Thesis
Reference
Observing exoplanet populations with high-precision astrometry
SAHLMANN, Johannes
Abstract
This thesis deals with the application of the astrometry technique, consisting in measuring the position of a star in the plane of the sky, for the discovery and characterisation of extra-solar planets. It is feasible only with a very high measurement precision, which motivates the use of space observatories, the development of new ground-based astronomical instrumentation and of innovative data analysis methods: The study of Sun-like stars with substellar companions using CORALIE radial velocities and HIPPARCOS astrometry leads to the determination of the frequency of close brown dwarf companions and to the discovery of a dividing line between massive planets and brown dwarf companions; An observation campaign employing optical imaging with a very large telescope demonstrates sufficient astrometric precision to detect planets around ultra-cool dwarf stars and the first results of the survey are presented;
Finally, the design and initial astrometric performance of PRIMA, a new dual-feed near-infrared interferometric observing facility for relative astrometry is presented.
SAHLMANN, Johannes. Observing exoplanet populations with high-precision astrometry. Thèse de doctorat : Univ. Genève, 2012, no. Sc. 4441
URN : urn:nbn:ch:unige-228481
DOI : 10.13097/archive-ouverte/unige:22848
Available at:
http://archive-ouverte.unige.ch/unige:22848
Disclaimer: layout of this document may differ from the published version.
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UNIVERSITÉ DEGENÈVE FACULTÉ DESSCIENCES
Département d’Astronomie Professeur Didier Queloz
Observing Exoplanet Populations with High-Precision Astrometry
T
HÈSEprésentée à la Faculté des Sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences,
mention Astronomie et Astrophysique
par
Johannes SAHLMANN
de
Hamburg (Deutschland)
Thèse No 4441
GENÈVE
Observatoire Astronomique de l’Université de Genève 2012
THE JURY THAT ASSESSED THIS THESIS WAS COMPOSED OF
DIDIERQUELOZ
UNIVERSITÉ DEGENÈVE, SWITZERLAND
FRANCESCOPEPE
UNIVERSITÉ DEGENÈVE, SWITZERLAND
STÉPHANEUDRY
UNIVERSITÉ DEGENÈVE, SWITZERLAND
ALANP. BOSS
CARNEGIEINSTITUTION FORSCIENCE, WASHINGTON, DC, USA FABIENMALBET
INSTITUT DEPLANÉTOLOGIE ET D’ASTROPHYSIQUE DEGRENOBLE, FRANCE
SUBMITTED: MARCH13, 2012 ACCEPTED: JUNE28, 2012 FINAL CORRECTIONS: AUGUST1, 2012
Für meine Familie
Pour ma famille
Para a minha familia
Abstract
The research field of extrasolar planets emerged two decades ago and has since made tre- mendous progress, fueled by the discovery and characterisation of hundreds of planets orbiting stars other than the Sun. The predominant techniques used so far are the meas- urements of changes in the stellar radial velocity due to an orbiting planet and of stellar brightness variations caused by a transiting planet. The work presented here makes use of the astrometry technique, which consists of measuring the position of a star in the plane of the sky. At very high precision, the lateral reflex motion of a star caused by an orbiting planet can be detected. Each of those techniques is applicable within a certain range of para- meter space and gives access to a limited information on an individual planetary system.
To advance our understanding of the formation of stars and their planets, we need prefer- ably complete information for a large number of planets, which can only be achieved by combining different observation techniques and by observing large stellar samples.
Astrometric measurements are the ideal tool to determine the mass distribution of exoplanet populations. However, the detection of the astrometric motion of a star caused by an or- biting planet requires a very high measurement precision and only a few instruments are capable of achieving the necessary level today. In this context, my doctoral work covers all aspects of observational astronomy, i.e. it deals with the building and operation of an astronomical instrument dedicated to precision astrometry, it discusses various data reduc- tion and analysis efforts required to achieve the task, and it presents scientific results on exoplanets obtained with the astrometry technique.
One possible method to obtain precision astrometry is near-infrared interferometry, i.e. the coherent combination of a stellar wavefront observed with two telescopes. When observing two stars simultaneously, it becomes possible to measure their relative position with very high precision. The opto-mechanical system that realises the beam combination is central to this experiment, because it is where the interference of the stellar light is detected, which carries the astrometric information. The design and performance of such interference fringe sensors as implemented within the PRIMAastrometric facility are described. A successful astrometric measurement relies on the meticulous orchestration of the instrument facility that includes two telescopes separated by∼100 meters, beam relay optics, laser metrologies, and detectors, and it requires that the internal optical path lengths are controlled at the nano- meter level. Such a facility is realised byPRIMAat the Very Large Telescope Interferometer and a detailed description of its implementation and mode of operation is given. The results of test observations aiming at establishing its performance are presented and discussed.
A pattern that emerged early in the search for exoplanets is the low abundance of substellar companions that are more massive than about thirteen Jupiter-masses orbiting close to Sun- like stars. Because those brown dwarf companions are rare, it is difficult to measure their mass distribution, which is vital for the comparison with theoretical formation scenarios.
With the help of theCORALIEsurvey, which consists of precision radial velocity measure-
ments of 1650 Sun-like stars collected over more than a decade, the candidates for brown dwarf companions were identified. The mass ambiguity inherent to radial velocity meas- urements could be resolved for several systems by using astrometric measurements made by theHIPPARCOSsatellite. Consequently, a robust frequency of close brown dwarf com- panions to Sun-like stars could be derived and a first pattern in their mass distribution was discovered.
Brown dwarfs and very low-mass stars, collectively referred to as ultra-cool dwarfs, are situ- ated at the very-low mass end of the stellar spectrum and although many stars are found to host planets, the question whether the conditions for planet formation are met around ultra- cool dwarfs is open. Observations with high-resolution spectrographs for radial velocity measurements are difficult because ultra-cool dwarfs are intrinsically faint, and imaging sur- veys can probe the existence of planets only at large separations. A solution to this problem is found with astrometric measurements from optical imaging with a very large telescope.
The large light-collecting aperture makes it possible to mitigate the atmospheric noise on the astrometric measurement and to reach faint targets. When many reference stars are present in the field of view, the astrometric precision is sufficient to detect Neptune-mass planets around ultra-cool dwarfs, which is a unique opportunity to explore a previously inaccess- ible domain. Preliminary results of an astrometric search for planets around 20 ultra-cool dwarfs are presented and the principles of this technique are discussed.
Finally, the successful application of the astrometry technique to measure the reflex motions of three selected planet host stars is presented, which illustrates the immense capacities of the method.
Résumé
Le domaine de recherche s’intéressant aux planètes extrasolaires est apparu il y a une ving- taine d’années et il a connu un essor remarquable avec la découverte et la caractérisation de centaines de planètes au-delà du système solaire. Jusqu’à présent, deux techniques de détection prédominent: d’une part l’analyse des changements périodiques de la vitesse ra- diale stellaire causés par une planète en orbite et d’autre part l’analyse des variations de luminosité stellaire produites par une planète en transit. Ce travail de thèse s’intéresse à une autre technique de détection qui consiste à mesurer la position d’une étoile dans le plan du ciel, l’astrométrie. Le mouvement réflexe latéral d’une étoile causé par une planète en orbite peut être détecté seulement avec une très haute précision. Chacune de ces tech- niques permet des détections dans un certain domaine de l’espace des paramètres stellaires et planètaires et donne accès à une information limitée sur un système planétaire individuel.
Pour faire progresser notre compréhension sur la formation des étoiles et de leurs planètes, nous avons besoin d’une information la plus complète possible sur un grand nombre de systèmes planétaires. Cela ne peuvent être obtenu qu’en combinant différentes techniques d’observation sur de grands échantillons.
L’étude des mesures astrométriques est l’outil idéal pour déterminer la distribution en masse des populations d’exoplanètes. Cependant, la détection du mouvement astrométrique d’une étoile causée par une planète en orbite nécessite une précision de mesure très élevée, que seuls quelques rares instruments sont aujourd’hui capables d’atteindre. Dans ce contexte, mon travail de doctorat s’est porté sur tous les aspects de l’astronomie observationelle. Ce travail traite de la construction et de l’exploitation d’un instrument dédié à l’astrométrie de précision, il en examine les divers efforts de réduction et d’analyse des données, et il présente des résultats scientifiques sur les exoplanètes obtenus à partir de l’astrométrie.
Une astrométrie de haute précision peut-être obtenue par de l’interférométrie dans le do- maine de l’infrarouge. Cette technique consiste à combiner de façon cohérente le front d’onde stellaire observé avec deux télescopes. Lorsque l’on observe deux étoiles simul- tanément, il devient possible de mesurer avec grande précision leur position relative. Le système opto-mécanique qui réalise la combinaison du faisceau est au cœur de cette ex- périence, car il permet la détection de l’interférence de la lumière stellaire. La concep- tion et les performances de ces capteurs de franges d’interférence mis en œuvre lors de l’installation PRIMAsont décrits dans ce texte. Une mesure efficace astrométrique repose sur l’orchestration minutieuse de l’installation instrumentale qui comprend deux télescopes séparés d’environ 100 mètres, des relais de faisceaux optiques, des métrologies laser et des détecteurs. Il est de plus nécessaire que les longueurs internes de chemins optiques soient contrôlées à l’échelle nanométrique. Une telle installation est réalisée par PRIMAau Very Large Telescope Interferometer. Ce travail contient une description détaillée de la mise en œuvre de l’instrument et de son mode de fonctionnement. Les résultats des observations visant à établir la performance instrumentale y sont présentés et discutés.
La recherche d’exoplanètes a montré que les étoiles semblables au soleil abritent un faible nombre de compagnons sous-stellaires, c’est-à-dire des objets d’une masse entre environ treize et quatre-vingt fois celle de Jupiter. Du fait que ces compagnons, appelés naines brunes, soient rares, il est difficile de mesurer leur distribution en masse, ce qui est vital pour la comparaison avec les scénarios théoriques de formation. A l’aide du programme de rechercheCORALIEqui consiste à amasser depuis plus de dix ans des mesures précises de vitesse radiale de 1650 étoiles semblables au soleil, des compagnons naines brunes po- tentiels ont été identifiés. L’ambiguïté de masse inhérente aux mesures de vitesse radiale a pu être résolue pour plusieurs systèmes en utilisant des mesures astrométriques réalisées par le satellite HIPPARCOS. La fréquence de compagnons naines brunes proches d’étoiles semblables au soleil a donc pu être mesuré et une première particularité de leur distribution de masse découverte.
Les naines rouges très froides sont des étoiles de faible masse ou des naines brunes massives et se situent à l’extrémité du spectre stellaire. Si ces naines remplissent des conditions fa- vorable à la formation planétaire reste une question ouverte. Les observations faites à partir de spectrographes à haute résolution mesurant la vitesse radiale sont difficiles à réaliser, car les naines très froides sont intrinsèquement peu lumineuses. La méthode de l’imagerie directe ne peut sonder l’existence de planète qu’à grande séparation de l’étoile primaire.
Une solution à ce problème peut être trouvé grâce à l’astrométrie par imagerie optique ob- tenue avec un très grand télescope. Grâce à la grande ouverture d’un tel télescope, il est possible d’atténuer le bruit atmosphérique sur la mesure astrométrique et d’observer des objets de faible luminosité. Lorsque de nombreuses étoiles de référence sont présentes dans le champ de vision, la précision astrométrique est suffisante pour détecter des planètes de la masse de Neptune en orbite autour de naines très froides, ce qui est une occasion unique d’explorer un domaine jusqu’alors inaccessible. Les résultats préliminaires d’une recherche astrométrique de planètes autour de 20 naines très froides sont présentés et les principes de cette technique sont discutés.
Cette thèse présente aussi l’application réussie de la technique astrométrique pour mesurer les mouvements réflexes de trois étoiles causées par leurs planètes respectives. Ceci dé- montre la capacité de l’astrometrie à haute precision.
11
Contents
List of Figures 13
List of Tables 16
List of Acronyms and Definitions 17
1 Introduction 19
1.1 Star formation: from binary stars to planetary companions . . . 19
1.2 Observations of multiple stars and planetary systems . . . 21
1.3 Astrometric planet detection and characterisation . . . 28
1.4 The golden thread and organisation of this thesis . . . 30
2 Astrometry orbits at high precision 32 2.1 Coordinates, proper motion, and parallax . . . 32
2.2 Orbital motion . . . 34
2.3 Detecting orbital motion with astrometry . . . 38
2.4 Astrophysical sources of astrometric noise. . . 41
2.5 Instrumentation and techniques for high-precision astrometry . . . 42
3 Shaping the high-mass tail of the planet distribution 45 3.1 The frequency of close brown-dwarf companions to Sun-like stars . . . 46
3.2 Search for brown-dwarf companions of stars (Article) . . . 50
3.3 Sorting massive planets from brown-dwarf companions. . . 74
3.4 A possible dividing line between planets and brown-dwarf companions (Art- icle) . . . 75
3.5 Detection of a brown-dwarf companion using HIPPARCOS astrometry . . . . 79
3.6 HD 5388 b is a 69MJupcompanion instead of a planet (Article) . . . 82
3.7 Properties of brown dwarf companions to Sun-like and other stars . . . 86
3.8 Summary of Chapter 3 . . . 89
4 Precision astrometry with the FORS camera at the VLT 90 4.1 Data reduction . . . 90
4.1.1 Determination of the stellar photocentres . . . 91
4.1.2 Astrometric reduction . . . 94
4.2 The dependence of achievable precision on the telescope diameter. . . 100
4.3 The putative planet around VB 10 . . . 102
4.4 Astrometric search for a planet around VB 10 (Article) . . . 103
4.5 Measuring the mass of the planet GJ 676A b . . . 109
5 From brown-dwarf companions to planetary companions of brown dwarfs 111 5.1 Overview of the PALTA project . . . 112
5.2 Preliminary results . . . 117
5.3 Summary of Chapters 4 and 5 . . . 125
6.2 The ESPRI project . . . 148
6.3 Measurement principles and mathematical description . . . 148
6.4 The PRIMA facility at the Very Large Telescope Interferometer . . . 154
6.5 Astrometric observations. . . 162
6.6 Data reduction . . . 167
6.7 PRIMA astrometry data modelling . . . 173
6.7.1 Determination of the wide-angle baseline . . . 175
6.7.2 Main Delay fitting . . . 178
6.7.3 Astrometry fit of a separation vector . . . 182
6.7.4 Auxiliary data of critical elements . . . 187
6.8 PRIMA astrometry precision and accuracy as of February 2012 . . . 188
6.8.1 Amplitude and structure of the residuals about a simple fit. . . 189
6.8.2 Astrometric precision and the atmospheric limit . . . 190
6.8.3 Combination of multiple epochs . . . 192
6.8.4 Astrometric accuracy and plate scale. . . 194
6.8.5 Potential sources of systematic errors in the PRIMA astrometry . . . . 201
6.9 Summary of Chapter 6 . . . 204
7 Summary and conclusions 206 7.1 Outlook. . . 207
8 Appendix 209 8.1 Selected formulae . . . 209
8.2 Additional information on PRIMA . . . 210
8.2.1 Dual feed co-phasing . . . 212
8.2.2 Results from the VLTI-PRIMA fringe tracking testbed (Article) . . . . 216
8.2.3 First results from fringe tracking with the PRIMA fringe sensor unit (Article) . . . 228
8.2.4 Auxiliary data of critical elements . . . 240
8.2.5 The PRIMA metrology measurements before July 2011 . . . 244
8.2.6 Verification of FSU real-time algorithms . . . 244
8.2.7 Drawings and graphics . . . 246
Related publications 252
Acknowledgements 255
Bibliography 256
Thanks 270
13
List of Figures
1.1 Multiplicity fraction as a function of spectral type . . . 21
1.2 Mass histogram of close companions to Sun-like stars within 50 pc. . . 22
1.3 Statistics of planets detected withHARPS . . . 23
1.4 Statistics of planet candidates detected withKEPLER . . . 23
1.5 CORALIEradial velocities of HD 168443 . . . 24
1.6 HARPSradial velocities of HD 85512 . . . 25
1.7 KEPLERtransit photometry of theKEPLER-16 system . . . 26
1.8 Radial velocities and Rossiter-McLaughlin effect of Wasp-15 . . . 27
1.9 Astrometric signatures of known extrasolar planets . . . 29
1.10 Graphical breakdown of the thesis . . . 31
2.1 Definition of the orbital elements . . . 35
2.2 Relative and barycentric orbits ofαCen . . . 36
2.3 Astrometric offsets ofαCen B . . . 37
2.4 HIPPARCOSscan angle and VLTI projected baseline angle . . . 37
2.5 Parameter dependencies of the barycentric orbit size . . . 40
2.6 Planet detection limits of astrometry and RV . . . 40
2.7 Effects of a starspot on photometry, radial velocity, and astrometry . . . 41
2.8 Difference between accuracy and precision . . . 42
3.1 Radial velocity orbit of HD 184860A . . . 47
3.2 Eccentricities of the brown-dwarf companion candidates . . . 47
3.3 Metallicities of the brown-dwarf companion host stars. . . 48
3.4 HIPPARCOSastrometry S/N and orbit significance . . . 49
3.5 Dividing line between massive planets and brown dwarf companions . . . 74
3.6 Illustration of the unit-sphere of orbit orientations. . . 80
3.7 Probability of measuring the inclinationi . . . 80
3.8 Mass-radius diagram of transiting objects with masses of5−100MJ . . . 88
4.1 Example of a FORS2 image . . . 92
4.2 Theoretical dependence of atmospheric and photon noise on field radius . . . 96
4.3 Dependency ofhmaxon the telescope diameter . . . 101
4.4 Atmospheric turbulence profile at Paranal . . . 102
4.5 Expected barycentric motion of GJ 676A . . . 110
5.1 Planet detection limits around L dwarfs with RV and astrometry. . . 113
5.2 Distances and apparent magnitudes of thePALTAtargets . . . 115
5.3 Residuals to the astrometry fit for dw04 . . . 118
5.4 Sky motion ofDENIS-P J065219.7-253450 . . . 119
5.5 Sky motion ofDENIS-P J065219.7-253450and the PSF . . . 120
5.6 Planet detection limits forDENIS-P J065219.7-253450 . . . 121
5.7 FORS2 astrometric precision for ultra-cool dwarfs . . . 122
5.8 Sky motions of severalPALTAtargets . . . 123
5.9 Sky motions of severalPALTAtargets . . . 124
6.1 FSU cryostat intervention . . . 128
6.2 Principle ofPRIMAastrometry. . . 149
6.3 Atmospheric limits to the astrometric precision . . . 150
6.4 Structure and beam train of the auxiliary telescope . . . 155
6.5 Optical layout of the star separator . . . 156
6.6 Optical layout of the metrology endpoint . . . 157
6.7 Drawing of a main delay line . . . 157
6.8 Drawings of the DDL . . . 158
6.9 Layout of the fringe sensor unit . . . 159
6.10 PRIMAbeam paths in the VLTI . . . 160
6.11 Layout of the VLTI platform at the Paranal observatory . . . 163
6.12 Illustration of the swap procedure . . . 164
6.13 The differential delay of HD 10360 . . . 165
6.14 IRISH-band images of HD 66598 . . . 165
6.15 Images of the fourPRIMAbeam pupils . . . 167
6.16 PACMANdata format . . . 169
6.17 Illustration of the wavelet analysis . . . 171
6.18 Wavelet power as function of differential delay and scan number . . . 172
6.19 Example ofPRIMETBunwrapping . . . 173
6.20 Definition of the position angle . . . 174
6.21 Measured delays for nine stars observed for the baseline calibration . . . 176
6.22 Residuals of the baseline fits . . . 177
6.23 Schematic of an interferometer with baselineB~ observing a source at true posi- tion~s0through a plane-parallel atmosphere of refractive indexn1. . . 179
6.24 Internal delay measured with delay line andPRIMAmetrologies . . . 180
6.25 Main delay measured with the DL metrology andPRIMAmetrology . . . 181
6.26 Delay difference between the DL and PRIMET metrology . . . 183
6.27 Residuals of the astrometric fit. . . 184
6.28 PRIMAmetrology minus DDL feedback . . . 185
6.29 Derotator angle . . . 188
6.30 Effect of thePRIMAmetrology bug . . . 189
6.31 Dispersion of the binned residuals for 48 astrometric runs. . . 189
6.32 Illustration of the fit residuals for HD 66598 . . . 190
6.33 Fit residuals for different target separations . . . 191
6.34 Astrometric precision of the HD 202730 observation . . . 192
6.35 Result of the combined fit of four epochs spanning over 56 days for HD 66598 . . 193
6.36 Result of the combined fit of five epochs spanning over 5 days for HD 10360.. . . 194
6.37 Linear fit to the relative motion of HD 202730 . . . 195
6.38 PRIMAmeasurements of HD 202730 . . . 196
6.39 PRIMAresiduals of HD 202730 . . . 196
6.40 Updated orbital fit to the relative motion of HD 10360 . . . 197
6.41 Updated orbit of HD 10360 andPRIMAmeasurements . . . 198
6.42 Simultaneous observation of HD 10360 withPRIMAandNACO . . . 199
6.43 Measured separation of HD 10360 withPRIMAandNACO . . . 199
6.44 PRIMAmeasurements of HD 10360 in November 2011 . . . 200
6.45 Residuals ofPRIMAmeasurements of HD 10360 in November 2011 . . . 201
6.46 Fit result and field rotation angle for HD 66598 (run 3-13). . . 202
6.47 Comparison of residuals with the standard and modified model function. . . 203
8.1 PRIMAfreezes the atmosphere. . . 212
8.2 Atmospheric turbulence during dual feed fringe tracking . . . 213
8.3 PRIMET DOPD after correction of sidereal motion . . . 214
8.4 Allan deviation of thePRIMADOPD . . . 215
8.5 FSU-measured group delay for run 0-9 . . . 215
8.6 Telescope altitude and azimuth . . . 240
8.7 The motions of the FSM and STS-VCM mirrors . . . 241
8.8 Telescope secondary mirror motions . . . 242
8.9 Positions and jitter of the telescope image stabilisation mirror . . . 243
8.10 Delay line VCM pressure and curvature . . . 243
8.11 PRIMAmetrology corrections for January 2011 data . . . 245
8.12 Difference between real-time and recomputed FSU observables . . . 246
8.13 Drawing of a main delay line’s cat’s eye assembly . . . 246
8.14 Environmental conditions during run 3-12. . . 247
8.15 Environmental temperatures during run 3-12 . . . 247
8.16 PRIMAmetrology fringe counter overflows . . . 248
8.17 Data reduction of a fringe tracking file . . . 249
8.18 Data reduction of a fringe scanning file. . . 250
8.19 Aerial view of the Paranal observatory . . . 251
List of Tables
2.1 HIPPARCOScatalogue data ofαCen B . . . 33
2.2 The mass term of Kepler’s third law . . . 36
2.3 Observables accessible by measuring the astrometric and/or RV orbit. . . 38
3.1 Constraints for imaging of the stellar companions . . . 49
4.1 FORS2 setup for the astrometric programmes. . . 91
4.2 Nomenclatures . . . 97
4.3 Orbital parameters of GJ 676A b . . . 109
4.4 Astrometric signature of GJ 676A and companion mass as a function of inclination109 5.1 PALTAtarget list . . . 116
5.2 Preliminary results for dw04. . . 117
5.3 Correlation matrix of the linear fit for dw04 . . . 118
6.1 PRIMAmetrology constants . . . 159
6.2 Optical path differences measured with thePRIMAmetrology . . . 159
6.3 Number of reflections before injection into the FSU fibre when using ATs . . . 161
6.4 PRIMA-VLTIcontrol loops acting on the stellar beams . . . 162
6.5 Overview of astrometric runs since first light . . . 168
6.6 Selected binary table content ofPACMANfiles . . . 170
6.7 Characteristics of selected targets . . . 174
6.8 Targets used for baseline calibration . . . 175
6.9 Wide-angle baseline fit results . . . 177
6.10 Baseline fit results: Differences between NORMAL and SWAPPED . . . 177
6.11 Metrology wavelengths and calculated index of refraction for run 3-12 . . . 181
6.12 Results of main delay fitting . . . 182
6.13 Results of the astrometry fit for run 3-12 . . . 183
6.14 Results of PRIMET – DDL modelling . . . 185
6.15 Linear elements of the HD 202730 . . . 195
6.16 Updated orbital elements of HD 10360 . . . 197
6.17 Fit results with standard and modified model function (run 3-13) . . . 203
8.1 Confidence intervals for a 7–parameter model . . . 210
8.2 Milestones of the PRIMA project . . . 210
8.3 Basic information on the 51PRIMAastrometric runs since first light . . . 211
8.4 PRIMET constants and derived parameters for the January 2011 observations . . 244
List of Acronyms and Definitions
APD Avalanche photo diode
APES Astrometric observation preparation software AT VLT Auxiliary Telescope (1.8 m)
CCD Charge-coupled device
CORALIE High resolution spectrograph at the Swiss 1.2 m telescope DDL Differential delay line
DEC Declination
DIT Detector integration time
DL Delay line
DOPD Differential optical path difference
DOPDC Differential optical path difference controller ESO European Southern Observatory
FC Fringe Counter
FORS Focal reducer / low dispersion spectrograph (VLT instrument) FSM Field selector mirror
FSU Fringe Sensor Unit
FSUA FSU feed A
FSUB FSU feed B
GAIA Space astrometry mission (ESA)
GD Group delay
HA Hour angle
HARPS High resolution spectrograph at the ESO 3.6-metre telescope HIPPARCOS Space astrometry mission (ESA)
IP Input channel
IRIS Infra-red tip-tilt guiding camera for VLTI KEPLER Space photometry mission (NASA) LST Local sidereal time
MJD Modified Julian date (JD – 2 400 000.5)
NACO Adaptive optics assisted infrared camera (VLT instrument) NEAT Nearby Earth Astrometric Telescope
OB Observing block
OPD Optical path difference
OPDC Optical path difference controller PACMAN PRIMA astrometric instrument PAE Provisional Acceptance Europe
PALTA Search for planets around L dwarfs with astrometry project
PD Phase delay
PRIMA Facility for phase reference imaging and micro-arcsecond astrometry PRIMET PRIMA laser metrology
PRIMETB PRIMET feed B
PSF Point spread function
PTV Peak-to-valley
RA Right ascension
RMS Root mean square
RV Radial velocity
S/N Signal-to-noise ratio
SIM Space interferometry mission (NASA) STS Star separator module
UT VLT Unit Telescope (8.2 m) VCM Variable Curvature Mirror VLT Very Large Telescope
VLTI Very Large Telescope Interferometer WDS Washington double star catalogue
19
1 Introduction
Stars are the main building blocks of the directly accessible universe. They make up the majority of objects we can see when observing the night sky and it isourstar, the Sun, that is enlightening the days on planet Earth. The study of stars and planets is thus directly related to our day-to-day life and the question whether there exist other worlds similar to the one we inhabit arises naturally. Within the past two decades, mankind went on from discover- ing the first planet around another star to studying the detailed properties of thousands of extrasolar planets and therewith made a large step towards answering that question. Those discoveries have changed humanity’s picture of the universe and are touching at the self- conception of our society. It is advances like these that fascinate us and that continue to motivate the efforts and expenses of science in general and scientists like myself in particu- lar.
Along with the fundamental question of the origin of the universe and the intricate nature of dark energy and dark matter, the understanding of the observed properties of the stars and planets constitutes one of the major research fields pursued in modern astrophysics. As as- tronomers we are bound to observe physical processes without being able to reproduce them or to repeat an experiment, because the objects we study are beyond our reach and control.
Exploring the nature of fundamental particles and their interactions and the reproduction of Sun-like nuclear fusion are among the limited number of experiments that address astro- physical questions and are realisable on Earth. The investigation of the predominant fraction of astrophysical phenomena necessitates more and more powerful observation facilities and telescopes as the state of knowledge increases and the technological capabilities improve.
Because the time span of astronomical observations is usually very short compared to the time scales of astrophysical processes, the latter have to be studied by observing several objects of the same class but during different stages of their evolution. Based on those ob- servations, theories can be developed that attempt to explain the observed properties within a certain parameter space. In the beginning, this space is very limited and the eternal pro- cess of science consists in enlarging it through the mutual stimulation of observational and theoretical work, with the ultimate goal of reaching an universally applicable model.
Now, my dissertation deals with extrasolar planets and since planets usually do not exist without stars, the question of the origin of planets cannot be answered without understand- ing the mechanisms of star formation.
1.1 Star formation: from binary stars to planetary companions
A comprehensive theory of star formation has to account for the observed distribution of single stars across all masses, compositions, and ages, the observed binary and multiplicity
fractions, and the mass distribution of stellar companions. In this picture, the planets appear in the very low-mass tail of the stellar companion distribution. A third class of objects are the brown dwarfs which are substellar objects with properties intermediate between planets and stars and therefore occupy a particular place in the the current understanding of star formation.
Stars form by the fragmentation of an extended molecular cloud into cloud cores, which subsequently collapse to form protostars (Shu et al.,1987). Initial angular momentum present in the cloud induces rotation and the formation of circumstellar disks. The end of the protostellar phase is marked by the halt of accretion of gas and dust from the envelope and disk onto the young star.
Binary stars are the preferred mode of star formation, i.e. the frequency of binary stars is higher than the one of single stars (Mathieu,1994;Tohline, 2002). The binary form- ation process occurs in the protostellar phase mainly through two mechanisms: the fragmentation of a protostellar core and the fragmentation of the protostellar disk.
Brown dwarfs form in principle like stars but do not retain enough mass to fuse Hydrogen (Whitworth et al.,2007), i.e. their mass is limited by definition to.0.08Mfor solar metallicity (Hayashi & Nakano, 1963;Kumar,1963; Grossman et al., 1974). A lower limit to the sub-stellar mass function has yet to be found but it lies below six Jupiter masses (Caballero et al.,2007).
Planets form in the circumstellar disk. The most prominent theory is the core accretion mechanism, in which dust accumulates by collisions to form grains, small planetes- imals, and finally a larger core massive enough to accrete gas from the disk (Pollack et al.,1996). An alternative mechanism to form giant planets is the fragmentation of the circumstellar disk, occurring on a significantly shorter timescale (Boss,2000).
This broad picture is supported by observations, but many details are matter of lively de- bate and only further observations will offer clarification. Because mass is the determining parameter for stars, mass-based definitions were initially employed to distinguish stars, re- quired to haveM & 0.08M corresponding to the minimum mass permitting Hydrogen burning, from planets withM . 0.013M marking the minimum mass required to burn Deuterium, and the brown dwarfs located in between. The mass-based definition of brown dwarfs became tricky when observations revealed an overlapping mass range with planets, and objects with very similar properties in mass and size were found to belong to different classes. For instance, a 15 Jupiter mass (MJ) companion of a Sun-like star could form like a planet and therefore be considered as one, whereas an isolated5MJ object in a star forming region could be named ’sub-brown dwarf star’ (Boss,2001). Thus, the classification of such objects requires to consider the individual formation process, composition, or environment, i.e. additional information that is not easily accessible by observations.
One of the unsolved questions in star formation theory emanates from the observation that companions with masses in the brown dwarf range orbiting close to solar-type stars are extremely rare compared to planetary mass companions on one side and stellar companions on the other side, both being discovered abundantly. A comprehensive theory that is capable of explaining the physical processes leading to the lack of brown dwarf companions remains undefined.
1.2 Observations of multiple stars and planetary systems 21
1.2 Observations of multiple stars and planetary systems
The statistical study of multiple stellar systems is tightly linked to the development and ad- vancement of high-resolution optical spectrographs. Duquennoy & Mayor(1991) observed 164 Sun-like stars and binaries, carefully selected to obtain an unbiased sample, to derive the distribution of orbital elements and companion masses, and found an overall multipli- city fraction of∼57%. Extending this survey,Halbwachs et al.(2003) derived a close binary (P < 10 years) frequency of14±2 % and the frequency of planetary systems at 7+4−2 %.
Raghavan et al.(2010) reversed the situation in terms of multiplicity by finding that54±2% of solar-type stars are single and argue that the significant disagreement withDuquennoy &
Mayor(1991) stems in the latter’s overestimated completeness correction. The binary mass ratio distribution is found to be approximately flat but exhibits a decreasing trend with de- creasing mass ratio, i.e. low-mass companions are less frequent. A generally recognised trend is that the multiplicity fraction decreases towards lower stellar masses, as shown in Fig.1.1.
Figure 1.1:The multiplicity fraction decreases towards late spectral types. FromRaghavan et al.(2010).
The search for low mass-ratio binaries eventually reached into the planetary mass domain, whenLatham et al.(1989) announced the discovery of aM2sini = 11±1MJ companion to the F9V star HD 114762. On the basis of the results discussed in Chapt. 3, HD 114762 b may mark the first discovery of an extrasolar planet. The discovery of a system of Earth- mass planets orbiting a pulsar was announced a few years later (Wolszczan & Frail,1992;
Wolszczan,1994).
Uncontestable evidence that extrasolar planets exist around Sun-like stars was put forward byMayor & Queloz(1995) with the discovery of a Jupiter mass planet in an uncomfortably close orbit around a G-type dwarf. Since then, the progress in planet detection and char- acterisation has been astonishingly fast and a large diversity of planetary systems has been uncovered. Solar-type stars were found to harbour rich planetary systems (Fischer et al., 2008; Lovis et al., 2011b) and planets were discovered around giant stars (e.g. Döllinger et al. 2009), around intermediate-mass stars (Lovis & Mayor, 2007; Sato et al., 2008), and
around M dwarfs (Delfosse et al., 1998). M dwarfs harbour very light planets, including Earth-mass planets (Mayor et al.,2009), but the frequency of giant planets around M dwarfs is lower than around G and K dwarfs (Bonfils et al., 2007;Endl et al., 2008; Bonfils et al., 2011).
Only a few dozen exoplanet candidate detections were needed to realise that objects with M2sini > 10MJ orbiting close to their solar-type host star are rare (Marcy & Butler,2000).
Observations of binary stars sinceDuquennoy & Mayor(1991) have shown that high-mass ratio binaries withM2 < 0.1M are rare too. The paucity of brown dwarf companions to solar type stars (Fig.1.2) has been confirmed repeatedly (Heacox, 1999; Halbwachs et al., 2000; Mazeh et al., 2003; Grether & Lineweaver, 2006), but a detailed statistical study of those objects was inhibited by the very low number of detected objects.
Figure 1.2:Mass histogram of close companions to Sun-like stars within 50 pc revealing the lack of companions in the brown dwarf mass range. FromGrether & Lineweaver(2006).
Large surveys with state-of-the-art instruments provide us with estimates of the occurrence rates of the various planet flavours, reaching towards the much sought after Earth-twin category.Howard et al.(2010) surveyed 166 Sun-like stars with precise radial velocities and obtained an occurrence rate of 12 ±4 % for planets with M2 = 3-10M⊕ and an orbital periodP shorter than 50 days.Mayor et al.(2011) observed a large sample of 822 non-active stars over 8 years and found that the fraction of stars hosting a planet withM2 > 50M⊕ andP <10years is14±2%1and the fraction of stars hosting a planet of any mass within that period range is as high as75±7%. Mayor et al. (2011) also uncovered the extremely high frequency of low-mass planets (∼50 % of solar-type stars host one withM2 < 50M⊕ andP <100days) and the presence of two populations separated in mass at approximately 30−50M⊕ and exhibiting different characteristics in terms of occurrence rate, host star
1Interestingly, the frequency of binaries is the same in this period range (Halbwachs et al.,2003).
1.2 Observations of multiple stars and planetary systems 23 metallicity, and orbital eccentricity. The increasing occurrence of giant planets with host star metallicity is not observed for the low-mass planets, see Fig.1.3.
10.0 100.0
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Figure 1.3: Left: Mass-histogram of planets around Solar-type stars detected with HARPS (black) and after correction of detection biases (red). Right: Host star metallicities for giant planets (black), low-mass planets (M2 < 30M⊕, red), and the full sample (blue). From Mayor et al.(2011).
The statistics of planets around M dwarfs was studied byBonfils et al.(2011), showing that giant planets at a frequency of 1-2 % are rare compared to the low mass planets found with a frequency of∼35 %. The frequency of planets with massesM2= 1−10M⊕in the habitable zone of M dwarfs was determined to41+54−13% by the same study. A likely detection of a∼3 Earth-mass planet at∼0.66 AU around a late type M dwarf (M2 ∼0.084M) was claimed based on a microlensing event (Kubas et al.,2012), and may be indicative of planet formation around very cool M dwarfs.
0 2 4 6 8 10
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Feb 2011 New
30000 4000 5000 6000 7000 8000 0.05
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Figure 1.4:Radius (left) and period (right) distribution of∼2300 planet candidates detected withKEPLERand the comparison betweenBorucki et al.(2011b) (dark histograms) and the update by Batalha et al.(2012) (light histograms). The increase of small planet candidates with sizes smaller than two Eart-radii is most prominent.
Initial statistical analyses of the properties of transiting planets discovered with theKEPLER mission were carried out byBorucki et al.(2011a,b) andBatalha et al.(2012). Several thou-
sand planet candidates around main-sequence stars were detected to date and more than
∼91 % of those have a radius smaller than Neptune, see Fig.1.4. TheKEPLERresults yield a first estimate for the frequency of Earth-size candidates of∼5 % and indicate that∼34 % of the planet candidates are found in multi-planetary systems. An early comparison indicates that the frequency of low-mass planets detected withKEPLER is low compared to the ra- dial velocity results. However, transit searches measure essentially the planet radii whereas radial velocimetry measures the planet minimum masses, which may explain the discrep- ancy. Certainly this comparison will give new insights into the properties of the low-mass planets.
So far, the only techniques that allowed us to study exoplanet populations with large samples, especially in the low-mass regime, are radial velocimetry and transit photometry. However, alternative and complementary techniques such as astrometry are gaining importance and are providing crucial information, in particular for the massive planet population.
Observing planetary systems via the stellar radial velocity
The gravitational pull of an orbiting planet on its host star results in a periodic Doppler frequency shift of the star’s spectral lines, which can be used to estimate its radial velocity (RV). The effective radial velocity variation is related to the masses of star and planet by Kepler’s laws, however, the orbit inclinationiis generally unknown, which results in the siniambiguity of RV-planet masses. A planet candidate with minimum massM2sinimay have a considerably higher mass, if the orbit is seen close to face-on.
−500 0 500
1500 2000 2500 3000 3500 4000 4500 5000
−50 0 50
HD168443 CORALIE
. .
RV [m/s]
JD − 2450000.0 [days]
O−C [m/s]
. .
Figure 1.5:Top: Eleven years of radial velocity measurements of HD 168443 withCORALIE showing the signature of two companions: am2sini = 17.5MJ companion on a 1750 day period orbit and am2sini= 7.8MJ planet on a 58 day orbit (Marcy et al.,2001;Udry et al., 2002).Bottom: Residuals of the Keplerian model fit with two companions.
The possibility of measuring the stellar radial velocity variations caused by a planet was
1.2 Observations of multiple stars and planetary systems 25
Figure 1.6:Phase-folded radial velocities of HD 85512 obtained withHARPS, revealing the significant detection of a m2sini = 3.6 ±0.5M⊕ planet in a 58 day orbit. The best-fit semiamplitude is K1 = 0.77±0.09 ms−1 and the residual dispersion is0.77 ms−1. From Pepe et al.(2011).
pointed out byStruve (1952) but it took more 40 years to reach an accuracy of ∼10 ms−1 and to detect 51 Peg b using this technique (Mayor & Queloz, 1995). Figure 1.5shows an example of radial velocity observations obtained with theCORALIEspectrograph attached to the Swiss telescope in La Silla. The quest for low-mass planets leads towards small amp- litudes, thus requiring increasingly sophisticated instruments, observing techniques, and data reduction strategies. At present, the state-of-the-art is realised by the HARPSoptical spectrograph (Mayor et al.,2003) which achieves stellar velocity measurements accurate to below the meter per second level, see Fig.1.6. Those performances rely on a careful minim- isation and calibration of the instrumental drifts and the modelling of stellar noise sources, among various other aspects (Pepe & Lovis,2008).
Using the radial velocity technique, planets were discovered around a large variety of stel- lar types, but the highest precisions are obtained around bright and quiet stars with a large number of narrow spectral lines. By implication, the performance degrades for faint stars, fast rotators, and young and active stars. Most observations are made in the optical wavelength range and the infrared domain is beginning to be explored. The next generation of optical and infrared spectrographs for radial velocities are expected to push the tech- nique’s capabilities even further.
Photometry and spectroscopy of planetary transits
The periodic dimming of the light received from an eclipsing binary when the stars align with the line of sight has been studied for centuries. The detection of a planetary transit with precision photometry was proposed byStruve(1952) and eventually reported byCharbon- neau et al.(2000) andHenry et al.(2000). This discovery motivated a large number of plan- etary transit searches both from the ground and from space, among which are the ground basedWASPandHATsurveys and theCoRoTandKEPLERspace missions. A drawback of the transit technique is the majority of discovered planet candidates have to be confirmed with independent measurements, usually radial velocity measurements, to eliminate false positives caused for instance by a background eclipsing binary (e.g.Torres et al. 2011).
The WASP survey (Pollacco et al., 2006) uses arrays of small-aperture optical cameras on the ground to search for transiting planet candidates around bright stars and has detec- ted dozens of large planets so far (Triaud, 2011). The KEPLER mission is an optical space telescope that continuously and simultaneously measures the brightness of∼150 000 main sequence stars (Borucki et al.,2010). It began operations in 2009 and was designed to de- termine the frequency of Earth-sized planets in the habitable zone of Sun-like stars from the measurement of planetary transit events. Based on the first years of data, statistical studies were undertaken (Borucki et al., 2011a,b; Batalha et al.,2012) and many discoveries were made, e.g. the detection of multiple transiting planets (Lissauer et al.,2011) and of the first circumbinary planets (Doyle et al.,2011;Welsh et al.,2012), see also Fig.1.7.
Figure 1.7:KEPLERphotometry of the multiple transit events in theKEPLER-16 system. Mu- tual stellar eclipses and the transit of a planet in front of each star are detected. FromDoyle et al.(2011).
Transiting planets offer the possibility to study a large spectrum of the system’s properties.
The measurement of the Rossiter-McLaughlin effect makes the measurement of the projec- ted spin-orbit angle possible (Queloz et al.,2000;Winn et al.,2010a;Triaud et al.,2010), see Fig.1.8. The atmospheric composition of an exoplanet can be constrained by transmission spectroscopy, i.e. by comparing the stellar spectrum in and out of transit (Richardson et al., 2007). The measurements of the occultation, i.e. the passing of the planet behind the star thus removing the planets thermal and reflected radiation from the total flux, and the phase curve gives access to the planet’s temperature and albedo (Knutson et al.,2009). Despite the few directly accessible physical parameters of extrasolar planets, it is possible to constrain their internal structure, essentially from the measurement of the planet’s mass and size, thus density. Based on our knowledge of the solar system planets, the common model for the in- ternal structure of an exoplanet is that it is composed of a rocky/icy core of heavy materials surrounded by an envelope of Hydrogen and Helium (Fortney et al. 2011and the references therein). Due to the incident flux from the star the outermost region is an irradiated at- mosphere (Barman et al.,2005). Solar system planets like Jupiter and Saturn (Young,2003) and some extrasolar planets are found to be rich in heavy materials (Baraffe et al., 2008), which supports the core-accretion model of planet formation. Many transiting exoplanets are found to be larger in radius and lower in density than expected, and these inflated plan-
1.2 Observations of multiple stars and planetary systems 27 ets remain difficult to explain, although several theories have been proposed (Showman &
Guillot,2002;Chabrier & Baraffe,2007).
Figure 1.8: Rossiter-McLaughlin observations of the transiting planet host Wasp-15, illus- trating the spectroscopic anomaly caused by the planet masking regions of varying effective Doppler velocity while crossing the disk of a rotating star.Left: Phase-folded radial velocity curve. Right: Close up of the transit window. The bottom panels shows the residuals after model fitting. FromTriaud et al.(2010).
In multi-planet systems the planets’ orbital motions are influenced by their mutual grav- itational interactions. As a consequence, the transit times of transiting planets in multiple systems are no longer strictly periodic, but show variations of their temporal separation.
The measurement of transit timing variations can be used to measure the masses of the indi- vidual planets, thereby confirming their nature (Holman et al.,2010), or to detect additional planets (Holman & Murray, 2005). Another method to determine the mass of a transiting planet is to measure the beaming effect in the stellar photometry, a relativistic effect result- ing in the small brightness variations of a moving light source (Mazeh et al.,2012).
Direct imaging and other planet detection techniques
Because the brightness ratio between a Sun-like star and a Jupiter-like planet is of the order of106, the direct imaging of extrasolar planets is extremely challenging, especially at close projected separations. Therefore, direct detection has yet only been achieved for giant plan- ets around low-luminosity primaries or at wide separations. Using ground-based adaptive optics imaging,Chauvin et al.(2004) detected a∼5MJobject at 0.800from the∼25MJbrown dwarf primary. It is debatable whether such a very low mass binary with a mass ratio of 0.2 may be called a planetary system, but the discovery demonstrated the direct detection of a planetary mass object.Marois et al.(2008) discovered a system of three planets with masses
∼5−13MJ at > 0.600 from the A5 star HR8799. Using HST imaging, Kalas et al. (2008) detected a planetary companion candidate at3.800 from Fomalhaut (A3V). Those planetary mass objects are separated by tens of astronomical units, thus have correspondingly long orbital periods, and their detection is predominantly based on common proper motion in- stead of the measurement of the orbital motion. This class of planets is therefore in a way separated from the previously discussed exoplanets. An intermediate object is the∼9MJ
planet aroundβPic (Lagrange et al.,2010) with a reported period of 17-35 years.
When a massive body passes in the line of sight to a distant luminous source, the relativistic light deflection refocuses the stellar light and causes a characteristic increase in the bright- ness of the background source called a gravitational microlensing event. If the foreground lens object is a star, these events offer the opportunity to detect planets around this star from the irregularities of the microlensing light curve (Bond et al.,2004). This technique is sensit- ive to planets in wide orbits, which is a good complement to the RV and transit techniques, but the discovered planets can hardly be followed up due to the chance alignment required for their detection. The results of large microlensing surveys over several years with the detection of ten events indicating planetary-mass lenses suggest that Jupiter-mass objects are about twice as common as main-sequence stars (Sumi et al.,2011). This large population would be composed of unbound objects and planets on wide (&10 AU) orbits around their host stars. In addition, a large population of bound (within 0.5–10 AU) low-mass planets is inferred (Cassan et al.,2012;Quanz et al.,2012).
A recently proposed planet detection technique in the context ofKEPLERis the detection of non-transiting planets using the beaming effect on high-precision photometric light curves, which was demonstrated by detecting stellar companions ofKEPLERtargets (Faigler et al., 2012).
1.3 Astrometric planet detection and characterisation
Today’s most important contributions to the field of observational exoplanet research are of statistical nature and beyond the discovery of one particular object. New directions in our understanding of exoplanets have emerged from the observational insight that some type of planets seem to be rare or conversely very common compared to another type. As an example, the realisation that hot Jupiters have high chances to exhibit high obliquities instead of being rationally aligned with their host star’s spin axis (Triaud et al., 2010), has led to a vibrant effort to explain the observations and to propose new theories.
So far, the contributions of astrometry work to the study of exoplanet populations have been limited and it is one of the motivations of my dissertation to change that situation.
I specify that when I write of astrometryin the context of planet detection, I usually refer to measurements that resolve the astrometric motion of the host star due to the orbiting planet. The single large survey undertaken with precision astrometry was theHIPPARCOS mission with a single measurement precision of∼1mas. If we assume a HD 114762 b type object, i.e. a10MJ companion in a 100 day orbit around a 1Mstar, and place it at 10 pc, the astrometric signature of the star’s barycentric orbit is 0.4 mas, thus difficult to detect withHIPPARCOS. Decreasing the companion mass to1MJ shrinks the orbit to 0.04 mas and sets it out of reach for a possible detection. An intrinsic difference compared to the radial velocity and transit techniques is that the astrometric signature caused by a planet depends on the observer’s distance from the host star, specifically it is decreasing reciprocally with distance2.
2Concerning HD 114762 b, the star is at 39 pc and the minimum barycentric orbit is thus 0.11 mas, which explains why the orbit was not detected with HIPPARCOS (Halbwachs et al.,2000;Zucker & Mazeh,2001).
1.3 Astrometric planet detection and characterisation 29 For illustration, I queried the extrasolar planets encyclopaedia3 (Schneider et al.,2011) on January 2, 2012. Of the 718 entries, 561 planets had a record of planet mass, orbital period, stellar mass, and distance, which I used to compute the minimum semimajor axis of the barycentric stellar orbit4, i.e. the minimum astrometric signature, which is shown in Fig.1.9.
Without requiring this planet census to be exhaustive, it shows that only ten planets, i.e. a very small fraction of<2 %, introduce an astrometric signal larger than 1 mas with a period shorter than 20 years and the smallest planet mass in this subsample is1.5MJ. On the other hand, 109 planets (∼20 %) create a signal larger than 0.1 mas.
10
−310
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010
110
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31e−6
0.001 0.01 0.1 1 10
Period (years) a
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Figure 1.9: Minimum astrometric signature as a function of orbital period for 561 planets (blue circles). The red circle marks HD 114762 b with a minimum signature of 0.11 mas and a period of 84 days.
This demonstrates that the astrometric accuracy has to increase by one order of magnitude compared to HIPPARCOS, i.e. from 1 mas to 0.1 mas, to peer into the regime where the detection of Jupiter-mass companions to Sun-like stars becomes possible. In this disserta- tion, I will present and discuss several approaches, techniques, and instruments that can achieve this task at present. They rely on large facilities, specialised observing and analysis techniques, and a considerable amount of resources, which explains why comprehensive astrometric surveys in the planet domain are sparse.
The astrometry technique offers several advantages over the previously discussed planet detection methods. For instance, all orbital elements of a detected planet are determined by it, including the orbit inclination. Therefore, astrometry yields a direct measurement of the system’s mass ratio, thus determining the planet mass if the stellar mass is known.
This capability makes astrometry the ideal tool to determine the mass function of extrasolar planets. Another advantage is that any star hosting a planetary system shows an astromet- ric signature, which in principle can be observed. Unlike radial velocities, where the signal amplitude depends on the orbit inclination (it may even vanish), and transit photometry, where the detection probability for a single star becomes unreasonably small for long orbital
3http://exoplanet.eu/
4When the inclination was unknown, I assumedM2=M2 sini. For the math see Chapter2.
periods, the astrometric signal is always present. Finally, high-precision astrometry opens a unique planet detection space, which is hardly accessible by other techniques. Chapter2 gives a detailed overview of astrometric orbit measurements, therefore I conclude this sec- tion by listing the domains in which I see the most important contributions of astrometry in the field of extrasolar planets in the short-term future:
• The detection and characterisation of giant planets around Sun-like stars, in particular the determination of their accurate mass function.
• The study of long-period planets.
• The search for planets around stars either impossible or difficult to access by other techniques, e.g. the active young stars or the faint brown dwarfs.
1.4 The golden thread and organisation of this thesis
As outlined before, the statistical study of exoplanet systems is central to fully taking ad- vantage of any new technique. High-precision astrometry has the unique capability of dir- ectly yielding precise companion masses. The combination of those elements leads me to highlight the power of observational astrometry and to identify the golden thread of this thesis:
The determination of the comprehensive companion-mass function across the primary star mass range.
Throughout the dissertation, I will refer to this central theme and demonstrate how my work contributes to achieve this major challenge.
The content of my dissertation reflects how I made my way towards the field of astronomy.
To complete my studies of pure physics, I engaged in the laboratory development and test- ing of an interferometric instrument, which included a mode dedicated to high-precision astrometric measurements. With the aim of pursuing my growing interest in astronomical instrumentation and its astrophysical applications, I engaged in the doctoral programme at the Observatoire de Genève, of which I spent the first two years being hosted by ESO in Europe and Chile. During five years, I worked on the implementation and putting into operation of a new astrometric instrument, I became knowledgeable in the astrophysical research related to extrasolar planets, and I initiated new projects aiming at using high- precision astrometry to advance this field of research. Consequently, this dissertation covers the outcome of my instrumentation work, presents the results of my research, and contains preliminary results of new projects not yet concluded.
Chapter1 gives an overview of the field of extrasolar planets and the motivation for this thesis, whereas Chapter2is a specific introduction to the formalism of and the instruments for high-precision astrometry. Chapter3presents the results of the combination ofHIPPAR- COS astrometry with a radial velocity survey aiming at determining the high-mass tail of the planet distribution function. The technique of imaging astrometry capable of very high pre- cision is described in Chapter4, along with its application to planets around two M dwarfs.
1.4 The golden thread and organisation of this thesis 31 The astrometric search for planetary-mass companions around twenty ultra-cool dwarfs is presented in Chapter5. Finally, the technique and instrumentation for high-precision astro- metry using a dual-field interferometer is described in Chapter6in the context ofPRIMAand the results of commissioning work are shown. Chapter7concludes the thesis with a discus- sion and outlook, and the Appendix8gives supplementary information.
To ease the reader’s orientation within the admittedly diverse topics, a graphical breakdown of the dissertation is given in Fig.1.10.
Observing Exoplanet Populations With High-Precision Astrometry
Results Techniques Instrumentation
The frequency of close brown dwarf compan- ions to Sun-like stars from theCORALIEsur- vey and HIPPARCOS astrometry. Chapter3.
Multi-reference astro- metry at 0.1 mas level using optical imaging.
Chapter4.
ThePRIMAfringe sensor unit.
Section6.1.
Astrometric search for planetary companions of ultra-cool dwarfs.
Chapter5.
Single-reference astrometry with in- frared interferometry.
Chapter6. Commissioning the
PRIMA facility for as- trometry. Section6.5.
Astrometric charac- terisation of massive planets around M dwarfs. Sections 4.3 and4.5.
Figure 1.10:Breakdown of the thesis showing its logical division into three categories.
2 Astrometry orbits at high precision
Astrometry consists in taking the measure of stellar positions and is one of the oldest dis- ciplines within astronomy. Besides the scientifically motivated pre-Christian efforts, pre- cision astrometry gained importance during the15th century when accurate stellar maps were required for navigation, which had direct impact on economic and political power.
Meridian telescopes were used to measure the transit times of stars and to establish first reference frames. The use of photographic plates for astronomy in the19th century led to the production of the first modern-scale catalogues and high-precision studies. Based on as- trometric measurements with photographic plates the detection of an extrasolar planet was announced byvan de Kamp(1963), a detection which later was shown to be spurious and caused by unrecognised systematic errors. The appearance of CCDs and the computer, com- bined with the technical and observational experience gained over several centuries, pushed on the progress and led to the first space astrometry mission, theHIPPARCOSsatellite.
There are a number of details which become important when dealing with astrometry below the arcsecond level. To introduce the basic types of astrometric motions I will illustrate them with the visual binaryαCentauri (α Cen), which is very nearby, has large proper motion, and a well-measured orbital motion that is considerable over time scales of years.
2.1 Coordinates, proper motion, and parallax
The position of a stellar object is described by its coordinates. Since known exoplanet host stars usually are located in the solar neighbourhood, the equatorial system of right ascen- sion (RA,α) and declination (DEC,δ) is conveniently used. A common source of oversight is the relation between a change∆of a star’s position along the celestial equator and the associated change of the star’s right ascension∆α. Because of the projection onto the celes- tial sphere, the two are related by∆ = ∆α? = ∆αcosδ and to simplify the equations one definesα? =αcosδ.
The reference astrometric catalogue for nearby and bright stars is theHIPPARCOScatalogue, of which there are two flavours today, the originalHIPPARCOScatalogue (HIP1,ESA 1997) and the new HIPPARCOS reduction (HIP2, F. van Leeuwen 2007). Table 2.1 lists the ba- sic data of α Cen B in both catalogues. The coordinate difference in RA and DEC is 13.4 mas and 32.7 mas, respectively, whereas the differences in parallax and proper motion are more significant. This is specific to targets likeαCen, i.e. an extremely bright binary with intermediate separation, which perturbed theHIPPARCOSsolution and therefore the astro-
