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Caractérisation Des Disques De Dé- Bris Par Imagerie Directe Et Haute Résolution Angulaire Les Performances De Naco Et SPHERE

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Caractérisation Des Disques De Dé- Bris Par Imagerie Directe Et Haute Résolution Angulaire Les Performances De Naco Et SPHERE THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ DE GRENOBLE Spécialité : Astrophysique Arrêté ministériel : 7 août 2006 Présentée par M. Julien Milli Thèse dirigée par Dr. David Mouillet et codirigée par Dr. Dimitri Mawet préparée au sein de l’Institut d’Astrophysique et de Planétologie de Grenoble et de l’Observatoire Européen Austral et de l’École doctorale de physique Caractérisation des disques de dé- bris par imagerie directe et haute résolution angulaire Les performances de NaCo et SPHERE Thèse soutenue publiquement le 23 Septembre 2014, devant le jury composé de : M. Olivier ABSIL Directeur de recherche à l’Université de Liège, Examinateur Mme Anne-Marie LAGRANGE Directrice de recherche à l’Institut d’Astrophysique et de Planétologie de Gre- noble, Examinatrice M. Alain LECAVELIER DES ETANGS Directeur de recherche à l’Institut d’Astrophysique de Paris , Rapporteur M. Dimitri MAWET Astronome à l’Observatoire Européen Austral, Co-Directeur de thèse M. Jean-Louis MONIN Professeur à l’Université Joseph Fourier, Président M. David MOUILLET Astronome à l’Institut d’Astrophysique et de Planétologie de Grenoble, Directeur de thèse M. Damien SÉGRANSAN Directeur de recherche à l’Université de Genève, Rapporteur Résumé Les vingt-cinq dernières années ont connu une véritable révolution dans notre connaissance des systèmes planétaires avec plus de 1800 planètes extrasolaires connues à ce jour. L’étude observa- tionnelle des disques de débris constitue l’approche proposée dans ce travail de thèse pour éclairer les processus de formation et d’évolution des systèmes planétaires. Ces disques circumstellaires sont constitués de particules de poussière générées par des collisions de petits corps appelés pla- nétésimaux, en orbite autour d’une étoile de la séquence principale. La lumière stellaire qu’elles diffusent représente une observable particulièrement riche en informations sur l’architecture du système, mais difficile d’accès en raison du contraste élevé et de la faible séparation angulaire avec leur étoile. Le développement récent de nouveaux instruments à haut contraste équipés d’optique adaptative extrême représente un formidable potentiel pour l’étude de ces systèmes. Cette thèse se place dans le cadre de ces nouveaux développements et porte sur la caractérisation des disques de débris grâce à deux instruments qui équipent le VLT (Very Large Telescope) : NaCo et SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch). NaCo est en opé- ration depuis plus de 10 ans et a connu plusieurs améliorations successives. SPHERE a été conçu et développé dans la même période, testé intensivement en laboratoire en 2013 et est actuellement en cours de vérification opérationnelle sur le télescope. Le caractère novateur de ce travail consiste à associer à l’étude des propriétés physiques des disques de débris, une expertise instrumentale poussée pour tirer le meilleur profit des observations. La première partie vise à développer et caractériser des méthodes de réduction de données innovantes adaptées aux ob- servations de disques en lumière diffusée et au comportement de l’instrument. En particulier les atouts, performances et biais des techniques d’imagerie différentielle angulaire, polarimétrique et de soustraction de référence sont quantifiés. Ces méthodes sont appliquées, dans une seconde partie, à l’étude et la caractérisation de deux prototypes de disques de débris entourant les étoiles β Pictoris et HR 4796A. Elles permettent une analyse poussée de la morphologie de ces disques et révèlent de nouvelles asymétries, interprétées en terme de perturbateurs gravitationnels ou de propriétés de diffusion de la lumière par la poussière. Enfin une évaluation prospective des performances attendues et observées avec l’instrument SPHERE est détaillée dans la dernière section, basée sur des simulations et des mesures en laboratoire ou sur le ciel. Une comparaison avec NaCo révèle les points forts de SPHERE avant de conclure sur les questions scientifiques auxquelles les observations de disques de débris avec SPHERE pourront apporter des réponses. Abstract Over the last two and a half decades, the discovery of more than 1800 exoplanets has been a major breakthrough in our understanding of planetary systems. To shed light on the formation and evolution processes of such systems, I have chosen an observational approach based on the study of debris discs. These circumstellar discs are composed of dust particles constantly generated by collisions of small rocky bodies called planetesimals, orbiting a main-sequence star. The stellar light they scatter can be studied from the Earth and reveal a wealth of information on the architecture of the system. These observations are challenging because of the high contrast and the small angular separation between the disc and the star. The recent developments of new high-contrast instruments with extreme adaptive optic systems are therefore bringing new expectations for the study of these systems and set the framework of this PhD thesis. My work aims at characterising debris discs thanks to two instruments installed on the Very Large Telescope: NaCo and SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch). NaCo has been in operation for more than a decade and has undergone many improvements. SPHERE has been designed and assembled in the same period, was intensively tested in laboratory in 2013, and is currently being commissioned on the telescope. The innovative approach of this PhD work is to combine the study of debris discs with strong instrumental expertise to get the best science results from the observations. The first part of the study aims at developing innovative data reduction techniques adapted to the observations of discs in scattered light and to the behaviour of the instrument. I quantify in particular the performances, advantages, and biases of the angular, polarimetric and reference-star differential imaging technique. In a next step, I apply those techniques to characterise two prototypes of debris discs, around the stars β Pictoris and HR 4796A. A detailed analysis of the morphology is carried out, which reveals new asymmetries interpreted in terms of gravitational perturbers or of dust scattering properties. Lastly, I detail the expected and measured performances of SPHERE, from simulations, laboratory and on-sky measurements. A comparison with NaCo reveals the assets of SPHERE and I conclude with the scientific questions SPHERE will be able to answer with new debris disc observations. Remerciements Je dédie cette thèse à mes parents qui, dès mon plus jeune âge, ont su entretenir et développer mon émerveillement devant la nature et les lois physiques qui la gouvernent, à travers le ciel étoilé bien sûr, à Valloire par exemple, mais aussi les volcans, la montagne, ou les grandes antennes de télécommunication à Mulhouse. Je dois aussi en grande partie mon parcours aux professeurs de physique et de mathématiques de l’enseignement secondaire et de classe préparatoire qui m’ont transmis leur goût pour la science et je souhaite en particulier saluer maître Petitjean, Marc Schaul et Jean-Pierre Demange ! Je souhaite remercier toutes celles et ceux qui ont pu me guider, m’écouter et me conseiller dans ce choix de m’orienter vers la recherche scientifique, après une première expérience dans l’ingénierie et la recherche et développement. Je ne regrette pas ce choix, bien au contraire ! Trois années se sont déjà écoulées depuis le début de ces travaux de thèse, un an dans la capitale des Alpes au sein de l’IPAG et deux ans dans celle des Andes à l’ESO. Elles se terminent maintenant par une intense période de rédaction et se concrétisent par ce mémoire. Ces trois années ont été inoubliables d’un point de vue scientifique et personnel. Elles m’ont donné l’occasion de côtoyer des personnes de grande qualité pour qui je garderai toujours un sentiment de profonde gratitude. Tout d’abord, je suis immensément redevable de ma progression à mes encadrants et col- laborateurs proches David, Dimitri et Anne-Marie. Leurs exceptionnelles qualités humaines, scientifiques et leur eur expertise complémentaire m’ont permis de trouver ma place dans le monde de la recherche sur les exoplanètes et les disques. Ils ont su guider mes efforts et ma démarche scientifique avec brio, et ce malgré la distance et les implications de chacun à haut niveau. Je me réjouis à présent de pouvoir poursuivre l’aventure avec eux pour les années qui viennent. Je suis particulièrement reconnaissant envers Jean-Charles, Christophe, Olivier, Gael, Steve et Zahed pour l’expertise sur les disques et les planètes dont ils m’ont fait bénéficier tout au long de ma thèse, ainsi que Julien G. pour l’approche instrumentale. Les conditions de travail ont été très bonnes, aussi bien à l’IPAG où Jean-Louis maintient un très bon dynamisme, qu’à l’ESO où Claudio et Paulina sont très attentionnés envers les doctorants et rendent ce cadre de vie stimulant. J’ai vécu à l’IPAG de très agréables moments, avec Julien R., Fabien, Jacques, retrouvés plus tard dans la forêt vierge canadienne, Virginie et Marie, sans oublier Christophe et Romane pour les courses le long de l’Isère. A l’ESO, je remercie les thésards Joanne, Jos, Amy, Anna, Bruno, Romain, Michael, Florian qui ont contribué à rendre mon séjour ici très chaleureux. Difficile d’être exhaustif, mais j’aimerais également saluer à ce stade mes amis Yannick, Béa, Fabien, Jonathan, Simon et Nicolas pour les bons moments passés au Chili, ainsi que Adrien, Uddhab. — 2 — Cette thèse est rédigée principalement en français, mais comporte les articles scientifiques dans la langue de Shakespeare, comme la plupart des figures. Les collaborations que j’ai menées pendant ces travaux ont dépassé le cadre francophone. Ce choix me paraît judicieux et permet ainsi de rendre la science présentée ici accessible à tous, tout en valorisant l’usage de la langue française. — 3 — Table des matières Glossaire 7 Liste des abbréviations 10 Introduction 13 1 Les disques de débris par imagerie directe 15 1.1 La recherche et caractérisation des systèmes planétaires et les moyens de détection 15 1.1.1 La poussière, premier indice de système extrasolaire découvert .
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