4,362 research outputs found

    The evolutionary state of young protoplanetary disks

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    Observations of protoplanetary disks have been focused on low-mass, classical T-Tauri stars and on intermediate mass Herbig Ae/Be stars. The observations of the Herbig stars have introduced a bias in the intermediate mass range since they exclude the earlier stages of disk evolution. The heaviest T-Tauri stars, the intermediate mass T-Tauri stars (IMTT stars), with spectral type from F to early K and with masses ≥1.5 M☉, are the younger precursors of the Herbig stars. To get a complete picture of the evolution of protoplanetary disks IMTT stars needs to be studied. Many IMTT stars have already been included in samples of classical T-Tauri stars where they are the most massive stars in the samples. This thesis seeks to remove some of this bias by focusing on the IMTT stars and observations of the disks around them. This thesis presents the research of a sample of 49 identified IMTT stars with infrared access. Their disks are compared with those of the known Herbig Ae/Be stars to examine their evolutionary status. The thesis also presents spatially resolved near-infrared scattered light observations of the IMTT star RY Tau. Using radiative transfer modelling the observations are recreated and features reminiscent of a dusty disk wind is assessed.Finally, it presents near-infrared spatially resolved scattered light observations of 23 optically bright stars in the Orion high mass star forming region. The observations are analysed in context of the stellar parameters, stellar multiplicity and the environment of a high star forming region

    Climate Change and Critical Agrarian Studies

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    Climate change is perhaps the greatest threat to humanity today and plays out as a cruel engine of myriad forms of injustice, violence and destruction. The effects of climate change from human-made emissions of greenhouse gases are devastating and accelerating; yet are uncertain and uneven both in terms of geography and socio-economic impacts. Emerging from the dynamics of capitalism since the industrial revolution — as well as industrialisation under state-led socialism — the consequences of climate change are especially profound for the countryside and its inhabitants. The book interrogates the narratives and strategies that frame climate change and examines the institutionalised responses in agrarian settings, highlighting what exclusions and inclusions result. It explores how different people — in relation to class and other co-constituted axes of social difference such as gender, race, ethnicity, age and occupation — are affected by climate change, as well as the climate adaptation and mitigation responses being implemented in rural areas. The book in turn explores how climate change – and the responses to it - affect processes of social differentiation, trajectories of accumulation and in turn agrarian politics. Finally, the book examines what strategies are required to confront climate change, and the underlying political-economic dynamics that cause it, reflecting on what this means for agrarian struggles across the world. The 26 chapters in this volume explore how the relationship between capitalism and climate change plays out in the rural world and, in particular, the way agrarian struggles connect with the huge challenge of climate change. Through a huge variety of case studies alongside more conceptual chapters, the book makes the often-missing connection between climate change and critical agrarian studies. The book argues that making the connection between climate and agrarian justice is crucial

    Dashing through the cluster: An X-ray to radio view of UGC 10420 undergoing ram-pressure stripping

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    We present multi-wavelength data and analysis, including new FUV AstroSat/UVIT observations of the spiral galaxy UGC 10420 (z=0.032), a member of the cluster Abell 2199. UGC 10420 is present on the edge of the X-ray emitting region of the cluster at a distance of ~ 680 kpc from the centre. The FUV data shows intense knots of star formation on the leading edge of the galaxy, accompanied by a tail of the same on the diametrically opposite side. Our analysis shows that the images of the galaxy disk in the optical and mid-infrared are much smaller in size than that in the FUV. While the broadband optical colours of UGC 10420 are typical of a post-starburst galaxy, the SFR derived from a UV-to-IR spectral energy distribution is at least a factor of nine higher than that expected for a star-forming field galaxy of similar mass at its redshift. A careful removal of the contribution of the diffuse intracluster gas shows that the significant diffuse X-ray emission associated with the inter-stellar medium of UGC 10420 has a temperature, T_X = 0.24^{+0.09}_{-0.06} keV (0.4-2.0 keV) and luminosity, L_X = 1.8+/-0.9 x 10^{40} erg/s, which are typical of the X-ray emission from late-type spiral galaxies. Our analysis favours a scenario where the interaction of a galaxy with the hot intra-cluster medium of the cluster, perturbs the gas in the galaxy causing starburst in the leading edge of the disk. On the other hand, the turbulence thus developed may also push some of the gas out of the disk. Interactions between the gas ejected from the galaxy and the intracluster medium can then locally trigger star formation in the wake of the galaxy experiencing ram-pressure stripping. Our data however does not rule out the possibility of a flyby encounter with a neighbouring galaxy, although no relevant candidates are observed in the vicinity of UGC 10420. (abridged)Comment: Accepted for publication in the Publication of the Astronomical Society of Australia (PASA). 16 pages, 11 figure

    Machine learning applications in search algorithms for gravitational waves from compact binary mergers

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    Gravitational waves from compact binary mergers are now routinely observed by Earth-bound detectors. These observations enable exciting new science, as they have opened a new window to the Universe. However, extracting gravitational-wave signals from the noisy detector data is a challenging problem. The most sensitive search algorithms for compact binary mergers use matched filtering, an algorithm that compares the data with a set of expected template signals. As detectors are upgraded and more sophisticated signal models become available, the number of required templates will increase, which can make some sources computationally prohibitive to search for. The computational cost is of particular concern when low-latency alerts should be issued to maximize the time for electromagnetic follow-up observations. One potential solution to reduce computational requirements that has started to be explored in the last decade is machine learning. However, different proposed deep learning searches target varying parameter spaces and use metrics that are not always comparable to existing literature. Consequently, a clear picture of the capabilities of machine learning searches has been sorely missing. In this thesis, we closely examine the sensitivity of various deep learning gravitational-wave search algorithms and introduce new methods to detect signals from binary black hole and binary neutron star mergers at previously untested statistical confidence levels. By using the sensitive distance as our core metric, we allow for a direct comparison of our algorithms to state-of-the-art search pipelines. As part of this thesis, we organized a global mock data challenge to create a benchmark for machine learning search algorithms targeting compact binaries. This way, the tools developed in this thesis are made available to the greater community by publishing them as open source software. Our studies show that, depending on the parameter space, deep learning gravitational-wave search algorithms are already competitive with current production search pipelines. We also find that strategies developed for traditional searches can be effectively adapted to their machine learning counterparts. In regions where matched filtering becomes computationally expensive, available deep learning algorithms are also limited in their capability. We find reduced sensitivity to long duration signals compared to the excellent results for short-duration binary black hole signals

    Beyond the Ultra-deep Frontier Fields And Legacy Observations (BUFFALO): a high-resolution strong + weak-lensing view of Abell 370

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    The HST treasury program BUFFALO provides extended wide-field imaging of the six Hubble Frontier Fields galaxy clusters. Here we present the combined strong and weak-lensing analysis of Abell 370, a massive cluster at z=0.375. From the reconstructed total projected mass distribution in the 6arcmin x 6arcmin BUFFALO field-of-view, we obtain the distribution of massive substructures outside the cluster core and report the presence of a total of seven candidates, each with mass 5×1013M\sim 5 \times 10^{13}M_{\odot}. Combining the total mass distribution derived from lensing with multi-wavelength data, we evaluate the physical significance of each candidate substructure, and conclude that 5 out of the 7 substructure candidates seem reliable, and that the mass distribution in Abell 370 is extended along the North-West and South-East directions. While this finding is in general agreement with previous studies, our detailed spatial reconstruction provides new insights into the complex mass distribution at large cluster-centric radius. We explore the impact of the extended mass reconstruction on the model of the cluster core and in particular, we attempt to physically explain the presence of an important external shear component, necessary to obtain a low root-mean-square separation between the model-predicted and observed positions of the multiple images in the cluster core. The substructures can only account for up to half the amplitude of the external shear, suggesting that more effort is needed to fully replace it by more physically motivated mass components. We provide public access to all the lensing data used as well as the different lens models.Comment: 29 pages, 17 figures, 3 table

    Business Functions Capabilities and Small and Medium Enterprises’ Internationalization

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    Ineffective global expansion can adversely affect small and medium enterprises (SMEs) business outcomes. Business leaders are concerned with developing effective global expansion strategies to penetrate potential international markets, thus enhancing sustainability. Grounded in the business management systems theory, the purpose of this qualitative multi-case study was to explore strategies that leaders of Sub-Saharan Africa manufacturing SMEs use for global expansion. The participants were five manufacturing value-adding SME leaders participating in export markets. Using Yin’s five steps data analysis process, six themes emerged: (a) enterprise characterization, (b) understanding the enterprise’s product, (c) intra-enterprise factor-based strategies for export participation, (d) the enterprise’s external factor-based strategies for successful export venture, (e) global expansion strategies, and (f) serendipitous findings. A key recommendation for SME leaders is to analyze the critical components of their products and prepare to adjust them to the demand dimensions of the target market. The implications for positive social change include the potential to increase the enterprise’s wealth, increase employment, reduce poverty for all value chain participants, and growth in gross domestic product

    The ALMA-ALPAKA survey:I. High-resolution CO and [CI] kinematics of star-forming galaxies at z = 0.5- 3.5

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    Context. Spatially resolved studies of the kinematics of galaxies provide crucial insights into their assembly and evolution, enabling one to infer the properties of the dark matter halos, derive the impact of feedback on the interstellar medium (ISM), as well as measure and characterize the outflow motions. To date, most of the kinematic studies at z = 0.5 - 3.5 have been obtained using emission lines tracing the warm, ionized gas (e.g., Hα, [OII], and [OIII]). However, whether these provide an exhaustive or only a partial view of the dynamics of galaxies and of the properties of the ISM is still debated. Complementary insights into the cold gas kinematics are therefore needed. Aims. We present the Archival Large Program to Advance Kinematic Analysis (ALPAKA), a project aimed at gathering high-resolution observations of CO and [CI] emission lines of star-forming galaxies at z = 0.5 - 3.5 from the Atacama Large Millimeter Array (ALMA) public archive. With ≈ 147 h of total integration time, ALPAKA assembles ∼0.25″ observations for 28 star-forming galaxies, which is the largest sample with spatially resolved cold gas kinematics as traced by either CO or [CI] at z ≳ 0.5, spanning 7 Gyr of cosmic history. A large fraction of ALPAKA galaxies (19 out of 28) lie in overdense regions (clusters, groups, and protoclusters). Methods. By combining multiwavelength ground- and space-based ancillary data, we derived the stellar masses (M∗) and star-formation rates (SFRs) for the ALPAKA targets. We exploited the ALMA data to infer the dynamical state of the ALPAKA galaxies and derive their rotation curves and velocity dispersion profiles using 3DBAROLO. Results. ALPAKA probes the massive (M∗ ≳ 1010 MO), actively star-forming (SFR ≈ 10 - 3000 MO yr-1) part of the population of galaxies at z ∼ 0.5 - 3.5. Based on our kinematic classification, we find that 19 out of 28 ALPAKA galaxies are rotating disks, two are interacting systems, while for the remaining seven sources the classification is uncertain. The disks have velocity dispersion values that are typically larger in the innermost regions than in the outskirts, with a median value for the entire disk sample of 35-9+11 km s-1.</p

    Multi-messenger astronomy with black holes: tidal disruption events

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    This chapter provides an overview of tidal disruption events, aiming to provide an overview of both the theoretical and the observational state of the field, with the overarching goal of introducing them as tools to indirectly observe massive black holes in the Universe. We start by introducing the relevant theoretical concepts, physical scales and timescales with an emphasis on the classical framework and how this has been (and continues to be) improved since the inception of the field. We then cover the current and future prospects of observing TDEs through a variety of messengers, including photons across the electromagnetic spectrum, as well as gravitational waves and neutrino particles. More recent advancements in the field, including repeating TDEs as well as TDEs by stellar-mass black holes, are also highlighted.Comment: To appear in Chapter 5 in the book Black Holes in the Era of Gravitational Wave Astronomy, eds. Arca Sedda, Bortolas, Spera, pub. Elsevier. All authors equally contributed to the Chapter writin

    Black holes and how to find them : on the detection of compact objects at several scales

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    This dissertation is structured in four chapters. First, the reader is introduced to current detection methods for astrophysical black holes at different scales. The following three chapters correspond to different research projects regarding black hole detection. The second chapter presents the kinematic detection of a supermassive black hole at the center of the dwarf spherical galaxy Leo I. Chapter three presents a novel idea for the detection of supermassive black hole binaries using the effect their gravitational waves produce on lower frequency gravitational waves in our vicinity. Finally, chapter four describes the spectroscopic follow-up of LIGO-Virgo-Kagra gravitational wave sources conducted using the Hobby Eberly Telescope at McDonald Observatory.Physic
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