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Analysis of Stellar Activity and Orbital Dynamics in Extrasolar Planetary Systems

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Analysis of Stellar Activity and Orbital Dynamics in Extrasolar Planetary Systems Analysis of Stellar Activity and Orbital Dynamics in Extrasolar Planetary Systems Dissertation zur Erlangung des Doktorgrades des Department Physik der Universität Hamburg vorgelegt von Sebastian Schröter aus Lübeck, Schleswig-Holstein Hamburg 2012 Gutachter der Dissertation: Prof. Dr. J. H. M. M. Schmitt Prof. Dr. A. Reiners Gutachter der Disputation: Prof. Dr. P. H. Hauschildt Prof. Dr. R. Banerjee Datum der Disputation: 04.05.2012 Vorsitzender des Prüfungsausschusses: Dr. R. Baade Vorsitzender des Promotionsausschusses: Prof. Dr. P. H. Hauschildt Dekan der MIN Fakultät: Prof. Dr. H. Graener Zusammenfassung Das Forschungsgebiet der extrasolaren Planeten, dessen Anfänge weniger als zwei Jahr- zehnte zurückliegen, erlebt zur Zeit eine rasante Entwicklung ausgelöst durch Weltraum- teleskope wie CoRoT und Kepler. Diese Observatorien überwachen Tausende von Ster- nen über lange Zeiträume mit bisher unerreichbarer Genauigkeit und Zeitauflösung, um Sternbedeckungen durch Planeten zu beobachten. Magnetische Aktivität des Sterns kann einen bedeutenden Einfluss auf die Entwicklung eines Sternsystems mit eng umlaufenden Planeten haben. Die hochenergetische Strah- lung führt zu Evaporationsprozessen und Massenverlust in der Atmosphäre des Planeten. Extrasolare Planetensysteme mit einem aktiven Stern sind dann besonders interessant, wenn der Planet die Sternscheibe bei einem Transit bedeckt. In diesem Fall ermöglicht der Planet, die räumliche Auflösung von durch Aktivität bedingten Strukturen in pho- tosphärischen, chromosphärischen und koronalen Schichten der Sternatmosphäre. Diese Arbeit ist der Untersuchung von Orbitdynamik und stellarer Aktivität in drei außergewöhnlichen Exoplanetensystemen vermöge boden- und weltraumgestützter Pho- tometrie, sowie optischer und Röntgenspektroskopie gewidmet. Der erste Teil dieser Arbeit handelt von photometrischen Untersuchungen des Exo- planetensystems TrES-2. Aufgrund der fast streifenden Bedeckung durch seinen heißen Jupiter und der geringen Aktivität des Sterns ist dieses System besonderes geeignet, um Veränderungen der Orbitgeometrie über größere Zeiträume zu verfolgen. Unsere Analyse von Vielfarben-Photometrie weist auf eine Inklinationsänderung des Planeten hin, die sich auf den Einfluss eines äußeren Störkörpers von Jupitermasse zurückführen lässt. Nachun- tersuchungen anderer Autoren konnten unsere Ergebnisse jedoch nicht bestätigen. Wir klären den Sachverhalt durch eine Analyse der ersten vier Quartale von Kepler-Daten von TrES-2, die keine Hinweise auf eine Inklinationsänderung entsprechender Größe zeigen. Im nächsten Teil wird der außergewöhnlich aktive Zentralstern im Planetensystem CoRoT-2 betrachtet. Wir charakterisieren die Eigenschaften des Sterns im optischen und Röntgenbereich mittels einer Zeitserie von VLT/UVES-Spektren und einer 15 ks Auf- nahme des Chandra-Teleskops, die beide während eines Transits gewonnen wurden. Die optischen Daten werden durch verschiedene Verfahren hochauflösender Spektralanalyse untersucht und erlauben Rückschlüsse auf die stellaren Parameter, Entfernung und Alter des Systems. Die Daten bestätigen, dass CoRoT-2A einen Begleitstern späten Spektral- typs hat, der gravitativ an das System gebunden ist. Die Röntgendaten zeigen koronale Emission von CoRoT-2A, der vermutlich gleichaltrige Begleitstern bleibt jedoch undetek- tiert. Der heiße Jupiter ist dem intensiven hochenergetischen Strahlungsfeld des Sterns ausgesetzt. Dies führt zu einer langsamen Evaporation der planetaren Atmosphäre. Bei der Untersuchung der zeitaufgelösten Spektren ist es erstmals gelungen, zeitabhän- gige Deformationen der chromosphärischen Emissionskerne der Ca II H- und K-Linien während des Transits nachzuweisen. Wir benutzen diesen Effekt, um die räumliche Struk- tur und Ausdehnung der Chromosphäre von CoRoT-2A zu bestimmen. Die vorliegende Arbeit schließt mit einer Untersuchung des Planetensystems CoRoT-7, in dem der erste Transit einer Supererde beobachtet worden ist. Wir kombinieren Röntgenbeobachtungen von XMM-Newton und Archivdaten von optischer VLT/UVES- Spektroskopie des Systems, um nachzuweisen, dass die beiden Gesteinsplaneten CoRoT-7b und CoRoT-7c starker UV- und Röntgenstrahlung des Sterns ausgesetzt sind. Wir schätzen den Einfluss der hochenergetischen Strahlung ab, den diese über den Mas- senverlust des Planeten auf die Entwicklung des Systems gehabt haben kann. Abstract The area of exoplanet research, which emerged less than two decades ago, was revo- lutionized with the advent of space-based photometers like CoRoT and Kepler. These observatories provide long-term monitoring of thousands of stars with unprecendented accuracy and high time resolution to detect transiting planets. Magnetic activity of the host star can have a substantial influence on the evolution of a planetary system with close-in planets. The high-energy radiation strongly heats the planetary atmosphere leading to evaporation and mass loss. Exoplanetary systems hosting an active star are particularly interesting if the planet transits the stellar disk, because in this case the planet allows to spatially resolve activity-related photospheric, chromospheric and coronal features in the host star’s atmosphere. This work is dedicated to the study of orbital dynamics and stellar activity in three landmark exoplanetary systems using ground- and space-based photometry as well as optical and X-ray spectroscopy. The first part of this work presents photometric studies of the TrES-2 exoplanetary system. The nearly grazing transit of its hot Jupiter and the low level of variability of the host star make this system particularly suited to study long-term changes in the orbital geometry of the planet. Our analysis of multi-color ground-based photometry indicates a secular change in the inclination of the planetary orbit, which we interpret as the result of an external Jovian mass perturber in the system. Follow-up studies by other authors were, however, unable to confirm our results. We resolve the issue with an analysis of the first four quarters of Kepler data of TrES-2, which clearly rule out an inclination change of the expected size. The following part focuses on the exceptionally active planet-hosting star CoRoT-2A. We provide a characterization of the star in the optical and X-ray regime using a se- ries of high-quality VLT/UVES spectra and a 15 ks Chandra exposure, both obtained during planetary transits. The optical data are analyzed to derive the fundamental stellar parameters, distance, and age of the system using complementary methods of high-resolution stellar spectroscopy. They confirm the presence of a late-type stellar companion that is gravitationally bound to the system. The X-ray data provide a clear detection of coronal X-ray emission from CoRoT-2A, however, unexpectedly, its likely coeval stellar companion remains undetected. The close-in hot Jupiter is found to be immersed in an intense field of high-energy radiation from its host star, which leads to a gradual evaporation of the planetary atmosphere. We further provide an analysis of our time-resolved spectroscopic data and report the first detection of a planet-induced deformation of the Ca II H and K emission-line cores during the transit. We use this effect to resolve the spatial structure and extent of the chromosphere of CoRoT-2A. The work at hand closes with a study of the CoRoT-7 system, known for hosting the first transiting super-earth. X-ray observations obtained with XMM-Newton are com- bined with archival optical VLT/UVES spectra to show that the rocky planets CoRoT-7b and CoRoT-7c are prone to ultraviolet and X-ray emission from their host star. We es- timate the effect that planetary mass loss due to high-energy irradiation could have had on the evolution of the system. Contents I. Introduction 1 1. Preface 3 1.1. The formation of the Solar System . .3 1.2. Outline of this work . .5 2. Extrasolar planets 7 2.1. History of the observation of planets . .7 2.2. Hunting extrasolar planets . .9 2.3. Transit modeling . 18 2.4. Time-dependent transit variations . 22 3. Solar and stellar activity 25 3.1. Solar activity and dynamo action . 25 3.2. Stellar activity . 35 3.3. The role of stellar activity in exoplanetary systems . 41 4. Analysis of high-resolution spectroscopy of solar-like stars 45 4.1. Basics of stellar line formation . 45 4.2. Stellar parameters inferred from strong lines . 47 4.3. Chemical abundance analysis . 48 4.4. Line-ratios of metal lines as temperature indicator . 54 4.5. Age estimates from lithium . 54 4.6. Distance estimates from spectral data . 55 4.7. Proxies for stellar activity . 57 II. Scientific contributions 59 5. Orbital dynamics of the transiting extrasolar system TrES-2 61 5.1. My contributions . 61 5.2. Multi-band transit observations of the TrES-2b exoplanet . 63 5.3. A consistent analysis of three years of ground- and space-based pho- tometry of TrES-2 . 72 6. Chromospheric and coronal stellar activity in extrasolar planetary systems 83 6.1. My contributions . 83 V VI Contents 6.2. The corona and companion of CoRoT-2A . 85 6.3. The extended chromosphere of CoRoT-2A . 97 6.4. The high-energy environment in the super-earth system CoRoT-7 . 106 III. Conclusions 113 7. Closing thoughts 115 7.1. Summary and conclusions . 115 7.2. Future work . 118 7.3. Epilogue . 120 Bibliography 122 Part I. Introduction 1 1. Preface “Le silence éternel de ces cloud. One slowly rotating and con- espaces infinis m’effraie.” tracting fragment, the pre-solar nebula, flattened to form a protoplanetary disk, – B. Pascal, Pensées about
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