgensatelliten¨ wie z.B. XMM-Newton, welcher eine hohe Empfindlichkeit sowie eine hohe spektrale Auflosung¨ besitzt, wurde die Untersuchung von Sternen in diesen Energiebereich mit bisher unbekannter Prazision¨ moglich.¨ Diese Arbeit beschaftigt¨ sich mit der Aktivitat¨ von kuhlen¨ Sternen und beruht auf Daten, welche mit den Instrumenten an Bord von XMM-Newton aufgenommen wurden. Verschiedene Aspekte der stellaren Rontgenemission¨ in unterschiedlichen Sterntypen werden untersucht. Nach einer Einfuhrung¨ werden im ersten Teil dieser Arbeit werden aktive M Sterne besprochen. Diese recht kuhlen¨ und massearmen Sterne sind starke und variable Rontgenquellen¨ mit extremen Eigenschaften ihrer Koronen. So sind die beobachteten koronalen Temperaturen wesentlich hoher¨ als bei der Sonne, der relative Anteil der im Rontgenbereich¨ abgestrahlten Energie ubertrif¨ ft den anderer Arten kuhler¨ Sterne oft deutlich und starke Energieausbruche¨ werden regelmaßig¨ beobachtet. Die Rontgeneigenschafte¨ n einer Auswahl von aktiven M Sternen werden in unterschiedlichen Aktivitatsphasen¨ untersucht, hierzu gehoren¨ die Doppelsternsysteme AT Mic und EQ Peg sowie die Einzelsterne AD Leo und EV Lac. Obwohl sich ihr Aktivitatsni¨ veau deutlich unterscheidet, sind wesentliche Eigenschaften ihrer Koronen vergleichbar. Im zweiten Teil wird die Rontgenemission¨ von jungen Sternen behandelt, wobei der Schwerpunkt auf klassischen T Tauri Sternen liegt. Diese sehr jungen Sterne haben ihre Entwicklung zum Hauptrei- henstern noch nicht vollstandig¨ abgeschlossen, sind ebenfalls als starke Rontgenstrahler¨ bekannt und be- sitzen eine den Stern umgebende Scheibe aus Gas und Staub. Zusatzlich¨ zu koronaler Aktivitat¨ kann ihre Rontgenstrahlung¨ daher auch durch magnetische Akkretion des den Stern umgebenden Materials, welche zu starken Schocks auf der Sternoberflache¨ fuhrt,¨ erzeugt werden. Die Entstehungsmechanismen der Rontgenstrahlung¨ werden in klassischen T Tauri Sternen unterschiedlichen Alters und spektraler Eigen- schaften anhand der Sterne BP Tau, CR Cha, SU Aur und TW Hya untersucht. Akkretionsschocks und koronale Aktivitat¨ tragen zu der beobachteten Rontgenstrahlung¨ bei, allerdings unterscheidet sich die Be- deutung der beiden Prozesse in den verschiedenen Objekten deutlich. Es wird weiterhin der Nachweis von Beta Pictoris im Rontgenlicht¨ berichtet und deren mogliche¨ Erzeugungsmechanismen diskutiert. Der dritte Teil dieser Arbeit beschaftigt¨ sich mit alteren¨ und nur maßig¨ aktiven, sonnenahnlichen¨ Sternen und ist insbesondere der Untersuchung von zyklischem Aktivitatsv¨ erhalten gewidmet. Einer der wesentlichen Merkmale der Sonnenaktivitat¨ ist ihr 11-jahriger¨ Zyklus, welcher mit seinen Einfluss auf das Erdklima von wesentlicher Bedeutung ist. Koronale Aktivitatszyklen¨ von Sternen sind dagegen nur bei wenigen Objekte bekannt, da entsprechende Langzeitbeobachtungen in Rontgenbereich¨ fehlen. Die Sternensysteme Alpha Centauri A/B und 61 Cygni A/B wurden uber¨ Jahre regelmaßig¨ mit XMM-Newton beobachtet. Die beobachtete Veranderlichk¨ eit der Rontgenleuchtkraft¨ und damit verbundene Veranderungen¨ der koronalen Eigenschaften werden untersucht. Anzeichen fur¨ moglich¨ koronale Aktivitatszyklen,¨ ins- besondere des solaren Zwillingssternes Alpha Cen A, werden in dieser Arbeit vorgestellt. iv v Abstract The discovery and succeeding detailed studies of sunspots and the solar corona first in visible light and later at higher energies gave birth to a new field in astronomy. The formation of a corona around the Sun and other cool stars is directly linked to the existence of strong magnetic fields, which are generated by dynamo processes in their interior. Magnetic fields and its changing configurations are responsible for a variety of phenomena, summarised as solar or stellar activity. The high temperatures of the coronal plasma make the X-ray domain ideal to investigate stellar activity and its underlying physics. Since cool stars have in common a comparable internal structure and their X-ray emission and activity is likewise related to magnetic processes, they are described in a similar way and solar results can often transfered into the stellar context. The levels of activity observed in stars exceed the solar one by up to a factor of thousand, but only on the Sun coronal structures can be spatially resolved. Investigation of stellar activity is therefore based on the study of light curves, X-ray brightness and especially on spectral analysis. With the advent of the new generation X-ray missions like XMM-Newton and its ability to provide high sensitivity and high spectral resolution, the study of the energetic phenomena on other stars was put on a new level. This thesis deals with the activity of cool stars and is based on data obtained with the instruments onboard XMM-Newton. Several aspects of stellar X-ray emission are investigated in different types of cool stars. These include among others active M dwarfs, classical T Tauri stars and solar-like stars. After introducing the topic, in the first part of this thesis the coronal activity of active M dwarfs is investigated. These low-mass stars at the cool end of the main sequence are strong and variable X-ray sources with extreme coronal conditions. Coronal temperatures are much higher than on the Sun, the relative amount of energy emitted in X-rays exceeds that of most of the others cool star and flaring is observed on a regular basis. Their coronal properties are investigated for different phases of activity for a sample of mid M dwarfs, which includes the binary systems AT Mic and EQ Peg as well as in the single stars AD Leo and EV Lac. Despite the observed different level of activity, many coronal properties of the sample stars are found to be comparable. The second part of this thesis deals with the X-ray emission from very young stars, mainly from classical T Tauri stars. These are the pre-main sequence equivalents of cool stars which are in the process of evolving towards the main sequence. Classical T Tauri stars still possess a disk with significant amounts of matter and are actively accreting. They are strong X-ray sources and additionally to coronal activity X-rays may be generated by magnetically funneled accretion, a mechanism that leads to strong shocks at the stellar surface. High and medium resolution spectra from several classical T Tauri stars are studied to investigate their X- ray generation mechanisms and plasma properties. The sample contains objects of various age, namely BP Tau, CR Cha, SU Aur and TW Hya, which exhibit quite different X-ray properties. Accretion shocks and coronal activity contribute to the observed X-ray emission, whereas the fraction for each process differs significantly between the individual objects. In the context of young stellar systems also the X-ray detection of Beta Pictoris is presented and implications on possible X-ray generating mechanisms are discussed. Part three of this thesis deals with older and only moderately active solar-like stars and is mainly devoted to the investigation of cyclic activity behaviour. The 11-year cycle is one of the characteristic features of the Suns activity and is of great importance, since solar activity also influences terrestial climate conditions. The indications for coronal activity cycles on stars are still limited to very few objects due to nonexistent long- term X-ray monitoring data. The stellar systems Alpha Centauri A/B and 61 Cygni A/B were monitored over several years with XMM-Newton. The observed variations in their X-ray luminosity and related changes of coronal properties are studied. Indications for cyclic activity, especially of the solar twin Alpha Cen A, are presented
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