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Astrometric Search for Extrasolar Planets in Stellar Multiple Systems

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Astrometric Search for Extrasolar Planets in Stellar Multiple Systems Astrometric search for extrasolar planets in stellar multiple systems Dissertation submitted in partial fulfillment of the requirements for the degree of doctor rerum naturalium (Dr. rer. nat.) submitted to the faculty council for physics and astronomy of the Friedrich-Schiller-University Jena by graduate physicist Tristan Alexander Röll, born at 30.01.1981 in Friedrichroda. Referees: 1. Prof. Dr. Ralph Neuhäuser (FSU Jena, Germany) 2. Prof. Dr. Thomas Preibisch (LMU München, Germany) 3. Dr. Guillermo Torres (CfA Harvard, Boston, USA) Day of disputation: 17 May 2011 In Memoriam Siegmund Meisch ? 15.11.1951 † 01.08.2009 “Gehe nicht, wohin der Weg führen mag, sondern dorthin, wo kein Weg ist, und hinterlasse eine Spur ... ” Jean Paul Contents 1. Introduction1 1.1. Motivation........................1 1.2. Aims of this work....................4 1.3. Astrometry - a short review...............6 1.4. Search for extrasolar planets..............9 1.5. Extrasolar planets in stellar multiple systems..... 13 2. Observational challenges 29 2.1. Astrometric method................... 30 2.2. Stellar effects...................... 33 2.2.1. Differential parallaxe.............. 33 2.2.2. Stellar activity.................. 35 2.3. Atmospheric effects................... 36 2.3.1. Atmospheric turbulences............ 36 2.3.2. Differential atmospheric refraction....... 40 2.4. Relativistic effects.................... 45 2.4.1. Differential stellar aberration.......... 45 2.4.2. Differential gravitational light deflection.... 49 2.5. Target and instrument selection............ 51 2.5.1. Instrument requirements............ 51 2.5.2. Target requirements............... 53 3. Data analysis 57 3.1. Object detection..................... 57 3.2. Statistical analysis.................... 58 3.3. Check for an astrometric signal............. 59 3.4. Speckle interferometry.................. 62 i Contents 4. Calibration 63 4.1. Calibration clusters................... 64 4.2. Iterative calibration cycle................ 65 4.3. Geometric field distortions............... 67 5. Results 71 5.1. Astrometric precision - lower limits........... 72 5.2. Astrometric calibration cluster - 47 Tuc........ 74 5.3. Target system - HD 19994................ 81 5.3.1. Relative astrometry............... 82 5.3.2. Speckle Interferometry............. 84 5.3.3. Radial velocity.................. 86 5.3.4. Companion HD 19994 C............. 88 5.3.5. Comparison with theoretical models...... 92 6. Summary and outlook 97 A. Calculation of ephemeridesI B. Calculation of atmospheric refraction indexV C. HD 19994 BC - Complex visibilities VII D. HD 19994 C - Orbital solutionIX E. HD 19994 C - χ2 MapsXV F. Extended target list XIX G. Acknowledgment XXXV H. Ehrenwörtliche Erklärung XXXVII ii List of Figures 1.1. Mass-orbit diagram of exoplanets............ 11 1.2. Properties of exoplanets around single stars and in stel- lar multiple systems................... 20 1.3. Statistical analysis of exoplanets around single stars and in stellar multiple systems............. 21 1.4. Properties of exoplanets in stellar multiple systems.. 23 1.5. Mass and multiplicity dependency of exoplanets in stel- lar multiple systems................... 26 2.1. Astrometric signal in a stellar binary.......... 31 2.2. Reflex motion of the flux center around the common center of mass...................... 32 2.3. Differential parallaxe effect for a binary........ 34 2.4. Turbulences of earth’s atmosphere........... 37 2.5. Differential chromatic refraction............ 42 2.6. Uncertainty of the differential refraction correction.. 44 2.7. Uncertainty of the differential aberration correction.. 48 3.1. Analysis strategy.................... 60 3.2. Examples of complex visibilities............ 62 4.1. Old globular cluster 47 Tuc............... 64 4.2. Iterative calibration cycle................ 66 4.3. Field distortions on the NACO S13 camera...... 68 5.1. Chosen cluster stars of 47 Tuc............. 77 5.2. Measurements of the Master-Baseline ......... 77 5.3. Intrinsic instability of the chosen cluster stars..... 79 5.4. Binary measurements of HD 19994........... 82 iii List of Figures 5.5. Visibilitiy and phase of HD 19994 BC for 2004..... 85 5.6. Astrometric measurements of HD 19994 A&BC.... 90 5.7. Speckle measurements of HD 19994 B&C........ 90 5.8. Radial velocities of HD 19994 B&C........... 91 5.9. Influence of the Brγ narrow band and the Ks broad band filter on brightness ratio measurements...... 94 5.10. Theoretical evolutionary tracks for HD 19994 B&C.. 95 A.1. Orbit of a celestial body................ II C.1. Complex visibilities of HD 19994 BC (2004 & 2006).. VII C.2. Complex visibilities of HD 19994 BC (2007 - 2009).. VIII D.1. HD 19994 A&BC - orbital solution...........X D.2. HD 19994 B&C - orbital solution............XI D.3. HD 19994 B - astrometric reflex orbit.......... XII D.4. HD 19994 B&C - binary orbit.............. XII D.5. HD 19994 B&C - radial velocities............ XIII E.1. Two dimensional χ2 maps................XV E.4. One dimensional χ2 maps................ XVIII iv List of Tables 1.1. Exoplanets in closer binaries.............. 15 1.2. Exoplanets in wider binaries.............. 16 1.3. Exoplanets in stellar systems with more than two com- ponents.......................... 17 1.4. Multiplicity of exoplanet host stars........... 18 1.5. Critical semi-major axis of close stellar binaries.... 24 2.1. Astrometric jitter caused by stellar activity...... 35 2.2. Parameter uncertainties regarding the differential re- fraction correction.................... 43 2.3. Parameter uncertainties regarding the differential aber- ration correction..................... 47 2.4. Maximum gravitational light deflection........ 50 2.5. Stellar systems observed on the northern hemisphere. 54 2.6. Stellar systems observed on the southern hemisphere. 55 5.1. Summary of remaining astrometric uncertainties... 73 5.2. Lower limits for astrometric measurement presicion.. 73 5.3. Measurements of the Master-Baseline ......... 78 5.4. Separation correction terms............... 80 5.5. Position angle correction terms............. 80 5.6. HD 19994 A&B separation measurements....... 83 5.7. HD 19994 A&B position angle measurements..... 84 5.8. HD 19994 B&C speckle measurements......... 86 5.9. Radial velocities of HD 19994 B&C........... 87 5.10. HD 19994 corrected measurements........... 89 5.11. Orbital elements of HD 19994 C............. 91 5.12. Ks band magnitudes of the HD 19994 system..... 94 v 1. Introduction 1.1. Motivation Since the first indisputable extrasolar planet1 around a solar like star was discovered in 1995 by Mayor and Queloz [116], more than 450 exoplanets are detected so far (Schneider [171]). Almost all of them were detected indirectly, mainly by the radial velocity (RV) method, where the Doppler shift in the spectra of a star (introduced by an orbiting object) is measured. Another very successful method is the transit method, which measures the decreasing star flux while an ob- ject is moving in front of the star through the line of sight. Both methods detected more than 90 % of all known exoplanets today and are a perfect example of how the combination of different observation techniques could gain much more information than each method on its own. Due to the unknown orbital inclination of the planetary compan- ion the RV method just measured its minimum mass, whereas transit measurements delivers the inclination angle and the radius of a sur- rounding object, but no information about its mass. Combining both techniques the true mass, the radius, and the density of the exoplanet can be obtained. That allows an insight into the interior structure 1 An extrasolar planet (exoplanet) is a planetary object orbiting another star than our sun. 1 1. Introduction of the exoplanet, which is an important key to distinguish and char- acterize them as e.g. solid earth-like or Jovian planets. However, transit measurements are only applicable for nearly edge-on2 orbits and are mainly sensitive for exoplanets very close to their host star. Almost all transiting exoplanets have orbital periods of less than one week. Looking at the mass-orbit diagram of exoplanets found so far (Fig. 1.1), one can identify areas with a higher rate of exoplanet detections. The reason for that is not only an astrophysical back- ground of planetary formation, but also different sensitivities of the detection methods. By improving the measurement precision and in- cluding other types of target stars one can enter neighboring regions in the parameter space (e.g. RV measurements recently entered the re- gion of lower mass exoplanets with smaller orbital periods, especially around low mass stars). But exoplanets with larger orbital periods are still undiscovered. The reasons are the observational timeline of current search programs and the decrease of the sensitivity for RV measurements toward larger orbital periods. As one can see by the lines of sensitivity in Fig. 1.1 that parameter region can be perfectly filled by astrometric observations. Astrom- etry is complementary to the RV technique and measures the two transverse dimensions instead of the radial dimension of the stellar reflex motion (also called wobble), which is induced by an orbiting exoplanet. In contrast to RV or transit observations astrometry is applicable for exoplanets with larger orbital periods and delivers the true planetary mass independently from its orbital inclination. Es- pecially in order to detect planetary system similar to our own solar system one has to consider outer planets with
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