Theses - Institute of Astronomy

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Open Access
New insights into primeval galaxies in the era of JWST
Witten, Callum; Witten, Callum [0000-0002-1369-6452]
The properties of galaxies that existed in the early Universe hold the key to understanding not only their formation and evolution but also the broader reionisation process. While the Hubble Space Telescope (HST) was able to identify a small sample of candidate z>7 galaxies, the James Webb Space Telescope (JWST) has revolutionised the field. The sensitivity of the near infrared camera (NIRCam) and spectrograph (NIRSpec) instruments have facilitated the detection and spectroscopic confirmation of galaxies out to z∼14. Moreover, the study of the properties of not just individual, but samples of high-redshift galaxies is now possible. As astronomers utilise the capabilities of this telescope multiple key questions, left over from the HST-era, require answering. Here I present the results of several studies that aim to answer some of these important unanswered questions. I utilise ground-based observations of the intrinsically strongest hydrogen emission line, Lyman-α (Lyα), to constrain the escape fraction of UV-bright galaxies in the early Universe. This shows that UV-bright galaxies appear to have low escape fractions of ionising photons, setting the scene for JWST studies of the ionising capabilities of different galaxy populations. I then use NIRCam observations of Lyα-emitting galaxies to explain the escape of Lyα photons from galaxies by showing that all high-redshift Lyα emitters (LAEs) have close companions, in contrast to typical high-redshift galaxies. The combination of observations and simulations, in this work, shows how interactions between close companion galaxies can both enhance Lyα emission, but also facilitate its escape. The following work I present focuses on observations of one of the highest redshift LAEs with the integral field unit on JWST. These observations reveal both a Lyα halo around this galaxy as well as an abundance of candidate UV-faint emission line galaxies, evidencing the complex escape of Lyα but also the environments within which high-redshift galaxies exist. Following this, I identify a galaxy that shows both strong emission lines and a Balmer break indicating the presence of both a young and old stellar population, and hence a rejuvenating star-formation history (SFH), revealing the complex SFHs of early galaxies. Finally, I discuss these results and future avenues for this work, especially in the context of next generation instruments and simulations. I also point the reader to some additional research conducted as part of my PhD, but not included in this thesis, in Witten et al. 2022. This work focuses on the low-resolution spectrographs on-board the Gaia satellite. This work, completed before the release of Gaia data release 3, addresses the potential information available in these low-resolution spectra to measure the temperature, surface gravity and metallicity of observed stars. I also show that these spectra can be used to identify candidate carbon-enriched metal-poor stars for follow-up high-resolution observations.
Open Access
Magnetic Field Evolution in Black Hole Accretion Disks and Relativistic Jets
Rodman, Payton; Rodman, Payton [0000-0002-1624-9359]
Supermassive black holes (SMBHs) catalyse change in their host galaxies, mediated through the inflow and subsequent outflow of material through the massive body. While an SMBH does not—indeed, cannot—hold its own magnetic field, the material it interacts with, feeds upon, and expels outwards is naturally magnetic, and this embedded magnetic field is expected to have a complex effect upon these processes. Due to the inherent difficulties in studying magnetic fields there is still much to learn about their role around SMBHs generally, and in accretion and the launching of relativistic jets in particular. This thesis aims to tackle both topics using three-dimensional magnetohydrodynamics (MHD) and relativistic magnetohydrodynamics (RMHD) simulations. In the first half (Chapters 2 and 3), I focus on the role of magnetic fields in simulations of thick accretion disks. At stake is a core question within the field of disk simulations: can a simulated accretion disk self-generate the strong, large-scale poloidal field needed to launch relativistic jets? To assess this question, I simulate disks with initially toroidal fields of both high (plasma beta β = P_gas /P_mag = 5) and low (β = 200) field strength and at high and moderate resolution, using the code Athena++. I find that both weak and strong initially toroidal field disks can generate a poloidal field, with the initially strong case producing a more ordered and large-scale field. These findings are tempered by caution on the resolution, as an m = 1 mode overdensity is created at the crossroads of a weak field, moderate resolution, and resolution step-change. I additionally perform a minor study of the disk dynamo, covered in Chapter 3. Using mean-field dynamo theory, I calculate the diagonal, symmetric α_ij components under both a global azimuthal average and a local Gaussian filter average, finding that local averaging returns more robust numerical signals. We retrieve a northern hemisphere α_φφ sign that is consistent with previous works and is in line with expectations of an αΩ-type dynamo. The mean-field formalism returns a better fit later in the disk evolution, suggesting that additional dynamo factors are present in the early phases of field evolution and growth. In the second half of the thesis, in Chapter 4, I move my focus to relativistic jets and investigate whether magnetised head-tail (HT) jets launched in a realistic turbulent magnetised background can form magnetic linkages between the lobes as have been observed in recent high-resolution radio images. To assess this, I simulate a suite of low density (density ratio η = ρ_jet /ρ_amb = 10^−3 ), pressure-matched jets at both low (v_j = 0.24c) and high (0.94c) velocity into pre-evolved turbulent backgrounds generated using the PLUTO code and subject them to strong, transonic crosswinds. I find that magnetic linkages do occur, but are between the jet lobes and the ambient medium, with the region between lobes being too turbulent to support long filaments. I additionally find that the location and direction of the existing filaments depend on the particular turbulent background realisation, implying that observations of filaments could be used to constrain the ICM field.
Open Access
Formation and dynamics of the Galactic disc and halo
Dillamore, Adam; Dillamore, Adam [0000-0003-0807-5261]
The treasure trove of recent data from Gaia and spectroscopic surveys has revealed an abundance of features in the Milky Way that must be interpreted dynamically. The disc, the bar, the halo and the satellites are all components of the Galaxy's dynamical system which are able to interact to create, perturb and destroy substructure. By using tailored and cosmological simulations, this substructure can be modelled and interpreted to learn about the formation and past evolution of the Milky Way. A detailed understanding of our own Galaxy's assembly also provides a test for the cosmological models implemented in hydrodynamic simulations of Milky Way analogues. In Chapter 1 I give an overview of the principles of Galactic dynamics which form the foundation of this thesis. I proceed to outline past research into Galactic archaeology in the Milky Way, including studies into its substructure, evolution and accretion history. Chapter 2 is a study of the dynamical effects of the Sagittarius Dwarf Galaxy (Sgr) on stellar streams produced by tidally disrupting globular clusters, with a focus on the GD-1 stream. This reveals a plethora of perturbations that may result from interactions with a massive Sgr, including asymmetry and folding of the tails. In Chapter 3 I run simulations of a stellar halo-like population of particles in the presence of a rotating bar. I show that the bar is capable of trapping stars in its resonances from highly eccentric and inclined orbits. These form overdensities in phase space which closely resemble similar features seen in data from Gaia. Chapter 4 continues to investigate the trapping of halo orbits in resonances with the bar. I develop an analytic toy model of resonant orbits to predict the appearances of action space (J_φ, J_r) and radial phase space (r, v_r). Both this model and test particle simulations successfully predict the appearance of Gaia data in these spaces, in particular the asymmetry in Galactocentric radial velocity v_r. Chapter 5 is a study of globular clusters trapped in resonances with the bar. I select 10 whose orbits are likely to be significantly altered by a bar with a slowing pattern speed, most of which are trapped by the corotation resonance. I show that a slowing bar is capable of transporting these clusters to higher energies and radii, indicating that it may have reshaped the Galaxy's globular cluster system. In Chapter 6 I use the ARTEMIS suite of cosmological simulations to investigate the angular momenta of dark matter haloes of Milky Way analogues. I find a correlation between the spin of a halo and its accretion history. Haloes with a highly radial accreted stellar component (like Gaia Sausage-Enceladus in the MW) tend to have lower spin than those with more isotropic accreted stellar velocity distributions. Chapter 7 uses the ARTEMIS simulations to study the formation (spin-up) of the Milky Way's disc. Most simulated galaxies spin up at higher metallicity than the Milky Way, suggesting our galaxy formed its disc unusually early. Galaxies with an analogue of Gaia Sausage-Enceladus spin up earlier than those without on average. The early spin-up galaxies tend to have both greater virial masses at early times and smaller present-day fractions of accreted stars.
Open Access
Connecting models and observations of disc evolution and planet formation
Zagaria, Francesco; Zagaria, Francesco [0000-0001-6417-7380]
Planets are assembled in and inherit their properties and composition from gas- and dust-rich discs orbiting young stars. Over the last decade, the Atacama Large Millimeter/submillimeter Array revolutionised our understanding of planet formation, opening up the possibility of determining the physical-chemical properties of protoplanetary discs and their secular evolution. Both are essential ingredients to understand the disc's potential to form planets and interpret the enormous diversity of features among the fully-formed exoplanets around main-sequence stars observed today. The first part of my thesis deals with disc evolution and aims to assess what mechanism, turbulent angular momentum transport or angular momentum removal by magneto-hydrodynamic winds, drives disc evolution. This is a long-standing issue with crucial consequences for many aspects of planet formation, from dust growth, pebble and gas accretion on planetary embryos, to planet migration. Forward modelling the available data, I showed that, although dust disc fluxes and sizes from ALMA snapshot surveys in nearby star-formation regions, and mass accretion rates from complementary VLT/X-Shooter spectroscopic surveys, cannot tell these two evolutionary scenarios apart, CO fluxes are both easy to measure in large disc samples with ALMA and a promising tracer of disc evolution. I also investigated the impact of stellar multiplicity on disc evolution, showing that, other than making discs fainter and smaller, it affects the correlation between disc dust masses and mass accretion rates, substantially reducing the disc lifetime. The second part of my thesis deals with combining ALMA and VLA broadband multi-frequency continuum observations to determine the temperature, size, density, and potentially internal properties, such as porosity and composition, of the large grains in the disc mid-plane. In the case of CI Tau, where high angular resolution observations are available, I showed that mm-sized compact amorphous carbonaceous grains are present in the system, and constrained radial variations of the density and size of dust on the scale of disc substructures. Instead, for the moderate-resolution observations of tens of discs in the Lupus star-formation region, my analysis provided global constraints on dust properties and the impact of dust self-scattering on continuum emission. Finally, in the case of HD 163296, one of the closest and best studied discs, I combined literature estimates of dust properties and the amount of dust trapping in the disc substructures, to show that favourable conditions to kickstart the formation of planetesimal under streaming instability are met in one of the rings.
Open Access
The Structure of Extrasolar Planetesimal Belts in Images
Han, Yinuo
Solid bodies in the form of planets and planetesimal belts familiar to us in the Solar System are also a prevalent feature around other stars. Their great diversity highlights a lack of complete theories for how they formed, which is further complicated by observational challenges to understand their full architecture in the first place. Currently, our best observational constraints on the outer planetary system come from imaging the planetesimal belts that population this region, known as debris disks. As planets dynamically interact with the disk throughout their formation and evolution, structures reflecting their history are imprinted on the disk. In this thesis, I develop systematic and unbiased methods to recover the three-dimensional structure of debris disks from their images applicable to a versatile range of imaging modes. By removing assumptions on the functional form of the disk structure, this method provides unbiased constraints on disk substructures with realistic uncertainties that enable us to interpret dynamical interactions shaping the planetary system. I apply this method to a sample of debris disks with resolved imaging to infer general three-dimensional structures of debris disks and their implications for planetary system formation. For the archetypal edge-on debris disk of $\beta$ Pictoris for which multiple epochs of high-resolution mid-infrared and millimetre images are available, I model in detail its vertical structure for different grain sizes and any variations in azimuthal substructures across time, constraining dynamical scenarios that could be shaping the evolution of the system. The approaches developed in this work to model debris disk structures from images will be important for interpreting upcoming observations with ALMA, JWST and the next generation of observatories at high sensitivity and resolution.
Embargo
Tracing the horizons of habitability with Venus and warm rocky exoplanets
Jordan, Sean
One of the primary objectives of astronomy and planetary science is the search for life beyond Earth. This search is conducted on two frontiers: one via exploration of extreme environments within the Solar System; the other via remote sensing of exoplanets that orbit distant stars. Venus, our nearest planetary neighbour in the Solar System, holds unique significance for the limits of habitability on both of these frontiers: first, Venus represents the paradigm of an Earth-like planet that either did not form or did not hold onto surface conditions habitable to known terrestrial life; second, the extreme environment of Venus's cloud layer may potentially be inhabited today by more exotic forms of life. For both of these reasons, Venus holds profound consequences for the horizons of habitability on exoplanets. In this thesis, I examine the horizon of extreme habitability in Venus’s clouds, and the horizon of surface hostility and the death of habitability that Venus epitomises for the wider population of rocky exoplanets. I begin by reviewing the history of Venus exploration, the current status of the Venus life hypothesis, and our present knowledge of Venus’s atmospheric chemistry and climate history. I explore how we can trace the possibility of extreme habitability in Venus’s cloud layer by modelling the metabolic signatures of an aerial biosphere that, if present, would be imprinted in the atmospheric chemistry. I then explore how we can trace the limit of surface habitability, using Venus as the paradigm of a warm rocky exoplanet. I investigate how the atmospheres of Venus-like exoplanets respond to irradiation by cool host stars and identify unique observational indicators that can break the degeneracy between Venus-like and Earth-like paradigms. Finally, placing these results into their astronomical context, I develop a self-consistent atmospheric model to determine how we can trace the inner edge of the liquid-water habitable zone across the known population of warm rocky exoplanets with current and future telescopes.
Open Access
Modeling Astrophysical and Large-Scale Structure Signatures in Axion Cosmologies
Döme, Tibor; Döme, Tibor [0000-0003-2586-3702]
The quest to understand the fundamental nature of dark matter (DM) remains a challenge in contemporary physics. This thesis aims at elucidating astrophysical and cosmological imprints of fuzzy dark matter (FDM), a promising class of models which find ample motivation within theories of particle physics. Notably, the string axiverse hypothesis posits the existence of a multitude of pseudoscalar particles called axions with logarithmically distributed masses, potentially addressing both the DM and dark energy problems simultaneously. Combining theoretical analyses with numerical simulations, we unravel the distinct signatures of FDM across intergalactic and group/cluster scales. We begin by studying density, shape and weak lensing statistics in FDM cosmologies, finding strong departures from the universality of density profiles and the monotonicity of shape profiles observed in the vanilla cold dark matter (CDM) model. Our first-ever analysis of geometric and intrinsic alignments in FDM cosmologies reveals increased alignment strengths compared to CDM, holding particular relevance for weak lensing surveys like Euclid, and we embed our findings into a wider weak lensing analysis to predict convergence and ellipticity spectra. By publishing the COSMICPROFILES Python package we enhance reproducibility and accessibility of our results. To reveal the morphology of the cosmic web, we then apply an advanced segmentation algorithm (NEXUS+). We report high mean densities of filaments, sheets and voids in FDM compared to CDM, while cosmic skewness estimates also increase, which could be used as a testbed for constraining FDM. Moving to astrophysical implications, we deploy generative machine learning models (normalizing flows) to characterize neutral hydrogen distributions in post-reionization FDM model Universes. Our findings indicate that extreme FDM models can be ruled out based solely on their low neutral hydrogen (HI) abundance. We quantify the HI and damped Lyman-𝛼 (DLA) biases, and show that many small-halos in FDM models have high DLA cross-sections, which we trace to the high column density of cosmic filaments. We construct mock radio maps for the Square Kilometre Array and present prospects for imaging the brightest HI peaks. By interpolating the latent space of axion masses, normalizing flows predict HI distributions even for synthetic FDM cosmologies. Addressing the degeneracy between baryonic physics and FDM physics, we also aim to advance our understanding of baryonic feedback and the star-formation histories of high-redshift galaxies. We identify instances of (mini-)quenching in several state-of-the-art galaxy formation models and implement advanced spectral energy distribution models to match spectrophotometric data from the James Webb Space Telescope, suggesting higher levels of burstiness in some observed galaxies compared to simulated galaxies. Central to our work is the development of a robust simulation framework tailored to bona fide FDM. We find that not only do common approximations for FDM lead to unreliable estimates of key observables, but by pairing the gravity solver with the ILLUSTRISTNG galaxy formation module, we find that baryonic processes (adiabatic contraction and feedback) alter the structure of ground-state eigenmodes in the centers of halos called solitons. In particular, solitons dynamically relax into modified eigenmodes and experience baryon-induced growth. Finally, we extend our simulation framework to accommodate for mixed dark matter (MDM), where FDM constitutes only a fraction of the total DM content, in agreement with recent observational constraints. We present non-linear power spectra and halo mass functions in selected MDM cosmologies for the first time, concluding our advancements in FDM/MDM modeling.
Open Access
Constraining reionization: Evidence from 21 cm limits and predictions for fast radio bursts
Heimersheim, Stefan; Heimersheim, Stefan [0000-0001-9631-4212]
In this thesis, I explore multiple constraints on the properties of early galaxies, the reionization history, and the global 21 cm signal. Specifically, I use upper limits on the 21 cm power spectrum measured by the HERA interferometer, current and future measurements of the global 21 cm signal, and forecasts for high redshift Fast Radio Bursts (FRBs). Firstly, I examine the influence of cosmic reionization on FRBs. They are recently discovered extra-galactic sources of strong radio signals, and the dispersion measure of these signals is sensitive to the ionization state of the intergalactic medium. This analysis has previously only been done for specific reionization models; I propose using a model-independent parameterization of reionization. I employ synthetic data of future FRB measurements at high redshifts z>5 to show that (i) the model-independent method removes a significant bias in the inferred optical depth, and (ii) that the observation of high-z FRBs can facilitate direct and model-independent measurements of the reionization history and associated cosmological parameters. Secondly, I use Bayesian methods for a model-independent parameterization of the sky-averaged 21 cm signal. One of the biggest challenges in that field is identifying the cosmological signal among other systematic contributions and foregrounds. In my work, I compare two model-independent methods to fit the 21 cm signal and to separate out the foregrounds: (i) a Gaussian Process modelling the foreground-orthogonal component of the data, and (ii) a spline-based FlexKnot interpolation utilising Bayesian evidence to find the simplest signal (agnostic of its cosmological or systematic nature) that fits the data. I apply these methods to both, a synthetic validation data set and the EDGES observations. I find that both methods fully recover the foreground-orthogonal component of the signal and that the FlexKnot method is able to separate the signal from the foreground in the synthetic data. Using this novel analysis I discover a set of four different shapes that can explain the EDGES observations, only one of which resembles the originally reported absorption signal. Finally, I derive constraints on the astrophysical properties of early galaxies using 21 cm power spectrum observations from the HERA telescope. I derive a likelihood function to compare the data with cosmological models, develop a neural network emulator to speed up the computation of those cosmological models and analyze the measurements of two HERA data releases. I derive constraints on astrophysical parameters based on semi-numerical models, in particular focusing on models with non-standard radio backgrounds. The main constraint I find is that early galaxies cannot simultaneously produce low X-ray and high radio emissions, as such scenarios would produce a signal larger than the upper limits set by HERA.
Open Access
Unveiling fundamental physics with high-resolution X-ray spectroscopy of Active Galactic Nuclei.
Sisk Reynes, Julia
This doctoral thesis explores the use of high-resolution spectroscopy of active galactic nuclei (AGNs) to probe fundamental physics. It focuses on the study of axion-like particles (ALPs) and the spin of supermassive black holes (SMBHs). Chapter 1 starts with a review of black holes as mathematical and astronomical objects and provides an account of the knowledge of the physics of the accretion flow onto SMBHs in AGNs as revealed by X-ray observations of AGNs. This is followed with a discussion on the use of X-ray reflection spectroscopy as a probe of the spin of moderately accreting SMBHs, and the use of spin diagnostics to probe the growth of SMBHs over cosmic timescales. Summaries of the Standard Models of cosmology (or the Lambda CDM paradigm) and particle physics are provided. Evidence of the need for physics beyond the Standard Model (BSM) is presented. This introductory chapter concludes by highlighting the role of ALPs as generic predictions in BSM theories and as compelling dark matter candidates and is accompanied by a description of plausible techniques towards their detectability with astronomical sources. Chapter 2 begins our discussion on astrophysical ALP searches by presenting the tightest bounds to date on the coupling of light ALPs to electromagnetism based on a spectral analysis of high-resolution archival *Chandra*/Grating observations of the luminous cluster-hosted quasar H 1821+643. Chapter 3 provides an exploration of how the next-generation *Athena* X-ray flagship observatory will improve on the current most sensitive limits presented in chapter 2. A promising technique to mitigate the effect of previously ignored systematic uncertainties is discussed. ALP projections from the AXIS probe-class concept proposed to NASA for a 2032 launch are also introduced. The future of ALP searches with upcoming missions is encouraging due to advances in detector technology. These advances include improvements in effective area, spatial resolution, and spectral resolution when compared with current observatories. In future, probing light ALPs with observations of bright AGNs located at the centres of rich clusters may be the only plausible observational test of string theories and will complement the search for ALP dark matter at light ALP masses. Chapter 4 presents the application of state-of-the-art X-ray reflection models on the *Chandra* spectral view of H 1821+643 introduced in chapter 2, pointing out that its colossal, central SMBH is rotating at moderate speeds. This chapter concludes by presenting the observed population of SMBHs whose spin has been estimated from such models. The observed population seems to feature two subpopulations: a population of low-mass SMBHs with maximal-to-extreme spins and a high-mass population of SMBHs whose spins cluster at moderate values. This notion is aligned with the predictions of semi-analytic and numerical models of hierarchical structure formation and black hole evolution over cosmic timescales. Therefore, assessing this hypothesis with Bayesian statistics may eventually help confirm what drives SMBH growth over cosmic timescales and help distinguish between the relative importance of growth powered by coherent and incoherent accretion and SMBH-SMBH mergers. Chapter 5 presents closing remarks and outlines possible future research directions.
Open Access
Exploring the Variability of Accretion Discs with Stochastic Models
Turner, Samuel; Turner, Samuel [0000-0002-8641-7231]
Across a large range of scales, accreting sources show a remarkable similarity in their observed variability. The theory of propagating fluctuations, in which stochastic fluctuations in the viscosity create corresponding fluctuations in the accretion rate which then propagate through the disc, is often used to explain these characteristic features in the variability. While this theory has been extensively explored analytically, there has thus far been little numerical work investigating the non-linear behaviour. In this thesis, I develop new numerical models for simulating variable accretion discs, and explore what the resultant simulations reveal about the physical origin of the variability. Firstly, I perform the most detailed 1D simulations of stochastic accretion discs, taking the analytic theory of propagating firmly into the non-linear regime. I find that the eponymous propagating fluctuations are present across a wide range of model parameters and that the model naturally produces variability with many of the same features as that observed in nature. Notably, the timescale on which the stochastic fluctuations in the viscosity occur is found to be closely related to the break frequency in the luminosity power spectral density (PSD), providing a possible observational probe for the magnetorotational instability (MRI). I then present a new 2D (vertically integrated) stochastic model for simulating variable accretion discs, generalising the previous work in 1D. This model has the potential to bridge the gap between constant viscosity models and those with full 3D magnetohydrodynamics (MHD), allowing for discs with realistic variability to be simulated with much less computational expensive. To explore the theory of propagating fluctuations in 2D, I then use this new model to run a series of simulations with varying model parameters. While many of the results from 1D translate well to 2D, I find that the presence of radial epicycles in 2D has an important effect on the local disc dynamics and thus on the local accretion rate in the disc. Further, there are suggestions that the expected log-normality of the observed luminosity (and the associated linear root mean square (rms)-flux relation) break down for sufficiently thin discs. Finally, I explore the classical thermal instability for radiation pressure dominated accretion discs by running 2D (vertically integrated) simulations of viscous discs. With these, I am able to explore the long duration behaviour of these systems, revealing interesting limit-cycle behaviour. I also apply the new 2D stochastic model, investigating the effect of stochastic viscosity on the nature of these outbursts.
Open Access
The Early Evolution of Terrestrial Planets: Impact Simulations and Planetary Chemistry
Itcovitz, Jonathan; Itcovitz, Jonathan [0000-0003-2079-8171]
Large impacts early in the lifetime of a terrestrial planet can transform its surface environment and interior composition, with the potential to forever change the evolutionary pathways available to that world. The chemically reduced metallic iron cores of these large impactors are key to such processes, as are the vast sums of energy that such impacts deliver. In this thesis, we explore the environments of such scenarios in the aftermath of impacts, modelling the chemical state of the interacting atmosphere and impact- generated silicate melt. We analyse the production of reduced atmospheric species (e.g., H₂, CH₄, NH₃), which are important to proposed prebiotic chemical pathways. We also analyse the geochemical signatures that such impacts leave in the planet’s mantle, namely in the form of highly-siderophile elements (HSEs), which are commonly cited as evidence for such impacts having occurred on Earth in the time after the Moon-forming impact. We find that the nature of how the impactor core accretes to the impact-generated melt (i.e., as large chemically inaccessible blobs, or as more accessible droplets) is an important but as of yet unanswered question in determining the consequences of these impacts. To answer this question on the fate of impactor core material, we perform impact simulations using the shock physics code, iSALE, with the primary purpose of determining where impactor metal accretes to within the planet, or indeed escapes the planet altogether. The inclusion of material strength is vital in the ability of these simulations to characterise the accretion process in ways that previous methods have been unable to. From these simulations, we formulate parametrisations of impactor core accretion as functions of impact parameters (mass, velocity, angle), advancing the limited such information currently available in the field for this regime of impacts. We demonstrate that impacts can be divided into two accretion modes: those that can generate a melt column down to the planet core, and those that cannot, and that the consequences of each impact mode are importantly distinct. We thus show that previous estimates of HSE retention in the postimpact mantle are overestimates, with greater mass fractions of the impactor core merging with the planet core than previously thought. Finally, we combine our simulations with experimentally derived laws governing the interaction between molten metal of the impactor core and the silicate melt through which it sinks. We find that such interactions are inefficient at depositing impactor metal into the planet’s mantle, leaving the rainout of vaporised impactor core material onto the planet surface as the predominant pathway for the retention of such material by the planet mantle. The liquid fragmentation of sinking molten metal, followed by the dissolution of the generated droplets, is an additional mechanism, but with a more minor role played. Therefore, in the absence of alternative proposed mechanisms for retention of impactor core material, we call into question the commonly cited evidence that the present-day abundances of HSEs in Earth’s mantle are geochemical proof of particular masses of large impactors on early Earth, thus challenging the currently held view of Earth’s Late Accretion, with implications for the evolution of the Solar System as a whole.
Open 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.
Open 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.
Open 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.
Open 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.
Open 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.
Open Access
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 Ω_K and lensing amplitude A_L. 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 Y_P and effective number of light particles N_eff . We find that the small-scale ACT data can improve limits on these parameters by 10−20%, and that an ACT preference for low N_eff leads to tighter upper limits on this parameter.
Open 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.
Open 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.
Open 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.