Modelling the Dust in the HD100546 Protoplanetary Disk Across All ALMA Scales
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Modelling the dust in the HD100546 protoplanetary disk across all ALMA scales Michael Stroet 11293284 Bachelor Thesis Physics and Astronomy Supervisor Prof. Dr. Michiel Hogerheijde Examiner Prof. Dr. Carsten Dominik Size 15 EC Conducted between May 2020 and August 2020 University University of Amsterdam Vrije Universiteit Amsterdam Faculty Faculty of Science Institute Anton Pannekoek Institute for Astronomy Date of submission 20 August 2020 Populairwetenschappelijke samenvatting Al ver voor de tijd van de oude Grieken en Egyptenaren bestudeerde men de planeten aan de hemel. Maar de enige planeten die zij konden waarnemen bevonden zich in ons eigen zonnes- telsel. Dit maakte het onderzoek doen naar de oorsprong van planeten vrij lastig aangezien onze planeten al gevormd zijn. Pas sinds de laatste jaren van de 20e eeuw, bezitten wij de tech- nologie om planeten te ontdekken die om andere sterren ver buiten ons zonnestelsel draaien. In de moderne tijd weten wij dat ons zonnestelsel acht planeten bevat. Vanaf de zon gezien zijn dat Mercurius, Venus, de Aarde en Mars die allemaal een vaste rotsachtige vorm hebben en de gasreuzen Jupiter, Saturnus, Uranus en Neptunus. De rotsachtige planeten en de gasreuzen zijn vanelkaar gescheiden door de astero¨ıdengordel. Maar astronomen hebben ontdekt dat de exoplaneten van andere sterren deze nette scheiding niet altijd hebben. Sterker nog, ons zon- nestelsel valt op tussen de andere zonnestelsels. Een veel voorkomend verschijnsel is een "hete Jupiter", planeten zoals Jupiter die heel dicht bij hun ster staan, dichter dan Mercurius bij onze zon. Om er achter te komen hoe zulke zonnestelsels kunnen ontstaan, moeten wij planeetvorm- ing onderzoeken. Wanneer er in de ruimte een nieuwe ster wordt geboren door het instorten van een enorme wolk van gas en stof, vormt het overgebleven materiaal een grote schijf om de ster heen. In zo'n schijf kunnen de kleine stofdeeltjes samenklonteren totdat zij de beginselen van planeten vormen, pro- toplaneten. Degelijke schijven worden daarom ookwel protoplanetaire schijven genoemd. Deze protoplaneten kunnen dan verder groeien tot rotsachtige planeten of kunnen gas uit de schijf verzamelen om uit te groeien tot gas reuzen. Oftewel, de verdeling van deze stofdeeltjes is erg belangrijk voor de vorming van planeten. Daarom richtte ik mij in mijn project op het bestuderen van de verdeling van stofdeeltjes in de protoplanetaire schijf rondom de ster HD100546. Aan de hand van een computermodel maak ik een afbeelding van de schijf die dan wordt vergeleken met echte waarnemingen van de ALMA telescoop hoog in de gebergten van Chili. Een algoritme kijkt naar de vergelijking van het model en de data en maakt kleine aanpassingen aan het model om te kijken of zij dan beter overeenkomen. Uit de uiteindelijke resultaten vind ik dat de schijf van HD100546 een ring van stofdeeltjes bevat tussen ongeveer 20 en 50 astronomische eenheden (AE), waar 1 AE gelijk staat aan de afstand tussen de zon en de aarde. Vooralsnog is het nog niet gelukt om de verdeling van stof in de ring zelf concreet vast te stellen. In de toekomst zal er meer onderzoek gedaan moeten worden om de stofverdeling duidelijker in kaart te brengen. 3 Abstract Context. The orderly lineup of planets in our solar system, with four inner terrestrial planets and four outer giants, is not the standard configuration across the universe. Planets form inside the interplanetary disks of newborn stars by growing from small dust particles to larger plan- etesimals. The distribution of the dust particles plays a major role in formation of planets. The ALMA telescope is able to resolve these disk and allows us to observe the planetary formation process directly. Aims. This work aims to model the dust distribution in the interplanetary disk of Herbig Ae/Be star HD100546 using two archival 1.3 mm continuum images taken by the ALMA telescope. The images have to be observed with different baseline configurations of the radio antennas, which enables the study of both the small and large scale dust structures of the disk. Methods. Using a Markov Chain Monte Carlo (MCMC) method, a variety of models will be fitted to the data images in order to find the model and fit parameters that best describe the dust distribution. The long baseline image will be modelled by a power law and a Gaussian model, whereas the small baseline image will be modelled by another power law extending from the long baseline power law model. Results. The small scale images were able to be fitted by a ring of dust extending from either +3:3 +12:1 +2:2 +28:6 19:8−5:5 to 55:0−8:8 AU for the power law model or from 22:0−6:6 to 60:5−12:1 AU for the Gaussian model. The fit results for the distribution of dust inside this ring, however, were in- conclusive, neither was any evidence found for an extended power law of dust beyond the ring. Conclusions. The results provide values for the extent of a ring of dust surrounding the HD100546 star, but were unable to confidently provide an understanding of the distribution the dust inside the ring. Further research could look into the data for an understanding of the distribution of flux of the u-v visibilities across the different scales. Another way forward is the use of more precise data to try and fit the same or different models to the dust distribution in the disk. 4 Contents Populairwetenschappelijke samenvatting 3 Abstract 4 1 Introduction 6 2 HD100546 7 2.1 Observational history . 7 2.2 Protoplanetary disks . 8 2.2.1 General structure . 8 2.2.2 The HD100546 system . 9 2.3 Observations . 10 2.3.1 ALMA telescope . 10 2.3.2 Database images . 11 2.3.3 Data processing . 12 3 Fitting the intensity profile 14 3.1 Modelling the dust continuum intensity . 14 3.1.1 Intensity model . 14 3.1.2 Radial intensity profile . 15 3.2 Fitting with a MCMC method . 16 3.2.1 Motive for MCMC . 16 3.2.2 Bayes' theorem . 17 3.2.3 Likelihood probability . 18 3.2.4 Prior probability . 18 3.2.5 Initialisation . 19 4 Results 21 4.1 Long baseline results . 22 4.2 Small baseline results . 25 5 Analysis 27 5.1 Small scale ring structure . 27 5.2 Large scale extended structure . 28 5.3 Future recommendations . 28 6 Conclusion 29 Acknowledgements 29 References 30 5 Chapter 1 Introduction For centuries the only laboratory astronomers had for the study of planets and planetary form- ation was our own solar system. Only since the first detection of a planet in orbit around a star other than the sun (Wolszczan and Frail, 1992) have we been able to study the planetary formation process by observing exoplanets. What we found, however, was that our orderly solar system with four inner terrestrial planets and four outer gas giants is not the standard configuration of solar systems. A popular example of such disparate systems is the existence and abundance of hot jupiters. Hot jupiters are gas giants similar to Jupiter, but in an orbit incredibly close to their star, closer than the orbit of Mercury around the Sun. A hot jupiter was the first exoplanet found orbiting a sun-like star (Mayor and Queloz, 1995). A discovery for which Mayor and Queloz were awarded the 2019 Nobel Prize in Physics. The existence of such planets suggested that they can either form in different places than we thought or are able to migrate to a radically different orbit. To understand the history of our own solar system and planetary formation as a whole, the formation of planets around newborn stars must be studied. When a molecular cloud of gas and dust collapses, leading to the formation of a star, the leftover material falls into a orbit around the newly born protostar, forming a circumstellar disk. Circumstellar comes from the latin words circum stella meaning 'around a star'. Tiny dust particles inside this disk are able to grow into larger planetesimals, the building blocks of all planets. For that reason, circumstellar disks like these are often called protoplanetary disks. If a large volume of gas is present around the orbit of a protoplanet, it is able to grow even larger into a gas giant. The initial distribution of the dusk particles inside of protoplanetary disks is therefore of profound importance to the formation of planets. In this project, my goal is to model the dust distribution inside the protoplanetary disk around the Herbig Ae/Be star HD100546. Several models of the distribution of dust will be created and compared to archival images taken by the Atacama Large Millimeter Array (ALMA) telescope in Chile. To be able to study both the small scale and large scale structures of the distribu- tion of dust, the archival images must have been observed with different configurations of the ALMA radio antennas. These models will then be fit to the data using an ensemble based Markov Chain Monte Carlo (MCMC) algorithm as implemented by the Python library emcee (Foreman-Mackey et al., 2013). This thesis is structured by the following outline: Chapter 2 portrays the history and characteristics of protoplanetary disks and of HD100546 in particular along with the ALMA observations and required data processing. Chapter 3 describes the dust distribution models and the methods used to fit the models to the data. In chapter 4 the results will be presented for each of the models. The results are then analysed in chapter 5. And finally, chapter 6 will conclude the thesis, summarising all that has been done and learned throughout the project. 6 Chapter 2 HD100546 In recent years, the protoplanetary disks around newborn stars have been an extensively studied topic (Grady et al.