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Observing Dust Grain Growth and Sedimentation in Circumstellar Discs Interpretations and Predictions of Their Observable Quantities in a Multi-Wavelength Approach

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Observing Dust Grain Growth and Sedimentation in Circumstellar Discs Interpretations and Predictions of Their Observable Quantities in a Multi-Wavelength Approach Observing dust grain growth and sedimentation in circumstellar discs Interpretations and predictions of their observable quantities in a multi-wavelength approach Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftliche Fakult¨at der Christian-Albrechts Universit¨atzu Kiel vorgelegt von J¨urgenSauter Kiel, 2011 Referent : Prof. Dr. S. Wolf Koreferent: Prof. Dr. C. Dullemond Tag der m¨undlichen Pr¨ufung: 7. Juli 2011 Zum Druck genehmigt: 7. Juli 2011 gez. Prof. Dr. L. Kipp, Dekan To my growing family Abstract In the present thesis, the observational effects of dust grain growth and sedimentation in circumstellar discs are investigated. The growth of dust grains from some nanometres in diameter as found in the inter- stellar medium towards planetesimal bodies some meters in diameter is an important step in the formation of planets. However, this process is currently not entirely un- derstood. Especially, in the literature several `barriers' are discussed that apparently prohibit an effective growth of dust grains. Hence, it is of particular interest to compare theories and observational data in this respect. State-of-the-art radiative transfer techniques allow one to derive observable quantities from theoretical models that allow this comparison. In this thesis, generic tracers of dust grain growth in spatially high resolution multi-wavelength images are identified for the first time. Further, a possibility to detect a dust trapping mechanism for dust grains by local pressure maxima using the new interferometer, ALMA, is established. By fitting parametric models to new observations of the disc in the Bok globule CB 26, unexpected features of the system are revealed, such as a large inner void and the possibility to interpret the data without the need for grain growth. In the same way, new observations and their interpretation of IRAS 04302+2247 provide evidence for the first time for radial dependent settling of dust grains in images. Moreover, in this thesis the applicability of the T Tauri disc paradigm to the high-mass disc candidate IRAS 18151-1208 is demonstrated. v Zusammenfassung In der vorliegenden Dissertation werden die beobachtbaren Effekte von Staubkorn- wachstum und -sedimentation in zirkumstellaren Scheiben untersucht. Das Wachstum von Staubpartikeln in der Gr¨oßevon einigen Nanometen, wie sie im interstellaren Medium zu finden sind, hin zu Planetesimalen mit einigen Metern Durchmesser ist ein wichtiger Prozess im Zuge der Entstehung von Planeten. Allerd- ings ist dieses Wachstum gegenw¨artignicht vollst¨andigverstanden. Insbesondere werden in der Fachliteratur einige Schranken diskutiert, die ein effektives Wachstum von Staubk¨ornernzu verhindern scheinen. Daher ist es von besonderer Bedeutung, Theorien und Beobachtungsdaten in diesem Zusammenhang miteinander zu verglei- chen. Strahlungstransportmethoden auf dem aktuellen Stand der Technik gestatten es, aus theoretischen Modellen Beobachtungsgr¨oßenabzuleiten, die einen solchen Vergleich erlauben. In dieser Arbeit werden zum ersten Mal generische Indikatoren des Staubkorn- wachstums in r¨aumlich hochaufgel¨ostenBildern mehrerer Wellenl¨angenidentifziert. Zudem wird eine M¨oglichkeit aufgezeigt, die es gestattet, mit Hilfe des neuen In- terferometers ALMA die Spuren von durch lokale Gasdruckmaxima eingefangenen Staubk¨ornernzu erkennen. Durch Bestimmung der Parameter geeigneter Modelle anhand neuer Beobachtungs- daten der Scheibe im Bok-Globulus CB 26 werden neue unerwartete Eigenschaften des Systems, wie ein großer, zentraler staubfreier Bereich und die M¨oglichkeit, die Beobachtungsdaten ohne die Annahme von Staubkornwachstum zu interpretieren, enth¨ullt. Ebenso erlauben neue Beobachtungen und deren Deutung von IRAS 04302+2247 zum ersten Mal einen Hinweis in Bildern auf radiusabh¨angigeSedimen- tation von Staubk¨ornern. Dar¨uberhinaus wird die Anwendbarkeit des Scheibenkonzeptes von T Tauri Ster- nen auf IRAS 18151-1208, einem massereichen Protostar, gezeigt. vii Contents 1. Introduction 1 2. Basic considerations 5 2.1. Protoplanetary discs: Setting the stage . 5 2.2. Dust: Building blocks of planets . 7 2.3. Observation: Looking yonder . 15 2.4. Radiative transfer . 20 2.5. Some remarks on methodology . 21 2.6. Aims of this work . 23 3. The disc in the Bok globule CB 26 25 3.1. Overview . 25 3.2. Observations . 26 3.3. The model and its parameter space . 30 3.4. Results . 36 3.5. Discussion . 39 4. IRAS 18151-1208: Exploring the high-mass regime 55 4.1. Discs' role in high mass star formation . 55 4.2. Observations . 56 4.3. Modelling . 57 4.4. Results & Discussion . 61 5. Observing grain growth and sedimentation 65 5.1. Method . 65 5.2. Results . 72 5.3. Other markers: Discussion . 89 6. Observing trapped dust 97 6.1. The drift barrier and dead zones . 97 6.2. The model with pressure maxima . 98 6.3. Observational consequences for millimetre images . 101 6.4. Interferometric observation of pressure maxima . 107 ix Contents 7. Dust settling in IRAS 04302+2247 113 7.1. The object . 113 7.2. Observation . 114 7.3. Analysis and Discussion . 115 7.4. Result . 118 8. Concluding remarks 121 8.1. Summary . 121 8.2. Outlook . 123 A. Configuration of the ALMA-Observatory 125 B. Allegory of the cave 131 List of Figures 137 List of Tables 139 List of Abbreviations 141 List of Symbols 143 Bibliography 146 Acknowledgements 159 x 1. Introduction Μετὰ tαῦτa δή, εἶποn, ἀπείκasvon toiούτú πάθει τὴν ἡμετέραν φύσvιn paideÐac te pèri kaÈ ἀπαιδευσvίας. ἰδὲ γὰρ ἀνθρώπους οἷon ἐn katageÐú οἰκήσvει svphlai¸dei, ἀναπεπτamènhn πρὸς tοφῶς τὴν εἴsvodon ἐχούσvῃ μακρὰν παρὰ πᾶν τὸ σvπήλαιon, ἐn tαύτῃ ἐκ paÐdwn ὄntac ἐn δεσvμοῖc kaÈ τὰ svkèlh kaÈ tοὺς aÎqènac, ¹svte mènein te aÎtοὺς εἴc te τὸ πρόσvθεn μόnon ὁρᾶν, κύκlú δὲ τὰς κεφαλὰς ὑπὸ tοῦ δεσvμοῦ ἀδυνάτouc περιάγειn.1 (Platon, PoliteÐa ) The questions concerning the origin of our very own existence might very well be as old as the ability of humans to pose questions in general. Attempts to answer these deep questions are provided by various enterprises of mankind such as religion, philosophy and also science. While the reasoning of the first two is confined to faith and exchange of (clever) arguments, it is the latter that gains its tremendous success by observing the physical nature as it surrounds us. Theories built on these observations are required to yield predictive power and these predictions are in turn subject to comparison with the physical world. One might argue that in this fashion rather the question How? is answered instead of the question Why?. The success of science is driven by its ability to connect subjects previously considered disjoint and to describe them by a common set of terms. In a plenitude of cases this property provides an answer to the first question and renders the necessity to pose the second one moot. Therefore, the question about our origin eventually leads to the question of the origin of our home, the planet Earth we dwell upon. No other planet is known that harbours life, and until the end of the last decade of the second millennium extra-solar planets in general had not been observed. Any understanding developed up until then on how planets come into existence rested solely on the observed properties of our own solar system. Five of the solar planets have been observed and studied since several thousand years. Their motion eventually gave rise to the powerful Newtonian Mechanics and 1A translation of all Chapter headings is given in the appendix B 1 1. Introduction also provided one impressive confirmation of General Relativity. The non-classical planets have all been discovered in the modern age by virtue of careful application of celestial mechanics. However, as the existence of extrasolar planets became evident, it was also clear that these newly discovered planetary systems differ greatly from the solar system. Previously, it was deemed necessary that small and rocky planets, such as Earth, should be found close to the host star while gaseous planets have been expected at larger radii. Nonetheless, among the first discovered extra-solar planets were `hot Jupiters': gaseous orbs at small distances to their star. Consequently, the established understanding of the formation of planetary systems needed serious revision. A fully consistent theory is still lacking. Circumstellar discs are understood to be the nursery of planets, providing both material and dynamic environment for their birth. The most accepted evolution- ary theory for planets in discs is the so-called \core-accretion gas-capture" scenario, featuring a build-up of planet cores first and gas-infall on sufficiently massive cores later (Adams et al., 1987; Lissauer, 1993; Klahr & Brandner, 2006; Williams & Cieza, 2011). In the evolution of T Tauri stars, circumstellar discs are the natural precursor of a planetary system. Key issues in the process of planetary core formation are dust grain growth and settling, which start the formation process of planetesimals (e.g. Youdin & Shu, 2002). A characteristic that is sometimes used to single out astronomy from other dis- ciplines of physics is its inability to conduct `hands-on' experiments. One cannot set up a protoplanetary disc in the laboratory and study its fate (much less galaxies or clusters thereof). In two particular senses this has changed, however. Today's experiments are carried out to study the coagulation, bouncing and fragmentation properties of dust grains that are expected to form planetesimals. And even more so, numerical experiments relying on considerable computing
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