7,303 research outputs found

    Hidden in the dark:Seeking the vanished polycylic aromatic hydrocarbons in planet-forming discs

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    The origin of life is closely linked to the formation of planetary systems, and both are fundamental drivers of modern astronomical research. Especially carbon is of interest as it is the building block of life as we know.In the interstellar medium, about 15 % of carbon is locked in the form of polycyclic aromatic hydrocarbons (PAHs). The infrared signals of these complex molecules have been observed in numerous astrophysical environments. Their detection in planet-forming discs is of particular interest, as these are the birth-sites of exoplanets. By understanding the evolution of PAHs during planet formation, it is possible to trace a large fraction of carbon. Additionally, the signals of PAHs can reveal crucial information about planet-forming discs themselves to better understand planet formation.This thesis particularly focuses on the formation of molecular clusters of PAHs bound by van der Walls forces in planet forming discs. We analysed the stability of PAH clusters against stellar UV radiation from young stars and modelled their dissociation rates. Further, we model the evolution of clusters in the presence of dust grains, as they interact through freeze-out. Then, we investigate the depletion of observable gas-phase PAHs which has been observed in many discs. Next, we simulate observations and discuss the amount of retrievable information from spectra. Finally, we investigate the interaction of PAHs with stellar X-rays from T Tauri discs and their influence on the destruction of PAHs and PAH clusters

    The Gemini Planet Imager Exoplanet Survey : giant planet and brown dwarf demographics from 10 to 100 au

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    We present a statistical analysis of the first 300 stars observed by the Gemini Planet Imager Exoplanet Survey. This subsample includes six detected planets and three brown dwarfs; from these detections and our contrast curves we infer the underlying distributions of substellar companions with respect to their mass, semimajor axis, and host stellar mass. We uncover a strong correlation between planet occurrence rate and host star mass, with stars M* > 1.5 M⊙ more likely to host planets with masses between 2 and 13MJup and semimajor axes of 3–100 au at 99.92% confidence. We fit a double power-law model in planet mass (m) and semimajor axis (a) for planet populations around high-mass stars (M* > 1.5 M⊙) of the form d2N/(dm da) ∝ mα aβ, finding α = −2.4 ± 0.8 and β = −2.0 ± 0.5, and an integrated occurrence rate of 9+5-4% between 5–13MJup and 10–100 au. A significantly lower occurrence rate is obtained for brown dwarfs around all stars, with 0.8+0.8-0.5% of stars hosting a brown dwarf companion between 13–80MJup and 10–100 au. Brown dwarfs also appear to be distributed differently in mass and semimajor axis compared to giant planets; whereas giant planets follow a bottom-heavy mass distribution and favor smaller semimajor axes, brown dwarfs exhibit just the opposite behaviors. Comparing to studies of short-period giant planets from the radial velocity method, our results are consistent with a peak in occurrence of giant planets between ∼1 and 10 au. We discuss how these trends, including the preference of giant planets for high-mass host stars, point to formation of giant planets by core/pebble accretion, and formation of brown dwarfs by gravitational instability.Peer reviewe

    Bayesian inference for challenging scientific models

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    Advances in technology and computation have led to ever more complicated scientific models of phenomena across a wide variety of fields. Many of these models present challenges for Bayesian inference, as a result of computationally intensive likelihoods, high-dimensional parameter spaces or large dataset sizes. In this thesis we show how we can apply developments in probabilistic machine learning and statistics to do inference with examples of these types of models. As a demonstration of an applied inference problem involving a non-trivial likelihood computation, we show how a combination of optimisation and MCMC methods along with careful consideration of priors can be used to infer the parameters of an ODE model of the cardiac action potential. We then consider the problem of pileup, a phenomenon that occurs in astronomy when using CCD detectors to observe bright sources. It complicates the fitting of even simple spectral models by introducing an observation model with a large number of continuous and discrete latent variables that scales with the size of the dataset. We develop an MCMC-based method that can work in the presence of pileup by explicitly marginalising out discrete variables and using adaptive HMC on the remaining continuous variables. We show with synthetic experiments that it allows us to fit spectral models in the presence of pileup without biasing the results. We also compare it to neural Simulation- Based Inference approaches, and find that they perform comparably to the MCMC-based approach whilst being able to scale to larger datasets. As an example of a problem where we wish to do inference with extremely large datasets, we consider the Extreme Deconvolution method. The method fits a probability density to a dataset where each observation has Gaussian noise added with a known sample-specific covariance, originally intended for use with astronomical datasets. The existing fitting method is batch EM, which would not normally be applied to large datasets such as the Gaia catalog containing noisy observations of a billion stars. In this thesis we propose two minibatch variants of extreme deconvolution, based on an online variation of the EM algorithm, and direct gradient-based optimisation of the log-likelihood, both of which can run on GPUs. We demonstrate that these methods provide faster fitting, whilst being able to scale to much larger models for use with larger datasets. We then extend the extreme deconvolution approach to work with non- Gaussian noise, and to use more flexible density estimators such as normalizing flows. Since both adjustments lead to an intractable likelihood, we resort to amortized variational inference in order to fit them. We show that for some datasets that flows can outperform Gaussian mixtures for extreme deconvolution, and that fitting with non-Gaussian noise is now possible

    Measuring and Correcting the Effects of Scintillation in Astronomy

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    High-precision ground-based time-resolved photometry is significantly limited by the effects of the Earth's atmosphere. Optical atmospheric turbulence, produced by the mixing of layers of air of different temperatures, results in layers of spatially and temporally varying refractive indices. These result in phase aberrations of the star light which have two effects: firstly the point spread function is broadened, thus limiting the resolution, and secondly the propagation of these aberrations results in spatio-temporal intensity fluctuations in the pupil-plane of the telescope known as scintillation. The first effect can be corrected with adaptive optics, however the scintillation noise remains. In this thesis, the results from testing a scintillation correction technique that uses tomographic wavefront sensing are presented. The technique was explored extensively in simulation before being tested on-sky on the Isaac Newton Telescope in La Palma, Spain. Scintillation noise also limits the signal-to-noise ratio that can be achieved for standard differential photometry as the random noise fluctuations in the comparison star and the target star light curves add in quadrature. A differential photometry technique that uses optimised temporal binning of the comparison star to minimise the addition of random noise fluctuations is presented and tested both in simulation and with on-sky data. Finally, an investigation into the use of sparse arrays of small telescopes to reduce scintillation noise in photometry is presented. The impact of several parameters on the correlation of scintillation noise measured between sub-apertures in the array is explored

    Purification of the neurodegenerative disease associated protein TDP-43 and development of TDP-43 aggregation inhibitors.

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    Transactive response DNA-binding protein-43 (TDP-43) is a protein that has been implicated in multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). In these diseases, TDP-43 is found aggregated in the cytoplasm of neurones in the brain and spinal cord and it is hypothesised that this aggregation leads to neuronal degeneration. Despite identification of TDP-43 as a constituent of pathological aggregates in 2006, progress in biochemical characterisation of TDP-43 and its aggregation has been limited by an inability to purify sufficient soluble protein to allow characterisation. In this study, a novel method for the purification of TDP-43 has been developed. The resulting purified protein exists in multiple oligomeric states depending on buffer conditions, displays evidence of secondary structural content by circular dichroism spectroscopy and in preliminary studies demonstrates DNA binding activity. A TDP-43 C-terminal fragment was also purified and a fluorescence-based assay developed to monitor its aggregation, with transmission electron microscopy (TEM) used to image the aggregates produced. In this assay, small molecules were tested as aggregation inhibitors. Following minimal success re-purposing generic aggregation inhibitor molecules, a series of targeted peptide-based inhibitors were designed. The third-generation peptide inhibitors, designed with the aid of the artificial intelligence system AlphaFold, reduced the aggregation of the C-terminal fragment, with TEM identifying changes to the morphology of the aggregates produced. Finally, a “druggable” Saccharomyces cerevisiae yeast cell model of TDP-43 proteinopathy was developed, in which molecules with potential as TDP-43 aggregation inhibitors can be tested further

    Future developments in ground-based gamma-ray astronomy

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    Ground-based gamma-ray astronomy is a powerful tool to study cosmic-ray physics, providing a diagnostic of the high-energy processes at work in the most extreme astrophysical accelerators of the universe. Ground-based gamma-ray detectors apply a number of experimental techniques to measure the products of air showers induced by the primary gamma-rays over a wide energy range, from about 30 GeV to few PeV. These are based either on the measurement of the atmospheric Cherenkov light induced by the air showers, or the direct detection of the shower's secondary particles at ground level. Thanks to the recent development of new and highly sensitive ground-based gamma-ray detectors, important scientific results are emerging which motivate new experimental proposals, at various stages of implementation. In this chapter we will present the current expectations for future experiments in the field.Comment: To appear in "Handbook of X-ray and Gamma-ray Astrophysics" by Springer (Eds. C. Bambi and A. Santangelo) - 59 p

    HETDEX Public Source Catalog 1 -- Stacking 50K Lyman Alpha Emitters

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    We describe the ensemble properties of the 1.9<z<3.51.9 < z < 3.5 Lyman Alpha Emitters (LAEs) found in the HETDEX survey's first public data release, HETDEX Public Source Catalog 1 (Mentuch Cooper et al. 2023). Stacking the low-resolution (RR \sim 800) spectra greatly increases the signal-to-noise ratio, revealing spectral features otherwise hidden by noise, and we show that the stacked spectrum is representative of an average member of the set. The flux limited, Lyα\alpha signal-to-noise ratio restricted stack of 50K HETDEX LAEs shows the ensemble biweight ``average" z2.6z \sim 2.6 LAE to be a blue (UV continuum slope 2.4\sim -2.4 and E(B-V) <0.1< 0.1), moderately bright (MUV19.7_{\text{UV}} \sim -19.7) star forming galaxy with strong Lyα\alpha emission (log LLyαL_{Ly\alpha} \sim 42.8 and WλW_{\lambda}(Lyα\alpha) \sim 114\AA), and potentially significant leakage of ionizing radiation. The restframe UV light is dominated by a young, metal poor stellar population with an average age 5-15 Myr and metallicity of 0.2-0.3 Z_{\odot}.Comment: 17 pages, 11 figures, 2 data files (ApJ Accepted

    Sensitivity of NEXT-100 detector to neutrinoless double beta decay

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    Nesta tese estúdiase a sensibilidade do detector NEXT-100 á desintegración dobre beta sen neutrinos. Existe un gran interese na busca desta desintegración xa que podería respostar preguntas fundamentais en física de neutrinos. O detector constitúe a terceira fase do experimento NEXT, colaboración na que se desenrolou esta tese. A continuación inclúese un resumo de cada un dos capítulos nos que se divide a tese. Comézase introducindo o marco teórico e experimental nas seccións Física de neutrinos, A busca da desintegración dobre beta sen neutrinos e O experimento NEXT. Posteriormente descríbense a parte principal do análise da tese en Simulación do detector, Procesamento de datos e Sensibilidade do detector NEXT-100

    Resolving particle acceleration and transport in the jets of the microquasar SS 433 with H.E.S.S. and HAWC

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    The microquasar SS 433 offers a unique laboratory to study the physics of mildly relativistic jets and the associated non-thermal processes. It hosts a compact binary system, from which a pair of counter-propagating jets is observed to emanate. The jets are resolved by observations out to distances of approximately 0.1 pc from the central source, but further out, they remain dark until they abruptly reappear at around 25 pc as bright X-ray sources. These outer jets were recently reported to be sources of TeV gamma-rays by the High Altitude Water Cherenkov (HAWC) observatory. This thesis presents a complete picture of the TeV emission from the jets of SS 433 including new data from the High Energy Stereoscopic System (H.E.S.S.) and the HAWC observatory. To fully exploit the capabilities of the H.E.S.S. observations, a new approach to background rejection is presented. It is based on the detection of Cherenkov light from muons by large Imaging Atmospheric Cherenkov Telescopes (IACTs), such as the telescope located at the center of the H.E.S.S. array. The application of this technique leads to a factor four reduction in background above several tens of TeV in the H.E.S.S. stereoscopic analysis. This thesis presents the detection of the SS 433 outer jets for the first time with an IACT array using H.E.S.S.. The superior angular and energy resolution of H.E.S.S. compared to HAWC allow for a detailed study of the emission from the jets, including a measurement of the physical extension of the emission and of the spectra out to tens of TeV. These observations also reveal the presence of striking energy- dependent morphology, ruling out a hadronic origin for the bulk of the gamma-ray emission. Photons above 10 TeV are observed only close to the base of the outer jets, implying efficient particle acceleration to very-high energies at that location. Evidence suggests that the acceleration is due to a shock, thus providing a clue to the long-standing question of the reappearance of the jets. The observed energy-dependent morphology is modeled as a consequence of the particle cooling times and the advection flow of the jet, which constrains the jet dynamics and, in particular, results in an estimate of the velocity of the outer jets at their base. This solves several issues concerning the non-thermal processes occurring in the jets and their dynamics, but also opens up new questions that highlight our incomplete understanding of the SS 433 system. A joint analysis of the H.E.S.S. and HAWC data would provide insights on the system across the entire range of TeV energies. To make this possible, a tool capable of reading and analyzing the data from both instruments is required. This thesis presents the extension and validation of an existing data format and analysis tool shared among IACTs to the data from particle detector arrays such as the HAWC observatory. This framework is then used to revisit the HAWC observations of the SS 433 region with the inclusion of additional data taken since the first detection was reported. The existence of this framework enables for the first time the joint analysis of the H.E.S.S. and HAWC data, the preliminary results of which are presente

    Deciphering Radio Emission from Solar Coronal Mass Ejections using High-fidelity Spectropolarimetric Radio Imaging

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    Coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic fields from the Sun into the heliosphere and are the most important driver of space weather. The geo-effectiveness of a CME is primarily determined by its magnetic field strength and topology. Measurement of CME magnetic fields, both in the corona and heliosphere, is essential for improving space weather forecasting. Observations at radio wavelengths can provide several remote measurement tools for estimating both strength and topology of the CME magnetic fields. Among them, gyrosynchrotron (GS) emission produced by mildly-relativistic electrons trapped in CME magnetic fields is one of the promising methods to estimate magnetic field strength of CMEs at lower and middle coronal heights. However, GS emissions from some parts of the CME are much fainter than the quiet Sun emission and require high dynamic range (DR) imaging for their detection. This thesis presents a state-of-the-art calibration and imaging algorithm capable of routinely producing high DR spectropolarimetric snapshot solar radio images using data from a new technology radio telescope, the Murchison Widefield Array. This allows us to detect much fainter GS emissions from CME plasma at much higher coronal heights. For the first time, robust circular polarization measurements have been jointly used with total intensity measurements to constrain the GS model parameters, which has significantly improved the robustness of the estimated GS model parameters. A piece of observational evidence is also found that routinely used homogeneous and isotropic GS models may not always be sufficient to model the observations. In the future, with upcoming sensitive telescopes and physics-based forward models, it should be possible to relax some of these assumptions and make this method more robust for estimating CME plasma parameters at coronal heights.Comment: 297 pages, 100 figures, 9 tables. Submitted at Tata Institute of Fundamental Research, Mumbai, India, Ph.D Thesi
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