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Astronomy and Astrophysics Astronomy and Astrophysics Observing the On-going Formation of Planets and its Effects on Their Parent Discs Mr Matthew Alexander Willson Submitted by Mr Matthew Alexander Willson to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Physics, June, 2017. This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signed: . M Willson Date: 07/09/2017 Abstract As the number of known exoplanetary systems has grown, it has become increasing ap- parent that our current understanding of planet formation is insufficient to explain the broad but distinct distributions of planets and planetary systems we observe. In particular, constructing a co- herent model of planetary formation and migration within a circumstellar disc which is capable of producing both hot Jupiters or Solar System-like planetary system is high challenging. Resolved observations of where planets form and how they influence their parent discs provides essential information for tackling this important question. A promising technique for detecting close-in companions is Sparse Aperture Masking (SAM). The technique uses a mask to transform a single aperture telescope into a compact interferometric array capable of reliably detecting point sources at the diffraction limit or closer to a bright star with superior contrasts than extreme AO systems at the cost of smaller fields of view. Applying image reconstruction techniques to the interferometric information allows an observer to recover detailed structure in the circumstellar material. In this thesis I present work on the interpretation of SAM interferometry data on protoplan- etary discs through the simulation of a number of scenarios expected to be commonly seen, and the application of this technique to a number of objects. Analysing data taken as part of a SAM survey of transitional and pre-transitional discs using the Keck-II/NIRC2 instrument, I detected three companion candidates within the discs of DM Tau, LkHα 330, and TW Hya, and resolved a gap in the disc around FP Tau as indicated by flux from the disc rim. The location of all three of the companions detected as part of the survey are positioned in interesting regions of their parent discs. The candidate, LkHα 330 b is a potentially cavity opening companion due to its close radial proximity to the inner rim of the outer disc. DM Tau b is located immediately outside of a ring of dusty material largely responsible for the NIR comment of the disc SED, similar to TW Hya b located in a shallow gap in the dust disc outside another ring of over-dense dusty material which bounds a deep but narrow gap. Both of these companion candidates maybe migrating cores which are feeding from the enriched ring of material. I conducted a more extensive study of the pre-transitional disc, V1247 Ori, covering three epochs and the H-, K- and L-wavebands. Complementary observations with VLT/SPHERE in Hα and continuum plus SMA observations in CO (2-1) and continuum were performed. The orientation and geometry of the outer disc was recovered with the SMA data and determine the direction of rotation. We image the inner rim of the outer disc in L-band SAM data, recovering the rim in all three epochs. Combining all three data sets together we form a detailed image of the rim. In H- and K-band SAM data we observe the motion of a close-in companion candidate. This motion was found to be too large to be adequately explained through a near-circular Keplerian orbit within the plane of the disc around the central star. Hence an alternate hypothesis had to be developed. I postulated that the fitted position of the companion maybe influenced by the emission from the disc rim seen in the L-band SAM data. I constructed a suite of model SAM data sets of a companion and a disc rim and found that under the right conditions the fitted separation of a companion will be larger than the true separation. Under these conditions we find the motion of 2 the companion candidate to be consistent with a near-circular Keplerian orbit within the plane of the disc at a semi-major axis of ∼6 au. The Hα data lack the necessary resolution to confirm the companion as an accreting body, but through the high contrast sensitivities enabled by the state of the art SPHERE instrument I was able to rule out any other accreting body within the gap, unless deeply embedded by the sparse population of MIR emitting dust grains previously inferred to reside within the gap. Through the combination of SAM and SMA data we constrain the 3-D orientation of the disc, and through multi-wavelength SAM observation identify a close- in companion potentially responsible for the gap clearing and asymmetric arm structures seen in previous observations of this target. During my PhD I have contributed to the field of planet formation through the identification of four new candidate protoplanets observed in the discs of pre-main sequence stars. To do so I have quantified the confidence levels of companion fits to SAM data sets and formed synthetic data from models of asymmetric structures seen in these discs. I have described for the first time the effects of extended sources of emission on the fitted results of companion searches within interferometric data sets. I have combined SAM data sets from two separate telescopes with different apertures and masks to produce reconstructed image of an illuminated disc rim with superior uv-coverage. I have used the expertise I have developed in this field to contribute to a number of other studies, including the study of the young star TYC 8241 2652 1, resulting in the rejection of a sub-stellar companion as the cause of the rapid dispersal of the star‘s disc. The companion candidates I have identified here should be followed up to confirm their presence and nature as accreting protoplanets. Objects such as these will provide the opportunity for more detailed study of the process of planet formation in the near future with the next generation of instruments in the JWST and E-ELT. Contents 1 Introduction 14 1.1 Background . 14 2 Star formation and disc evolution 17 2.1 Introduction . 17 2.2 Star formation paradigm . 17 2.2.1 Initial collapse . 19 2.2.2 Formation of a protostar . 20 2.2.3 Pre-main sequence evolution . 21 2.2.4 Formation of the circumstellar disc . 22 2.2.5 Angular momentum evolution . 22 2.3 Protoplanetary disks . 29 2.3.1 Classical . 29 2.3.2 Transitional/Pre-transitional . 31 2.4 Observational constraints on structure . 33 2.4.1 Spectral energy distribution model fitting . 33 2.4.2 Spectroscopic constraints . 35 2.4.3 Sub-millimeter and radio continuum long baseline interferometry . 36 2.4.4 Optical imaging . 37 2.4.5 Variability . 42 2.4.6 Disc clearing mechanisms . 45 2.4.7 Non-dynamical disc clearing mechanisms . 45 2.4.8 Dynamical disc clearing mechanisms . 47 2.5 Chapter Summary . 49 3 Planet formation and protoplanets 50 3.1 Introduction . 50 3.2 Planet formation theory . 50 3.2.1 From dust particles to pebbles . 51 3.2.2 From pebbles to planetesimals . 53 3.2.3 From planetesimals to planetary cores . 56 3.2.4 Giant planet formation . 61 3.3 Circumplanetary discs . 64 3 CONTENTS 4 3.4 Planet-disc dynamical interaction . 67 3.5 Planet-planet dynamical interaction . 70 3.6 Protoplanets . 72 3.6.1 Observational properties . 72 3.6.2 Previous protoplanet observations . 73 3.7 Chapter summary . 79 4 High angular resolution imaging techniques 82 4.1 Introduction . 82 4.2 Single-aperture imaging . 83 4.2.1 Principles of single aperture imaging . 83 4.2.2 Adaptive optics imaging . 85 4.3 Interferometric Imaging . 87 4.3.1 Principles of Interferometry . 87 4.3.2 Observables . 94 4.3.3 Calibrators . 97 4.3.4 Sparse Aperture Masking (SAM) . 98 4.4 Chapter summary . 99 5 Image reconstruction algorithms 101 5.1 Introduction . 101 5.2 Bayesian approach . 103 5.2.1 The likelihood function . 104 5.2.2 The regularisation function . 105 5.2.3 Regularisation weight parameter, µ .................... 107 5.3 Separating star from environment . 108 5.4 Employed image reconstruction algorithm . 109 5.5 Chapter summary . 109 6 Simulation of aperture masking observables 111 6.1 Introduction . 111 6.2 Generating synthetic observations . 111 6.2.1 Degeneracies between derived model parameters . 113 6.3 Numerical simulations of companion and disc scenarios . 114 6.3.1 Small-separation/unresolved companion scenario . 114 6.3.2 Marginally resolved companion scenario . 115 6.3.3 Fully-resolved companion scenario . 116 6.3.4 Asymmetries arising from a disc wall . 116 6.3.5 Asymmetries arising from disc inhomogeneities . 119 6.3.6 The effect on the position of a companion in the presence of a disc . 119 6.4 Summary . 122 CONTENTS 5 7 Keck-II/NIRC2 Pre-Transitional Disk Survey 123 7.1 Context . 123 7.2 Target selection . 124 7.3 Observations . 124 7.4 Results . 127 7.4.1 Detection statistics . 129 7.4.2 Degeneracies . 132 7.4.3 DM Tau . 132 7.4.4 FP Tau . 137 7.4.5 LkHα 330.................................
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