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Developing a Ground-Based Search System for Transits of Extrasolar Planets

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Developing a Ground-Based Search System for Transits of Extrasolar Planets Developing a ground-based search system for transits of extrasolar planets vorgelegt von Diplom-Physiker Holger Voß Berlin Eingereicht an der Fakultät II - Mathematik und Naturwissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.) genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Dr.-Ing. H. J. Eichler Berichter/Gutachter: Prof. Dr. H. Rauer Berichter/Gutachter: Prof. Dr. E. Sedlmayr Tag der wissenschaftlichen Aussprache: 15.06.2006 Berlin 2006 D 83 Angefertigt am Institut für Planetenforschung des Deutschen Zentrums für Luft- und Raumfahrt (DLR) e.V., Berlin-Adlershof II Exploration By TMHIII Off we go into the deep and black, Riding high bright as a sun, Leaving home riding on wings of thunder, Throttle up, staging is done, Putting out on a new sea full of wonders, For distant shores under our sun, Moon and Mars are just the start, Planets around new stars, We live to fly, we explore or die, Nothing can stop the spirit of exploration. III IV Abstract The search for extrasolar planets is a relatively new field in astronomical and astrophysical sciences that was boosted by the first discovery of the exoplanet 51 Peg b in 1995 by radial velocity measurements. The short orbital period of 4.2 days determined for this giant exoplanet encouraged the accelerated development of another search method due to the high probability that such a Hot Jupiter planet would cross its host star in the line of sight yielding a photometrically detectable signal during this crossing. This transit method (also known as the occultation or eclipse method) measures such a darkening signal as an evidence for an orbiting planet. The first detection of a transiting exoplanet HD 209458 b in 1999 and the discoveries obtained for this planet during follow-up observations demonstrated the high scientific potential of planets discovered by the transit method. The size of the exoplanet and the inclination of the orbit were determined. Combined with the information obtained with radial velocity measurements the mass and the density of the planet were calculated identifying the planet as a gas giant. Atmospheric components like Sodium, Hydrogen, Oxygen and Carbon were identified with spectroscopic measurements made with the Hubble Space Telescope. Hydrogen evaporating from the surface of the giant planet due the high stellar pressure of the nearby host star was discovered. The direct flux centered around the secondary eclipse was determined in infrared bands by measurements with the Spitzer Telescope. This unique opportunity to study detailed physical and chemical properties of exoplanets is advantageous compared to planets proven only by other search methods. In recent years only three additional transiting planets orbiting bright stars suitable for high-resolution follow-up spectroscopy have been discovered. A substantially higher number of detected transiting exoplanets are imperative to answer fundamental questions about Hot Jupiter planets. Deep surveys like OGLE (five transiting planets orbiting faint stars detected) will give higher number of detected transiting exoplanets, increasing the statistical database. Due to the faintness of the host stars the physical and chemical characterization of the detected planets will be limited. Furthermore the obtained large error estimations of the physical parameters for the planets will limit using the parameters for modeling purposes. The transit method offers the unique possibility to detect small rocky planets orbiting solar- like stars if observations are carried out from space. Satellite missions like COROT and Kepler will open this new field in the exoplanet search in the near future monitoring ten thousands of stars simultaneously from space without the disturbing noise introduced by the earth atmosphere. In this thesis the establishing of the ground-based transit survey Berlin Exoplanet Search Telescope (BEST) is described. This survey was initiated with the goal of widening knowledge about this relatively young search method for exoplanets to support the preparations for the French-led COROT satellite mission. The instrument of the search system, a 20cm Schmidt telescope, has a similar dimension as the telescope of the COROT satellite. The field of view (FOV) of BEST (3.1° x 3.1°) is comparable in size with the FOV of the COROT telescope. The set-up and the successful implementation of the BEST system are described in this work. Furthermore the developed strategies for the photometric observations, the calibration and analysis of the data, follow-up observations and their optimization are commented. Two transits of the exoplanet orbiting the star HD 209458 were already observed during the initial set-up phase of the BEST system demonstrating the feasibility that transit signals can be successfully measured with the system. During the 2-year observational campaign (2001-2003) from the Thüringer Landessternwarte (TLS) Tautenburg, Germany, three target fields were observed for 90 nights. The collected data was calibrated and analyzed by the developed automatic data V pipeline. Photometric precision sufficient for detections of transits by Jupiter-sized planets orbiting Solar-like stars was obtained for several thousand stars. Five transit candidates were discovered in the data set. Two candidates were identified as eclipsing binaries; one of the secondaries was determined to be one of the smallest main sequence stars with a known radius. A blending scenario was identified for one transit candidate: an eclipsing binary was blended by two more stars observed in the same point spread function (PSF) of BEST. A fourth candidate host star for a transiting planet was found to be a giant star too large that a transit signal of a planet-sized object could be detected. Candidate 5 is still under investigation; a planetary origin of the observed signal could not be confirmed so far. Additionally variable stars were identified in the data set of the three observed target fields. Altogether 83 stars were identified as variable stars, all discoveries are new discoveries. For 53 of these variables periodic variability could be confirmed. Among the variables 38 objects were classified as eclipsing binaries. The observational campaign from TLS turned out to be a useful investment for both collecting observational data and developing an automated data reduction process. Furthermore optimization of the photometric observations, data reduction and –analysis, follow-up analysis and observations could be conducted. Nevertheless the obtained orbital phase coverage during one observational season was not sufficient enough for detecting more than one transit event of short-periodic exoplanets eventually contained in the three target fields with high probability. Not optimal meteorological conditions were identified as the main limitation of the observations, thus that the planning for a movement of the search system to an observational site with better meteorological conditions was accelerated. More than 20 transit search programs have already been established during recent years. Most of these are wide-angle searches monitoring bright and moderately faint stars (10 mag < V < 15 mag). Only one transiting exoplanet has been detected by a wide-angle survey: TrES- 1. In this work it was analyzed how the detection rates of transit search systems can be maximized based on the knowledge obtained during BEST observations and from operational and proposed transit searches. Aspects of this optimization process were the analysis of the fraction of suitable targets stars in different magnitude ranges in typical target fields close and above the Galactic plane, the influence of stellar crowding, the maximization of the orbital phase coverage and the minimization of main noise sources worsening the photometric precision. The analysis showed that the BEST system allows higher detection rates for transits of exoplanets compared to other ongoing transit search systems. Nevertheless, as a result of this analysis, a new transit search system is proposed consisting of f/1 Schmidt telescopes with an aperture of 45cm able to monitor a FOV of 4.7° x 4.7° combined with a 4k CCD with a pixel size of nine µm. This system would combine the advantages of a wide FOV and a pixel scale well suited to monitor target fields close and above the Galactic plane. Nevertheless all analyzed search systems can reach sufficient detection probabilities if the target fields are well selected so that the pixel scale corresponds to the stellar density of the observed target fields. VI Zusammenfassung 1995 begann mit der Entdeckung des ersten Exoplaneten, welcher einen sonnenähnlichen Stern umkreist, ein neues Kapitel in der Astronomie und Astrophysik. Die unerwartet kurze Umlaufzeit dieses Planeten 51 Peg b von 4.2 Tagen und die damit verbundene hohe Wahrscheinlichkeit der Bedeckung des Zentralsterns durch den Planeten eröffnete neue Perspektiven für die Transitsuche nach Exoplaneten. Mit der Entdeckung des ersten Transitexoplaneten HD 209458 b im Jahre 1999 und den umfangreichen Informationen über den Planeten, welche bei Nachfolgebeobachtungen gewonnen werden konnten, wurde diese Entwicklung weiter forciert. Mit dem COROT-Satelliten wurde eine Weltraummission unter französischer Führung in Planung genommen, welche als ein Hauptziel die Suche nach Transits von Exoplaneten hat. Das Deutsche Zentrum für Luft- und Raumfahrt beteiligt sich unter anderem mit dem bodengebundenen Transit-Suchsystem
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