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Theses - Institute of Astronomy

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  • ItemOpen Access
    Missing methane: Machine learning for satellite remote sensing of methane
    Roberts, Clayton; Roberts, Clayton [0000-0002-5184-7485]
    It is well understood that planet Earth is undergoing a period of severe climate change, brought on by decades of anthropogenic emission of greenhouse gases into our atmosphere. The increased abundance of greenhouse gases like carbon dioxide and methane has warmed our planet dramatically above pre-industrial levels. Rising global temperatures lead to increasingly frequent severe weather patterns and habitat destruction, and so we now find ourselves faced with the daunting task of stemming the tide of greenhouse gas emissions as quickly as possible. Although it is desirable that we decrease anthropogenic emissions of all greenhouse gases, a case can be made that reductions in methane emissions should be prioritised in order to mitigate the worst near-term effects of global warming. Methane is a much stronger greenhouse gas than carbon dioxide, capable of trapping 80 times more energy in our atmosphere over a 20-year timescale after emission. Methane also has a shorter atmospheric lifetime than carbon dioxide (just over a decade for methane compared to centuries for carbon dioxide), and thus reductions in methane emissions today will lead to a reduction in the global atmospheric abundance of methane in the near future. Over the past two decades, satellites have begun to be used to monitor greenhouse gases, and are crucial for maintaining accountability as nations commit to reductions in emissions. The latest such satellite is the TROPOspheric Monitoring Instrument (TROPOMI), capable of observing methane globally on a daily basis. However, TROPOMI observations of methane are often spatially disrupted due to cloud cover and other factors that prevent accurate retrievals of methane abundances. In this thesis, I present a Bayesian model capable of learning the extent to which TROPOMI observations of methane are spatially correlated with observations of nitrogen dioxide, and we use this model to spatially augment TROPOMI methane observations over the Permian basin in Texas. We then explore the efficacy of this model when used with TROPOMI observations of a variety of fossil fuel producing regions around the globe. Additionally in this thesis, I explore the effect of spatially disrupted TROPOMI observations on regional methane emission rate estimation. Regional methane emission rate estimates are crucial for providing timely updates on progress made towards national reductions in methane emissions. We find that spatially disrupted data may result in underestimated methane emission rates, and develop an optimised methodology for producing non-negative spatial maps of regional methane emission.
  • ItemOpen Access
    Exoplanetary Atmospheric Retrievals with Transit Spectroscopy in the JWST Era
    Constantinou, Savvas; Constantinou, Savvas [0000-0001-6839-4569]
    Atmospheric spectroscopy of planets beyond our solar system can provide important insights into their atmospheric processes, formation histories and even the presence of life. The James Webb Space Telescope (JWST) has already led to the first of likely many breakthroughs with novel chemical detections. These expected advances in exoplanetary science in the JWST era necessitate atmospheric retrieval techniques to evolve to take full advantage of JWST observations. This thesis contributes towards that objective, considering what atmospheric inferences are possible through transmission spectroscopy with JWST and the advances in atmospheric retrieval techniques necessary to achieve them. I first present what atmospheric abundance constraints are achievable with JWST observations of cloudy, temperate sub-Neptunes. Considering two exoplanets as case studies and several instrument configurations, I find that JWST observations of suitable exoplanets over a broad wavelength range can yield precise abundance constraints for prominent molecules, even in the presence of high altitude clouds. I then consider atmospheric retrievals with the first JWST observations of the hot Saturn WASP-39 b, spanning a previously unexplored spectral region. I implement a physically-based Mie scattering calculation to model the spectral contributions of clouds. Using this new aerosol model, I constrain the atmospheric elemental abundances of O, C and S, finding them to be largely consistent with the inferred C abundance of Saturn. I subsequently present VIRA, a new atmospheric retrieval framework designed to make the most of JWST observations. VIRA implements several complementary models for atmospheric composition, aerosols and temperature structure, as well as rigorously accounting for correlated observations. I use VIRA to analyse JWST observations of WASP-39 b, confirming prior results while also finding spectral contributions from ZnS aerosols and robustly inferring CH4 depletion. I also present work carried out as a contribution to recent studies. This includes analyses of observations from space and ground for a variety of gas giant planets, assessing their compositions and impact of clouds and stellar heterogeneities. In doing so, I identify a number of themes that are characteristic of the last decade of transmission spectroscopy, which are set to persist in the JWST era. Lastly, I present work examining the observability of several chemical species in the atmospheres of Hycean planets. This work demonstrates that by taking advantage of the comparatively observable atmospheres of Hycean planets, a number of candidate biomarker species are readily detectable by JWST. This thesis highlights the wealth of information that can be encoded in precise observations of exoplanetary transmission spectra. At the same time, it demonstrates the pivotal importance of sophisticated and robust atmospheric retrievals in understanding the atmospheres, interiors and formation histories of exoplanets and the search for life beyond Earth.
  • ItemOpen Access
    Exotic Stars and Thorne-Żytkow Objects
    Hackett, Alexander
    The concept of a hybrid star, a stellar object that has some sort of atypical internal structure, particularly in regards to its energy budget, has been around for over a century. Arguably the pre-Gamow explanations offered for the source of luminosity for all stars correspond to a form of hybrid star models, from Kelvin's thermal explanation, to Landau's suggestion that the sun harboured a neutron degenerate core. This dissertation focuses on the study of exotic stellar objects, both a class of hybrid stars with a neutron core known as Thorne-Żytkow Objects (TŻOs), and highly magnetized, super-Chandrasekhar mass white dwarfs. A Thorne-Żytkow Object may form as a result of a Common Envelope Evolution (CEE) event between a giant or supergiant star with a neutron star companion. It consists of a large, diffuse giant envelope surrounding a neutron degenerate core. We investigate the structure and evolution of these objects here. Focusing on the central degenerate component of these objects themselves leads to the study of exotic compact objects in their own right, in this case, white dwarfs that harbour intense magnetic fields, which provide sufficient magnetic pressure support for them at masses above the Chandrasekhar mass, making them possible progenitors of overly luminous Type Ia supernovae. In Chapter 1, I provide a brief introduction to the venerable field of stellar evolution to provide the necessary context for the following Chapters of this work. In Chapter 2, I present an introduction to the physics, structure and evolution of Thorne-Żytkow Objects, the canonical models thereof as they exist in the literature and the challenges and some of the approaches taken to overcome them. I also discuss the formation and death of TŻOs. In Chapter 3, I provide a similar introduction to the study of highly magnetized compact objects, white dwarfs (B-WDs) and neutron stars (B-NS / magnetars) as well as the relevant microphysics that we must consider to study these objects. In particular I discuss the mechanisms by which thermal neutrinos can be produced in such objects. This is essential to understanding their cooling. In Chapter 4 I introduce and explain the numerical techniques and codes used throughout this dissertation, specifically the STARS and MESA Henyey-style one-dimensional stellar evolution codes. I also explain the modifications made to the codes in question to model the exotic objects I study. In Chapter 5 I present a novel series of solutions for envelopes of TŻOs which, while qualitatively similar to those of the canonical TŻO models that I discussed in Chapter 2, differ in a few key ways. The solutions resemble the canonical supergiant-like solutions, dominated by nuclear burning, even for masses that admit a giant-like solution, dominated by accretion on to the neutron core, in these earlier models. I have investigated the thermodynamic consistency of these models and how robust the qualitative structure of the solutions is to changing accretion rates and other boundary conditions. I found that our use of revised, updated tables of thermal neutrino loss rates compared those used in the canonical work serves to explain the majority of the structural differences between our models. I also present a series of hybrid-AGB models, in which the core exists in a state between that of a neutron star and a white dwarf, and is modelled in full. Anomalous surface chemical abundances in these models indicate a method by which TŻOs could be identified observationally. In Chapter 6, I investigate the structure and evolution of super-Chandrasekhar mass B-WDs, finding that solutions do exist at masses above the Chandrasekhar mass, given a sufficiently large magnetic field permeating the object. I also present a modified field prescription that addresses an issue regarding non-physical current sheaths in the B-WDs, by means of a saturation radius. This was shown to replicate the previous results and suggests that highly magnetized supermassive white dwarfs could indeed serve as progenitors for overly luminous Type Ia supernovae. In Chapter 7 I summarize the content of this dissertation, contextualising and expanding upon the results and providing a short review of possible future avenues for related research.
  • ItemOpen Access
    Exogeology as Revealed by Polluted White Dwarfs
    Buchan, Andrew; Buchan, Andrew [0000-0003-0105-5540]
    Astronomy has entered a new era in which a large number of exoplanets has been discovered (more than 5500). In the search for an Earth twin, and possibly alien life, understanding the composition and fate of these worlds is of paramount importance. Nature provides an unlikely source of information in the form of dead stars called white dwarfs. Many white dwarfs have accreted remnants of their own planetary systems, causing their atmospheres to become polluted by heavy elements. The relative quantities of these elements contain a wealth of information about the composition of the accreted planetary bodies. This thesis focuses on the interpretation of white dwarf pollution, with the aim of better understanding planetary composition and evolution. There are a large number of processes that can affect, and potentially explain, the composition of planetary bodies. For example, exposure to high temperature can remove volatile elements. The formation of an iron-rich core is another significant process: certain white dwarfs exhibit iron-rich (or iron-poor) pollution, which are often interpreted as the accretion of core (or mantle) material. However, given a set of elemental abundances, identifying the physical processes which best explain the data is a highly non-trivial exercise. Bayesian modelling is a powerful method to disentangle the most likely explanation from the myriad of possibilities. This approach reveals evidence that core formation and volatile loss, which shape Solar System bodies such as Earth, also occur in other planetary systems. In addition to their formation histories, polluted white dwarfs can be used to uncover the ultimate fate of planetary bodies as they are accreted. Different accretion scenarios alter the composition of detected pollution in a probabilistic way, which must be investigated at the population level. I use population synthesis to calculate the sample size required to distinguish between accretion scenarios. The number of white dwarfs with detected pollution will increase in the coming years, largely due to follow-up of candidate systems identified by the Gaia mission. Applying the methodologies presented in this thesis to the resulting data will help us learn more about how planets form, about how they are ultimately destroyed, and about the Solar System's significance within the galaxy.
  • ItemOpen Access
    Planet formation and evolution in protoplanetary disc
    Scardoni, Chiara Eleonora
    In the last decades, thousands of exoplanets have been detected, revealing a variety of characteristics different from those of the Solar System's planets - the only planets orbiting a main sequence star known until 1995. This enhanced the interest in processes of planet formation and evolution that can help to explain the observed exoplanets' characteristics. This thesis focuses on the formation of planets via core accretion in protoplanetary discs - disc-shaped structures made of dust and gas that form around newborn stars- and in their subsequent evolution as a consequence of disc-planet interaction from a numerical and theoretical perspective. In the core accretion scenario, planets form in protoplanetary discs by growing the initial μm-sized dust grains up to the size of a planet. The aerodynamic interaction between the dust grains and the disc gas component, however, causes the grains to lose angular momentum and drift inwards; for cm-sized grains, the drift is so fast that they are expected to rapidly go towards the star, becoming unavailable to form planets. This problem is called the `radial drift barrier', and it can potentially be solved by the action of the streaming instability that causes the rapid formation of dust clumps that can later collapse under the action of self-gravity. In this thesis, we investigate the emission of systems undergoing streaming instability that we simulate through 2D local simulations using the hybrid code ATHENA. By comparing the simulated systems before and after particle clumping to the data from the Lupus star forming region in the optically thick fraction - spectral index plane, we find that the action of streaming instability drives the simulations towards the area of the plane occupied by the data. We further analyse the azimuthal brightness asymmetries produced when systems undergoing streaming instability are observed at an inclination angle. We demonstrate that the optically thick fraction exhibits a peak along the minor axis when the disc containing unresolved annular optically thick substructures is inclined and that, for favourable system parameters, these are likely observable by ALMA. Once a planet is formed, it is subject to mutual gravitational interaction with the host disc, which modifies both the disc structure and the planet's orbital parameters. The second part of this thesis focuses on the migration of massive planets in the planet-dominated regime of Type II migration. By performing long-timescale, live-planet simulations, we revisit previous results about the existence of a direct correlation between the rate of change of the semi-major axis and the torques acting on the planet. We find that such a correlation breaks for live-planet simulations when planet eccentricity is excited, but it is recovered by disentangling the contribution to the torque due to the semi-major axis evolution from that due to the eccentricity evolution. We develop a toy model based on the existence of that correlation. By applying this model to investigate the planet migration in viscously evolving discs, we show that the planets tend to migrate towards a precise location in the disc (`stalling radius'); this effect, combined with the evolution of the disc, causes the planets to distribute in a band around the stalling radius, estimated to be around 1-10 AU, disfavouring the idea of hot Jupiter formation through Type II migration in the planet-dominated regime.
  • ItemEmbargo
    CMB analysis with ACT and Planck
    Rosenberg, Erik; Rosenberg, Erik [0000-0003-3484-5645]
    The Cosmic Microwave Background (CMB) and especially its anisotropies have been a key source of information for cosmology and have played a major role in establishing the now-standard ΛCDM model. Upcoming experiments aim to extend CMB measurements even further, hoping to measure the sum of neutrino masses, search for primordial gravitational waves, further exploit secondary anisotropies, and constrain extended models more generally. In this thesis I present a study of the CMB temperature and polarization anisotropies, their power spectra, and cosmological parameter constraints using recent data from the Atacama Cosmology Telescope (ACT) and the Planck satellite. This includes extensive discussion of CMB analysis methods and the challenges in analysing these datasets individually and together, as well as presentation of new constraints on multiple cosmological models using these data. We begin by presenting new angular power spectra and cosmological parameter constraints derived from the Planck PR4 (NPIPE) maps of the microwave sky. We conduct extensive internal checks for systematic errors, and compare these results to previous Planck products and external data. We find excellent consistency between NPIPE and the Planck 2018 maps at the parameter level, showing that the Planck cosmology is robust to substantial changes in the mapmaking. The lower noise of NPIPE leads to ∼10% tighter constraints on cosmological parameters, and we see both smaller error bars and a shift towards the standard ΛCDM values for beyond-ΛCDM parameters including the curvature of the Universe ΩK and lensing amplitude AL. Next we continue to study Planck data, now in comparison with the ACT 2020 data release, DR4. Motivated by observed discrepancies between power and cross spectra from ACT DR4 and Planck 2018, particularly in the cross-correlation of temperature and E-mode polarization, we study challenges that may be encountered in the comparison of satellite and ground-based CMB data. In particular we focus here on the effects of Fourier-space filtering and masking involving bright point sources. We show that the filtering operation generates cross-shaped artefacts in the map that stretch far outside typical point source masks. If not corrected these artefacts can add bias or additional variance to cross-spectra, skewing results. However we find that the effect of this systematic is not large enough to explain the ACT-Planck differences presented with ACT DR4. Finally, we combine the ACT and Planck likelihoods to study pre-recombination extensions to ΛCDM affecting the primordial helium fraction YP and effective number of light particles Neff . We find that the small-scale ACT data can improve limits on these parameters by 10−20%, and that an ACT preference for low Neff leads to tighter upper limits on this parameter.
  • ItemOpen Access
    Coherence and state engineering of an optically active central spin system
    Zaporski, Leon
    This thesis aims to pave the way for the experimental study of many-body physics in a dense central spin system. It focuses on the development of both the highly coherent quantum-optical platform - a droplet-etched GaAs Quantum Dot (QD) - as well as the techniques to optically induce and probe the collective dynamics of its resident nuclear spin ensemble. An all-optical quantum control of the electron spin is realised for the first time in a GaAs QD and subsequently used to refocus the strong hyperfine interaction between the spin and the nuclear ensemble. The measurement demonstrates a nearly hundred-fold improvement of the electron spin coherence over the state of the art in the conventional InGaAs QDs. This is owed to the reduced inhomogeneity of the nuclear quadrupolar interaction, and it further raises the prospects of turning the nuclear ensemble into a coherent quantum register - a host to collective non-classical phenomena. To complement these results, I analyse a series of proof-of-concept experiments on initialising and addressing the nuclear quantum register in an InGaAs QD. These entail cooling and polarising the nuclear ensemble using strong electron-nuclear feedback, as well as driving the collective nuclear spin excitations via the electron-nuclear interaction. The asymmetry in the collective transition rates probed at a partial nuclear polarisation is used as an entanglement witness to demonstrate the formation of a nuclear dark state: a highly-entangled many-body state protected from being polarised by the nuclear wavefunction symmetry. The thesis ends with a detailed proposal for controlling the structure of such nuclear entanglement exclusively via the electron spin. Specifically, the way to phase-engineer a many-body singlet state of the ensemble is outlined. In the hope of guiding the next generations of physicists, the exposure of the core topics is aimed to be complete and pedagogical.
  • ItemOpen Access
    The Importance of Photoevaporation in the Evolution of Protoplanetary Discs
    Sellek, Andrew; Sellek, Andrew [0000-0003-0330-1506]
    Protoplanetary discs consist of gas and dust - the remnants of the star formation process - found around stars in the first few million years of their life. Photoevaporation, whereby high-energy radiation from the central star heats disc material causing it to flow away in a wind, is one process thought to contribute to their ultimate dispersal. Previous studies have failed to reach a consensus on the main radiation which is responsible for this process, variably finding the X-ray, Extreme Ultraviolet, or Far Ultraviolet. These paradigms make very different predictions for the amount of mass lost to the winds, and consequently how important they are for disc evolution. The primary aim of the thesis is to tackle this uncertainty from the following directions: a) by understanding the microphysical processes that underpin the differences in existing models in order to establish a comprehensive methodology for future state-of-the-art photoevaporation simulations that resolve the present disagreements; b) by considering how different wind models appear in observations of atomic forbidden emission lines and so how both line profiles and spatially resolved emission may be used to constrain the wind's nature; c) by including photoevaporation in models of disc evolution on secular timescales that predict its interplay with other processes - and how this manifests in disc demographic surveys - and thus determine how it contributes to the disc's ultimate dispersal. I conclude that while EUV-driven models have underestimated the role of X-ray due to a lack of detail in the spectrum, the X-ray driven models have underestimated the cooling from molecular emission lines. Thus; the true picture may be expected to be somewhat intermediate between the two extremes. Constraints from disc demographics require low enough rates that discs survive to the age of older star forming regions even around low-mass stars, and there is time for dust to deplete considerably before the wind disperses the gas. Conversely, ratios of emission lines require a high enough mass-loss rate to ensure the wind is only weakly ionised.
  • ItemOpen Access
    A matter of life and death: the formation and destruction of planetary bodies
    Brouwers, Marc
    Since the discovery of the first exoplanets in the 1990s, our knowledge of planets has expanded far beyond the Solar System, and surveys like Kepler and Tess have revealed a huge diversity of other worlds. In response to this new information, a novel field of planetary astronomy has sprung up to deal with the major questions, including: How do planets form out of proto-planetary discs? What are the bulk and atmospheric compositions of planets, and what are their building blocks? In this thesis, I contribute to the literature around both of these questions, by studying accretion processes across the lifetime of planetary bodies. My thesis is organized chronologically, starting with the birth of planetary building blocks, and ending with the destruction of fully-formed planets. Besides the shared topic of planetary astronomy, a second unifying theme in this thesis is the use of simple analytical methods to pursue novel research ideas. The first strand of my research (Chapter 2) deals with the formation of planetesimals - a plausible starting point for planet formation. I develop a new theory that relates the formation of these planetesimals to the spinning motion around their own axis. Specifically, I show that a general mechanism exists, whereby objects that gravitationally collapse next to an external potential naturally acquire spin angular momentum that is aligned with their orbital angular momentum (prograde). Planetesimals in the Solar System have a strong prograde bias, and prograde spin-up, therefore, provides new evidence for the popular hypothesis that they formed via gravitational collapse. The second strand of my research (Chapter 3) deals with the formation of the planets themselves, which likely grow via the accretion of large planetesimals, as well as smaller particles called pebbles. In this work, I study how the accretion of pebbles changes the opacity of planetary envelopes during their formation, which crucially determines how quickly accretion heat is lost, and how much hydrogen and helium the planet can bind. I show that relatively low opacities are predicted from this process, unless the pebble accretion rate crosses a certain threshold. The implication of this work is that the accretion of nebular gas during planet formation might be more efficient than previously thought, especially during periods of slow pebble accretion. The final strand of my research (Chapters 4, 5, and 6) takes us to the end of a planet’s lifetime, when its host star has left the main sequence and has shed its outer layers to become a white dwarf star. Many of these white dwarfs show metal absorption lines in their spectra, indicative of pollution with accreted planetary material. From the analysis of such spectra, the composition of exoplanetary material can be recovered. In this work, I explore how planetary material could have accreted onto these stars, and try to link this process to observable features, such as the accretion rate and infrared excess. I also explore the possibility that different components of a pollutant could accrete onto these stars asynchronously, over different periods of time, which is a crucial process to understand for the pollutant composition to be correctly interpreted based on the measured stellar abundances.
  • ItemOpen Access
    Spectral Energy Distribution Modelling of X-ray Selected AGNs and Their Host Galaxies
    Marshall, Adam
    The nature of the relation between active galactic nuclei (AGN), and their host galaxies have been observed in detail throughout the Universe. Such work has found an intrinsic link between central supermassive black hole (SMBH) masses, and host galaxy properties such as the velocity dispersion of stars, and bulge mass. However, the difference in scale between SMBH and their host galaxies has led to debate on how this relation might form, and develop over time. In order to aid in understanding the relation between AGN and their host galaxies, the work throughout this thesis has therefore focused on the development and implementation of a new SED fitting code, using an up-to-date AGN SED to accurately infer both AGN and host galaxy properties. To this end, we explore the intricacies involved in producing useful property inferences using a Bayesian MCMC fitting method, whilst working to avoid common issues such as bimodality and lack of convergence. We then perform SED fitting using our methods to 711 luminous X-ray AGN at 0.7 < z < 4.5 using 10-bands of optical and infra-red photometric data for objects within XMMSERVS. Using these fits, we study the relation between AGN X-ray luminosity and host galaxy stellar mass, along with our ability to predict emission line strength and morphology from photometry alone. In order to further understand the intricacies of SED fitting, we also provide a case study into the effect of AGN SED choice on host galaxy and AGN property inferences, by comparing our AGN SED to another commonly used template. In this work, we show that it is important to consider host galaxy contamination when trying to produce a pure AGN template, and the effect that this contamination can have on AGN and host galaxy property inferences. We also find that the use of lower resolution SEDs can lead to repercussions on property inferences such as host galaxy stellar mass, which may provide incorrect assumptions on the relation between AGN and their host galaxies.
  • ItemOpen Access
    Investigating the Characteristics of Exoplanetary Atmospheres and Interiors
    Nixon, Matthew; Nixon, Matthew [0000-0001-8236-5553]
    The characterisation of exoplanets has made rapid progress in recent years, with observations of bulk properties such as mass and radius combining with detailed atmospheric spectroscopy to provide unprecedented insight into the nature of these remote worlds. However, these high-quality observations also require sophisticated modelling and analysis tools in order to maximise the scientific output from the data. In this thesis I present a number of advances in atmospheric modelling and retrieval, as well as internal structure models, which have been used to investigate the properties of a wide range of planets, from hot Jupiters to temperate mini-Neptunes. I conduct an assessment of the feasibility of supervised machine learning as a tool to carry out atmospheric retrievals of exoplanets. Retrieval methods commonly conduct Bayesian parameter estimation and statistical inference using sampling algorithms such as Markov Chain Monte Carlo or Nested Sampling. Recently several attempts have been made to use machine learning algorithms either to complement or replace fully Bayesian methods in order to improve computational efficiency. However, results from these algorithms sometimes disagree with contemporary Bayesian retrievals. To investigate this, I use the Random Forest supervised machine learning algorithm which has been applied previously for atmospheric retrieval. I extend the machine learning approach to develop a new algorithm, and demonstrate excellent agreement with a Bayesian retrieval of the transmission spectrum of the hot Jupiter HD~209458b. Despite this success, and achieving high computational efficiency, I still find that this machine learning approach is computationally prohibitive for high-dimensional parameter spaces that are routinely explored with Bayesian retrievals with modest computational resources. I discuss the trade offs and potential avenues for the future. I present \textsc{Aura-3D}, a three-dimensional atmospheric retrieval framework for exoplanet transmission spectra. \textsc{Aura-3D} includes a forward model that enables rapid computation of transmission spectra in 3D geometry for a given atmospheric structure and can, therefore, be used for atmospheric retrievals as well as for computing spectra from General Circulation Models (GCMs). In order to efficiently explore the space of possible 3D temperature structures in retrievals, I develop a parametric 3D pressure-temperature profile which can accurately represent azimuthally-averaged temperature structures of a range of hot Jupiter GCMs. I apply this retrieval framework to simulated JWST observations of hot Jupiter transmission spectra, obtaining accurate estimates of the day-night temperature variation across the terminator as well as the abundances of chemical species. I demonstrate an example of a model hot Jupiter transmission spectrum for which a traditional 1D retrieval of JWST-quality data returns biased abundance estimates, whereas a retrieval including a day-night temperature gradient can accurately retrieve the true abundances. The forward model also has the capability to include inhomogeneous chemistry as well as variable clouds/hazes. This new retrieval framework opens the field to detailed multidimensional atmospheric characterisation using transmission spectra of exoplanets in the JWST era. I also present a new internal structure model for super-Earths and mini-Neptunes that enables detailed characterisation of a planet's water component. I use my model to determine how the bulk properties and surface conditions of a water world affect its ocean depth, finding that oceans can be up to hundreds of times deeper than on Earth. I explore the region of mass--radius space in which planets with H-rich envelopes could host liquid H$_2$O. Such envelopes could contribute significantly to the planet radius while retaining liquid water at the surface, highlighting the exciting potential for habitable conditions to be present on planets much larger than Earth. I contribute to internal structure models of a number of sub-Neptunes whose atmospheres are set to be observed using JWST. Before such observations take place, it is vitally important to understand the interior structures of these planets, which strongly affects their possible atmospheric compositions. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the habitable-zone mini-Neptune K2-18b, for which I contribute the H$_2$O EOS. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be $\lesssim$6\%; spanning $\lesssim$10$^{-5}$ for a predominantly water world to $\sim$6\% for a pure iron interior. The thermodynamic conditions at the surface of the H$_2$O layer range from the supercritical to liquid phases, with a range of solutions allowing for habitable conditions. We also investigate the possible compositions of the pair of planets orbiting the star TOI-776. The bulk densities of TOI-776b and c allow for a wide range of possible interior and atmospheric compositions. However, the models indicate that both planets must have retained a significant atmosphere. Upcoming observations will revolutionise our understanding of these planets, helping to uncover the mysteries of the sub-Neptune population. Finally, I discuss the latest developments in exoplanet observations, and consider how these advances may further our understanding of worlds beyond our own.
  • ItemOpen Access
    Probing Late Stages of Stellar Evolution with Gaia-selected Planetary Nebulae
    Chornay, Nicholas; Chornay, Nicholas [0000-0002-8767-3907]
    This thesis concerns the use of data from the Gaia satellite to study Galactic planetary nebulae. Gaia is a recent ESA mission measuring positions, parallaxes, proper motions, and photometry of nearly two billion stars in the Milky Way. Several thousand of these stars are passing through the planetary nebula phase, an important phase that low and intermediate mass stars experience towards the ends of their lives before becoming white dwarfs. The thesis has three main contributions. The first is the development and application of an automated technique for identifying the central stars of planetary nebulae in the Gaia catalogue. The second is a statistical method for identifying photometric variability in these central stars. Periodic photometric variability is often an indicator of binarity, which is now known to be key to the formation and shaping of a significant fraction of planetary nebulae. The method is validated through ground-based follow up observations. Finally, more precise distances from Gaia parallaxes are combined with newly obtained ground-based narrowband imagery to determine and investigate the planetary nebula luminosity function of the local Milky Way. Besides being an important extragalactic indicator, this distribution of nebula magnitudes is related to the evolution of planetary nebula central stars across different masses. Distances and photometry from Gaia allow the brightness of the nebula and the evolution of the central star to be related directly in unprecedented detail.
  • ItemEmbargo
    Characterising Exoplanet Atmospheres with High-Resolution Transmission Spectroscopy
    Langeveld, Adam; Langeveld, Adam [0000-0002-4451-1705]
    Over 5000 exoplanets have been discovered in the last three decades, revealing a truly diverse population of worlds beyond our solar system. The field has accelerated rapidly towards characterising the atmospheres of exoplanets around the nearest and brightest stars with large observational surveys from ground-based and space-based instruments. This thesis contains an overview of state-of-the-art methods for detecting chemical species in the atmospheres of transiting exoplanets. In the first chapter, I briefly review the history, key insights, and modern techniques for detecting exoplanets and studying their atmospheres. The subsequent chapters are each based on specific aspects of the analysis of high-resolution transmission spectra in optical wavelengths, aimed towards answering key questions regarding detrending techniques and the atmospheric properties of highly irradiated giant exoplanets. I begin by assessing the robustness of telluric correction methods for removing contaminating telluric water and oxygen lines from high-resolution spectra. This is a challenging and critical task when observing from the ground since absorption from Earth’s atmosphere often overwhelms the exoplanetary signals by several orders of magnitude. I compare the performance of corrections made using a telluric spectrum derived empirically from airmass and using a model of Earth’s transmission spectrum, focusing specifically on sodium detections in the transmission spectrum of a hot Jupiter. Next, I explore the diversity of ten highly irradiated giant exoplanets with equilibrium temperatures ranging from around 1000 to 4000 K. I conduct a homogeneous survey of sodium absorption using high-resolution transmission spectra extracted from observations made with the HARPS and HARPS-N spectrographs. Using sodium as a tracer, I report uniformly measured atmospheric properties across the sample, including atmospheric heights, net day-night wind speeds, and sodium doublet line ratios. I report a new detection of sodium in one of the planets and confirm previously reported detections in the others. I also discuss how different assumptions and detrending processes can cause discrepancies in the measured absorption features, highlighting the importance of a homogeneous analysis when comparing results across multiple planets. I then use these results to perform a search for new global trends on planetary atmospheric properties, with the goal of understanding how the characteristics of the atmospheres change over a diverse sample of planets. I investigate how the homogeneously measured atmospheric heights (as probed by detections of sodium) vary with macroscopic planetary properties, revealing an empirical relationship describing the relative atmospheric heights as a function of planetary equilibrium temperature and surface gravity. I also use the sodium detections to measure net atmospheric wind velocities for all ten planets and discuss how these results can provide information about common underlying processes and dynamics. Finally, I investigate signatures of other atomic species in optical high-resolution transmission spectra of two ultra-hot Jupiters using the cross-correlation technique. In particular, I search for prominent metallic species that are expected to be present due to the extreme temperatures of these exotic atmospheres. I compare the atmospheric heights derived from high-significance detections of neutral iron between the two targets. I also discuss tentative inferences of asymmetric iron absorption features, where the signals from the atmosphere become progressively more blueshifted in the first half of the transit up to a constant value. This asymmetry is indicative of different atmospheric structure or chemistry between the morning and evening terminators. I conclude with final remarks on promising avenues for future research over the course of the next decade, where high-resolution spectroscopy will play a pivotal role in furthering our understanding about the atmospheric properties and processes of the diverse population of exoplanets.
  • ItemOpen Access
    On the evolution of gaseous haloes in cosmological simulations of galaxy formation
    Bennett, Jake
    The study of gaseous haloes holds the key to understanding gas flows in and out of galaxies, which in turn determines how galaxies form and evolve. However, recent observations have revealed them to be very complex, with the presence of significant amounts of multiphase gas, driven by a range of physical process within galaxies as well as by the larger-scale environment. Simulations that realistically model this gas are therefore crucial to interpreting this new data, and ultimately helping us understand how galaxies assemble. However, many cosmological simulations sacrifice resolution in gaseous haloes to reduce their computational cost, meaning their ability to accurately resolve some of the key physical processes, such as turbulence and instabilities, can be limited. Using cosmological simulations, in this thesis I have investigated how gaseous haloes evolve over time. I have implemented a new shock refinement scheme to boost numerical resolution in gaseous haloes. Applied to a massive cluster progenitor, I found significant increases in cold gas mass in filaments and clumps, leading to more widespread star formation. In the hot halo there is a notable boost in turbulent velocities, leading to a doubling of non-thermal pressure support. With the FABLE suite, I studied how the hydrostatic mass bias and turbulent pressure support level in galaxy clusters change as they undergo major mergers and are host to powerful feedback episodes. I found that clusters rarely tend to be in hydrostatic equilibrium for long, with variations being driven by both merger activity and interestingly, at higher redshift, AGN feedback. Such insights could be important for future cluster mass estimates from X-ray and SZ observations, for use as cosmological probes. Finally, I simulated the growth of an ultramassive black hole of mass ∼ 10^10 M⊙ by z = 6. I have studied its impact on the gaseous halo around it, including how powerful AGN feedback can disrupt inflowing cold filaments and increase the thermal pressure of the surrounding gas. I found that strong AGN-driven outflows can deposit significant quantities of metals at very large distances from the central galaxy, which could be a unique signature of the presence of such ultramassive black holes in the very early Universe. Many of the effects predicted by the simulations described in this thesis have the potential to be observed (or ruled out) in the coming decade or two, with an incoming wealth of observational data from observatories like JWST, Athena, CMB-S4, SKA, and LISA. These data, combined with further developments in theory and simulation, will therefore revolutionise our understanding of gaseous haloes.
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    Observational Insights into White Dwarf Planetary Systems
    Rogers, Laura
    In recent decades the number of known planets has escalated from the eight solar system planets to over 5000 exoplanets; the focus has now shifted to their characterisation. Spectroscopic studies of white dwarfs ‘polluted’ by planetary material are a unique laboratory allowing measurements of their bulk composition. This thesis focuses on these polluted white dwarfs to observationally investigate how the material ultimately accretes onto the white dwarfs, and the inferences made about the composition of exoplanetary bodies. These white dwarfs become polluted due to the scattering of exoplanetary bodies on star grazing orbits where they tidally disrupt, producing dusty debris detectable as excess infrared emission, and then subsequently accrete onto the white dwarf. The scattering and accretion are expected to be stochastic processes with variability predicted on human time-scales. Chapter 2 reports near-infrared (JHK) monitoring campaigns of the dust emission with the UKIRT/WFCAM and NTT/SOFI. Over timescales of hours, days, months, and years no statistically significant variation is found. Chapter 3 reports spectroscopic monitoring campaigns of the metal features in the white dwarfs using SALT/HRS and Magellan/MIKE. Across more than 10 years and thousands of sinking timescales, no unambiguous statistically significant variability in the amount of material in the photospheres of the white dwarfs is found. Both results agree that the processes driving the accretion do so at a constant rate. Polluted white dwarfs with infrared emission tend to be the most heavily polluted systems. Chapter 4 presents the first composition studies from a novel programme which identifies new heavily polluted white dwarfs discovered from their infrared excess. The planetary material that polluted these seven white dwarfs are broadly consistent with rocky material, but show some compositional and geological diversity. Some of the white dwarfs appear to have accreted a fragment of a larger core-mantle differentiated body. Using oxygen budgeting, two white dwarfs are discovered to have accreted oxygen-rich material which may imply water-rich bodies. Also, evidence points towards one of the white dwarfs accreting material from two distinct planetary bodies, this challenges the commonly used assumption that there is one body in the white dwarf's atmosphere at once.
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    Hierarchical Bayesian Spectro-temporal Models of Type Ia Supernovae in the Optical and Near-Infrared: Understanding the Properties of Dust in Supernova Host Galaxies
    Thorp, Stephen
    With the Vera C. Rubin Observatory Legacy Survey of Space and Time, and the Nancy Grace Roman Space Telescope both on the horizon, Type Ia supernova (SN Ia) cosmology is about to undergo a paradigm shift. Whilst this new era has the potential to bring unprecedented constraints on the nature of dark energy, this will only be possible if we are able to overcome the considerable challenge of controlling the astrophysical sources of systematic uncertainty that will dominate the error budget of these future experiments. In this thesis, I develop and deploy a robust new hierarchical Bayesian framework for modelling the spectral energy distributions (SEDs) of SNe Ia in the optical and near-infrared (NIR). I apply this framework to detailed studies of the dust in SN Ia host galaxies – particularly the distribution of the dust law R_V. This model is able to leverage the optical and NIR in a principled manner, whilst coherently marginalising over all sources of uncertainty. I use this model to analyse the Pan-STARRS grid light curves of 157 low-redshift SNe Ia from the Foundation Supernova Survey. I use these data to robustly model the dust laws in SN Ia host galaxies, and to place constraints on the extent to which R_V differs between low- and high-mass hosts. I do not find statistically significant evidence for strongly different R_V between low- and high-mass hosts, and am able to place upper limits on the R_V population variance. My results indicate that observed correlations between SNe Ia and host-galaxy mass cannot be fully explained by dust. I follow this with an analysis of optical and NIR (B–H band; ≈3500–18000 Å) data of 86 SNe Ia from the Carnegie Supernova Project (CSP). I use this independent sample to uphold my previous results, place tighter constraints on the R_V population variance, and to investigate the dust laws for more-highly reddened SNe Ia (beyond the apparent B–V≤0.3 colour cut typically used in cosmological analyses). I also argue for the critical importance of combining optical and NIR data to break degeneracies when investigating the effect of dust on SNe Ia, and present a simulation-based demonstration of the challenges inherent in estimating R_V and its population distribution. I show that hierarchical Bayes is suited to overcoming these challenges. Finally, I analyse high-redshift data from the RAISIN (SN IA in the IR) Survey. These unique data include Hubble Space Telescope F125W and F160W observations for 37 SNe Ia with 0.22≤z≤0.61, providing the first sample of rest-frame NIR (YJ-band) observations at such high redshifts. I use these data to place constraints on R_V in the host galaxies of high-z SNe Ia. By combining the high-z RAISIN data with a low-z CSP companion sample, I am able to place limits on the possible redshift evolution of SN Ia host galaxy dust laws.
  • ItemOpen Access
    Cosmology from the CMB and Lyman-α forest
    de Belsunce, Roger; de Belsunce, Roger [0000-0003-3660-4028]
    In this thesis, we study challenges arising in cosmological data analysis using data from the cosmic microwave background (CMB), remnant radiation from the Big Bang, and the Lyman-α forest, absorption features in spectra of distant quasars. A six-parameter standard model of cosmology, the ΛCDM model, can explain to great accuracy how the Universe evolved from a hot, dense state to the web of galaxies that we observe today. However, it leaves fundamental questions about the nature of dark matter and dark energy unanswered, despite these making up 95% of the observable Universe. The advent of large cosmological surveys present a unique opportunity to infer some of the fundamental laws governing our Universe. Extracting the full potential of this data set is an ongoing challenge because of its size and highly non-linear nature. In the first part, we present an end-to-end analysis pipeline for large-angular scale CMB data. We present novel foreground removal techniques, improved modeling of the noise and systematics in the data, and develop and extensively test novel likelihood-approximations. The accurate representation of likelihoods including systematics is challenging: exact likelihoods are either unknown or intractable. We present methods that show how to make reliable inference for the optical depth to reionization (τ) or primordial gravitational waves, parametrised by the tensor-to-scalar ratio (r), from large-angular scale CMB data from the Planck satellite. The methods presented range from exact pixel-based likelihoods, maximum-entropy-based semi-analytic likelihood-approximations to simulation-based, so-called likelihood-free, approaches to constrain cosmological parameters. We exhaust current CMB data sets with the developed methods and discuss their potential for next-generation surveys of the CMB. The upcoming Dark Energy Spectroscopic Instrument (DESI) survey will measure spectra for tens of millions of galaxies and quasars, constructing a three-dimensional map spanning the nearby Universe to 11 billion light years. The Lyman-α forest consists of a series of absorption lines that map the distribution of neutral hydrogen in the intergalactic medium. This allows us to probe the matter distribution of the Universe at intermediate redshifts 2 ≤ z ≤ 5. In the second part of this thesis, we present and develop new continuum fitting methods to extract the un-absorbed flux of a quasar spectrum. We will discuss methods to constrain cosmology using this rich new data set.
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    Characterizing sub-mm observations of protoplanetary disks at super-resolution scales
    Jennings, Jeffrey
    Current advancements in telescope and instrumentation technology enable us to observe planets as astrophysical objects across epochs that span Gyr of evolution, from their formation in protoplanetary disks and continued growth in debris disks to their dynamical evolution in exoplanetary systems and ultimate accretion onto white dwarfs. Measurements in each of these eras can be used to inform study in the others. Interferometry affords the highest angular resolution of any observing technique in astronomy, and the use of radio interferometry with instruments such as the Atacama Large Millimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) is markedly advancing the protoplanetary disk field. By further improving our methods to reconstruct high fidelity (in terms of both resolution and sensitivity) images from the interferometric observable, we can not only characterize these disks, but detect the dynamical effects of planets within them. Over a large ensemble of sources, this offers the potential to both progress disk science and connect inferences on the embedded planet population to the study of these objects in later epochs. This thesis centers on a new imaging framework for radio interferometric observations and its specific application to detect and characterize substructures in protoplanetary disks. Chapter 1 introduces the basics of protoplanetary disk theory and observations, with a focus on the principles of radio interferometry and its application for disk science. Chapter 2 then presents Frankenstein (frank), the open source code we have developed and applied to fit sub-mm observations of disks in order to search for substructure. Chapter 3 applies frank to the high resolution (30 mas) DSHARP mm sample of 20 disks to identify new substructure in these sources and more accurately constrain known disk features. Major results include discovery of more structured inner disks (at separations within 30 au of the host star). In Chapter 4 we apply frank to the moderate resolution (120 mas) Taurus mm survey, finding that compact disks (those with radii <50 au) routinely exhibit substructure. Chapter 5 concludes by summarizing the frank algorithm and our novel scientific results with this tool, then places this evolving imaging approach in the context of future disk science.
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    The complex interplay between AGN jet feedback and galaxy evolution
    Talbot, Rosemary
    Supermassive black holes (SMBHs), residing in the nuclei of most galaxies, are capable of imparting extreme amounts of energy to their surroundings. Observations indicate the presence of collimated, relativistic jets as well as powerful, multiphase winds emanating from these active galactic nuclei (AGN). Whilst the clearest observational signatures of AGN jet feedback are found in galaxy clusters, there is growing evidence that these jets are, in fact, largely ubiquitous. Indeed, AGN jets have now been detected in a wide variety of galaxy contexts including dwarfs, Seyferts and ellipticals. Looking towards the future, observational facilities such as JWST, SKA and Athena will greatly expand our understanding of AGN physics by facilitating investigations of SMBH jet feedback at earlier times, lower luminosities and with higher spatial and spectral resolution. Additionally, the LISA mission will expand the field of multimessenger astronomy to the low-frequency range, detecting gravitational waves from the coalescence of SMBHs all the way back to cosmic dawn. Exploring the coupled evolution of AGN and their host galaxies is a highly non-linear, multiscale, problem and, thus, lends itself to exploration via numerical simulation techniques. For numerical simulations to provide firm theoretical predictions for, as well as accurately interpret the wealth of data from these next-generation facilities, however, it is vital that physically motivated models of AGN feedback are applied in realistic galaxy formation scenarios. In this thesis I present a novel numerical scheme to model SMBH spin-driven jet feedback in galaxy formation simulations. I then apply this model, first to simulations of isolated Seyfert galaxies and then to simulations of the major merger of gas-rich galaxies at cosmic noon, exploring how AGN jet feedback affects the properties and evolution of the host galaxies and the SMBHs themselves. In Chapter 1, I provide an overview of the relevant scientific concepts and historical events that set the scene for the work which is presented in the subsequent chapters. In Chapter 2, I introduce my model for black hole accretion through a (warped) α-disc and feedback in the form of a Blandford-Znajek jet and describe its numerical implementation. In the remainder of Chapter 2, I perform benchmarking of the model and explore the effects of AGN jet feedback on the central regions of a typical radio-loud Seyfert galaxy. Chapter 3 follows on directly from Chapter 2 and significantly expands the simulation suite presented therein. Using these simulations, I explore how varying the initial black hole spin magnitude and direction, the density of the medium into which the jet propagates and the rate at which gas is funnelled towards the black hole affect the evolution and properties of the jet-driven outflows. Then, in Chapter 4, I apply my spin-driven AGN jet model to black holes at the centres of gas-rich galaxies undergoing a major merger at z ≈ 2. With these simulations, I explore how the AGN jets affect the stellar component, the gaseous haloes of the galaxies and the dynamics of the merger. Additionally, I examine how the AGN jets self-regulate when subject to the extreme environments present in such mergers. Finally, in Chapter 5, I bring together the main conclusions from these three studies, before discussing some future prospects for work in this field.
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    Automating Searches for Gravitationally Lensed AGN in Wide-Field Surveys
    Desira, Christopher
    Gravitational lensing is a rare phenomenon which requires the exact alignment between a distant source and a massive foreground galaxy. The light from the source is bent around the galaxy forming spectacular rings, arcs and multiple images which have captured the public imagination since their discovery in 1979. This thesis focuses on searches for lensed systems where the background source is an active galactic nucleus (AGN) - a galaxy powered by their central supermassive black hole (SMBH). Gravitational lensing of the most distant and luminous subclass of AGN, known as quasars, provide a unique way to study the universe. They have been used to constrain the Hubble constant (H0), probe the dark matter content of galaxies and uncover the proposed co-evolution between SMBHs and their hosts. Until recently, these studies have been hindered by low sample sizes. However, increased research into automated approaches such as machine learning, combined with a wealth of data from new wide-field surveys, will enable us to dramatically increase the lensed quasar population. In this thesis we develop a number of searches leveraging classic colour selection as well as supervised machine learning techniques. We target quadruply imaged and high-redshift lensed quasars using photometric data from DES, WISE and Pan-STARRS, together with Gaia astrometry. Candidates were confirmed with follow-up spectroscopy, resulting in the discovery of several new systems. A search for radio-loud AGN was conducted using newly released VLASS data and candidates were prepared for high spatial resolution radio confirmation. We discuss the serendipitous discovery of an exciting lensed radio-loud galaxy with extreme high ionisation UV line emission and damped Lyα absorption. We finally outline a deep learning approach to automate the visual inspection stage of our lens searches in preparation for the release of new surveys such as LSST and Euclid.