21,663 research outputs found

    Magnetic braking with MESA evolutionary models in the single star and LMXB regimes

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    Magnetic braking has a prominent role in driving the evolution of close low mass binary systems and heavily influences the rotation rates of low mass F- and later type stars with convective envelopes. Several possible prescriptions that describe magnetic braking in the context of 1D stellar evolution models currently exist. We test four magnetic braking prescriptions against both low mass X-ray binary orbital periods from the Milky Way and single star rotation periods observed in open clusters. We find that data favors a magnetic braking prescription that follows a rapid transition from fast to slow rotation rates, exhibits saturated (inefficient) magnetic braking below a critical Rossby number, and that is sufficiently strong to reproduce ultra compact X-ray binary systems. Of the four prescriptions tested, these conditions are satisfied by a braking prescription that incorporates the effect of high order magnetic field topology on angular momentum loss. None of the braking prescriptions tested are able to replicate the stalled spin down observed in open cluster stars aged 700 - 1000 Myr or so, with masses \lesssim 0.8 M\rm M_{\odot}.Comment: 20 pages, 5 figure

    A suite of quantum algorithms for the shortestvector problem

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    Crytography has come to be an essential part of the cybersecurity infrastructure that provides a safe environment for communications in an increasingly connected world. The advent of quantum computing poses a threat to the foundations of the current widely-used cryptographic model, due to the breaking of most of the cryptographic algorithms used to provide confidentiality, authenticity, and more. Consequently a new set of cryptographic protocols have been designed to be secure against quantum computers, and are collectively known as post-quantum cryptography (PQC). A forerunner among PQC is lattice-based cryptography, whose security relies upon the hardness of a number of closely related mathematical problems, one of which is known as the shortest vector problem (SVP). In this thesis I describe a suite of quantum algorithms that utilize the energy minimization principle to attack the shortest vector problem. The algorithms outlined span the gate-model and continuous time quantum computing, and explore methods of parameter optimization via variational methods, which are thought to be effective on near-term quantum computers. The performance of the algorithms are analyzed numerically, analytically, and on quantum hardware where possible. I explain how the results obtained in the pursuit of solving SVP apply more broadly to quantum algorithms seeking to solve general real-world problems; minimize the effect of noise on imperfect hardware; and improve efficiency of parameter optimization.Open Acces

    Theorising Christian Anarchism A Political Commentary on the Gospel

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    This thesis argues that there is a tradition in political theology and in political theory that deserves to be called "Christian anarchism." The various thinkers that contribute to this tradition have never before been considered to be part of a theoretical movement or tradition, and the originality of this thesis is to weave these thinkers together and present a generic theory of Christian anarchism. . Taken together, thinkers like Tolstoy, Ellul, Elliott and Andrews put forward a comprehensive exegesis of Jesus' teaching and example as implying a critique of the state and a vision of a stateless society. Based on this understanding of the Gospel, they accuse both the state and the church of contradicting, betraying and corrupting the essence of Christianity. Some Christian anarchists - Eller in particular - even see Romans 13 and the "render unto Caesar" passage as not discrediting but indeed confirming their interpretation, and although more activist Christian anarchists sometimes disagree on the potential role of civil disobedience, they do all stress that what matters above all is obedience to God Moreover, they all call for the "true" church to lead the Christian anarchist revolution by example, despite the very demanding sacrifices which this involves. They point to numerous examples of similar witness ever since the early church, and themselves strive to emulate such examples in their own lives - the Catholic Worker movement being perhaps the most notable example in this regard Thus, Christian anarchist thinkers' critique of the current order and appeal to follow God's radical commandments echoes the voices of the prophets of old, calling society to return to God's covenant. By weaving their scattered voices together - by theorising Christian anarchism - this thesis provides a political commentary on the Gospel which contributes as much to political theory as it does to political theology

    Synthesis and properties of some doped lithium transition-metal phosphates

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    Four structurally related materials; LiNiPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃, were synthesised, characterised and evaluated as potential cathode materials for lithium secondary batteries. The materials were synthesized using mainly solid-state techniques. Structural characterisation was performed using powder XRD with Rietveld refinement and Raman spectroscopy. Results showed that the pure forms of LiNiPO₄, LiCoPO₄ and Li₃Fe₂(PO₄)₃ could be synthesised in air at various temperatures. Li₃V₂(PO₄)₃ required a reducing atmosphere of 2%H₂/98%N₂ to achieve phase purity. Attempts were made to substitute all four materials with aliovalent dopants. Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃ underwent a phase change depending on dopant content to a higher ionic conducting phase. AC impedance spectroscopy was used to determine conductivity of the materials. In general the phosphates are poor conductors. There was a significant increase in conductivity when substituting the transition metal Ti⁴⁺ for M³⁺ in Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃, and V³⁺ for Co²⁺ in LiCoPO₄. The four materials and their highest conducting doped analogues were evaluated as cathodes in coin type lithium cells to determine their viability in Li secondary batteries. LiCoPO₄ showed a first discharge capacity of 130 mAh/g at 4.6V, but could be cycled over only a limited number of charge-discharge cycles owing to electrolyte instability at the high oxidation potentials (>5V) required for full charge. LiNiPO₄ could not be charged at all to accessible voltages. Ti doped Li₃Fe₂(PO₄)₃ had a relatively low discharge capacity of 60 mAh/g. Li₃V₂(PO₄)₃ and Ti doped Li₃V₂(PO₄)₃ showed a discharge capacity of 130 and 110 mAh/g respectively, although the Ti doped Li₃V₂(PO₄)₃ showed better cycling characteristics. Therefore, although aliovalent doping could increase the total conductivity of the phosphate materials, the accessible capacities of these materials remained limited

    The dynamics and ISM properties of high-redshift dusty star-forming galaxies

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    In this thesis we present a range of observations of submillimetre galaxies (SMGs), a subclass of dust-obscured star-forming galaxies (DSFGs) at redshifts of z~1-5. SMGs are among the most actively star forming sources ever observed, believed to contribute significantly to the star-formation rate density (SFRD) at its peak, so-called 'cosmic noon', at z~2. Given their extreme nature, SMGs provide a strong test of galaxy formation and evolution models. Advancements in instrumentation, in particular with the Submillimetre Common-User Bolometer Area 2 (SCUBA-2) and the Atacama Large (sub-)Millimeter Array (ALMA), have driven significant progress in SMGs studies over the last decade. We have now identified samples of hundreds of SMGs in survey fields with a plethora of photometric coverage, such as the Cosmic Evolution Survey (COSMOS), the UKIDSS Ultra Deep Survey (UDS) and the Extended Chandra Deep Field Survey (ECDFS). Indeed, the main motivation of this thesis is to exploit these samples of SMGs, with a particular focus on the molecular and ionised gas properties, using state-of-the-art instrumentation such as ALMA and the Northern Extended Millimeter Array (NOEMA) for the former, and the K-band Multi-Object Spectrograph (KMOS) mounted on the Very Large Telescope for the latter. Firstly, in Chapter 2 we present CO observations of 47 SMGs, providing one of the largest and highest quality samples of its kind. With this study we demonstrate the capability of ALMA and NOEMA to undertake blind redshift scans in the 3mm waveband, and in doing so add significantly to the number of SMGs with spectroscopic redshifts, which prior to the work presented in this thesis was small. We also exploit the multi-wavelength coverage of the samples, together with the robust new spectroscopic redshifts, to model their spectral energy distributions (SEDs) with the MAGPHYS code and subsequently estimate key physical properties such as stellar masses and star-formation rates. Perhaps more importantly, this survey has allowed us to characterise the molecular gas content in the SMG population, along with its excitation properties, results from which we present in Chapter 3. We also show that the gas depletion timescale in SMGs remains constant, and given that SMGs are significant contributors to the star-formation rate density (SFRD) at z~2, the global evolution of star-formation in SMGs appears to coincide with the evolution of the molecular gas content, as opposed to any variation in star-formation efficiency. We provide a new test of the SMG population as descendants of massive local early-type galaxies, using the derived CO linewidths and baryonic masses. In Chapter 4 we present our Large Programme with KMOS which, when completed, will have observed ~400 SMGs in the COSMOS, UDS and ECDFS fields. Expanding on the work of Chapters 2 and 3 this is designed to further add to the catalog of SMGs with spectroscopic redshifts by detecting the H_alpha and/or [OIII] emission, which probes ionised gas and can also be used to estimate star-formation rates. We detail the target selection and observing strategy of this survey, before presenting early results for 43 emission line-detected sources, including the H_alpha-derived star-formation rates, the mass-metallicity relation and BPT diagram. We also compare the H_alpha, rest-frame optical/near-infrared and dust sizes where available, finding median radii of R_e = 3.6+/-0.3 kpc, R_Halpha = 4.2+/-0.4 kpc and R_dust = 1.2+/-0.3 kpc. Additionally, the sample are consistent with a median Sersic index of n=1, i.e. with an exponential disc-like light profile. The integral field spectrograph (IFS) capabilities of KMOS allow us to spatially resolve the H_alpha/[OIII] emission when it is sufficiently bright and extended, and this provides valuable diagnostics of the galaxy kinematics. Therefore, in Chapter 5 we present resolved H_alpha/[OIII] velocity and velocity dispersion maps for 36 SMGs, from which we derive rotation curves and dispersion profiles. We compare the derived kinematics of our SMGs with less active galaxies at lower redshifts, and divide the sample into 28 'ordered' sources with clear velocity gradients, and rotation curves which can be modelled as Freeman disks, and eight 'disordered' sources with much more messy velocity maps, from which little reliable kinematic information can be obtained. We measure a median rotational velocity of v_rot = 190+/-20 km/s and a median intrinsic velocity dispersion of sigma_0 = 87+/-5 km/s from the 'ordered' subset, both of which are significantly higher than the less actively star-forming galaxies to which we compare. The median ratio of rotational velocity to intrinsic velocity dispersion in the 'ordered' sample is v_rot/sigma_0 = 2.2+/-0.5, indicating that our sources are somewhat rotationally supported, and we therefore suggest that our SMG sample likely represents 'scaled-up' versions of more 'normal' star-forming galaxies, rather than merger-dominated systems

    From algorithms to connectivity and back: finding a giant component in random k-SAT

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    We take an algorithmic approach to studying the solution space geometry of relatively sparse random and bounded degree kk-CNFs for large kk. In the course of doing so, we establish that with high probability, a random kk-CNF Φ\Phi with nn variables and clause density α=m/n2k/6\alpha = m/n \lesssim 2^{k/6} has a giant component of solutions that are connected in a graph where solutions are adjacent if they have Hamming distance Ok(logn)O_k(\log n) and that a similar result holds for bounded degree kk-CNFs at similar densities. We are also able to deduce looseness results for random and bounded degree kk-CNFs in a similar regime. Although our main motivation was understanding the geometry of the solution space, our methods have algorithmic implications. Towards that end, we construct an idealized block dynamics that samples solutions from a random kk-CNF Φ\Phi with density α=m/n2k/52\alpha = m/n \lesssim 2^{k/52}. We show this Markov chain can with high probability be implemented in polynomial time and by leveraging spectral independence, we also observe that it mixes relatively fast, giving a polynomial time algorithm to with high probability sample a uniformly random solution to a random kk-CNF. Our work suggests that the natural route to pinning down when a giant component exists is to develop sharper algorithms for sampling solutions in random kk-CNFs.Comment: 41 pages, 1 figur

    Pontus in Antiquity: aspects of identity

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    The purpose of this thesis is the presentation of the interaction between the successive inhabitants of Pontus in antiquity, indigenous Anatolians, Greeks, Persians and Romans. Limited archaeological evidence cannot determine the precise extent of interaction, although the available information substantiates the notion of a slow, but steady amalgamation. Initially, the intermingling was based on mutual trading links. Although the Hellenic cultural element tended to surface, Eastern factors remained visible. The Mithridatic dynasty was established around the vicinity of Pontus, creating the 'Kingdom of Pontus' which reached its height under Mithridates VI. His administrative and military policy appears to have placed the foundations for the later, Roman corresponding structures. His policies-propaganda reflected the GraecoEastern image of a king, which appealed to the Greek and Persian-Eastern inhabitants of his kingdom, Asia Minor and, to a lesser extent, mainland Greece. This GraecoEastern image might have nourished the concept of a shared history among the inhabitants of Pontus. Their interactions appear to have given rise to an unnamed, local culture, which was enriched with the relevant Roman practices. Around the third century A.D., the Roman administrative patterns might have established an externally defined appellation. During Roman times, Christianity started to be established in Pontus. Although it was not yet a socio-political factor, its non-racial nature prevailed in later centuries. The influence of the Roman-Christian elements can still be observed in the modern Ponti an identity. In antiquity, (lack of) evidence indicates that no group defined themselves as 'Pontics' or 'Pontians' and an internally defined Pontic identity is unlikely to have existed. However, people associated themselves with the geographical area of Pont us, cultural and religious concepts were frequently amalgamated, while the notion of a common descent and a shared history might have been unconsciously fostered. These factors can assist in the understanding of the 'Pontians' today

    Development, Validation, and Application of Methods for High Time-Response Measurement of Gaseous Atmospheric Chlorinated Species

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    Halogenated compounds that participate in catalytic cycles in the atmosphere can influence the fate of chemicals, including ozone, methane, and volatile organic compounds (VOCs). These halogen radicals, in particular atomic chlorine (Cl), can deplete ozone and will react rapidly with VOCs. Reliable, sensitive, and widely available hydrogen chloride (HCl) measurements are important for understanding Cl initiated oxidation in many regions of the troposphere. We configured a commercial HCl cavity ring-down spectrometer (CRDS) for sampling HCl in the ambient atmosphere and developed validation techniques to characterize the measurement uncertainties. The HCl analyzer was used to make continuous HCl measurements in the polluted marine boundary layer during the Halifax Fog and Air Quality Study (HaliFAQS). Bimodal HCl features in the high irradiance days indicated two photochemical processes; (1) morning time photolysis of Cl precursors, and (2) midday formation of nitric acid followed by acid displacement onto chloride (Cl) containing aerosols. A box model used measured HCl to estimate nitryl chloride mixing ratios at sunrise and assessed the contribution of photolabile Cl precursors to radical formation. Total gaseous chlorine (TClg) measurements can illuminate unknown sources of Cl to the atmosphere. Techniques for measuring TClg have been limited to offline analysis of extracted filters and do not provide suitable temporal information on fast atmospheric process. The utility of this novel TClg measurement technique will be crucial to future estimates and assessments of chlorinated compounds and their impact on air quality, climate, and health

    The Caribbean Syzygy: a study of the novels of Edgar Mittelholzer and Wilson Harris

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    The problem of racial inheritance - the "search for identity" - is a recurring theme in the criticism of Caribbean literature. It is a pre-occupation with Caribbean writers, affecting both subject matter and literary quality, as FM. Birbalsingh, for example, has shown with reference to the novels of John Hearne and E,R. Braithwaite (Caribbean quarterly Vols. 14, December 1968 and 16, March 1970). This study of the work of Edgar Mittelholzer and Wilson Harris will attempt to show that there are important areas still to be explored relating Caribbean literature to its complex racial and cultural background. Both Mittelholzer and Harris deserve close, critical study in their own right; but a parallel examination reveals similarities and differences which bring into sharper focus wider concerns of Caribbean literature. The two important directions of West Indian writing are more clearly seen: the one, pioneered by Mittelholzer, in which the writer looks outward towards a "parent" culture, and the other looking inward, seeking in its own, complex inheritance the raw material for new and original growth. Mittelholzer and Harris are both Guyanese of mixed racial stock, both deeply concerned with the psychological effects of this mixture, and both writers have a profound awareness of the Guyanese historical and cultural heritage. They also share a deep feeling for the Guyenese landscape which appears in their work as a brooding presence affecting radically -the lives of those who live within i-t. Mittelholzer's attitude to his mixed racial and cultural origins, however, produces in his work a schizophrenic Imbalance while Harris, by accepting racial and cultural complexity as a starting-point, initiates a uniquely creative and experimental art. Mittelholzer, in his approach to history, human character eM landscape, remains a vi "coastal" writer never really concerned (as Harris is) with. the deeper significance of the "Interior" and all that this implies, both in a geographical and psychological sense. The fact that Mittelbolzer's work reflects a psychological imbalance induced by a pre-occupation with racial identity has been demonstrated by Denis Williams in the 1968 Mittelholzer Lectures, and by Joyce Sparer in a series of articles in the Guyana Graphic. Mittelholzer's awareness of this imbalance, however, and his attempt to come to terms with it in his art remain to be examined and documented, as does Harris's attempt to create am "associative" art aimed at healing the breach in the individual consciousness of Caribbean Man. The aim of this study is to demonstrate that Mitteholzer and. Harris, although antithetical in impact and style (each representing an approach to fiction directly opposed to the other) are, in fact, the opposite elements of a dichotomy. Their work illustrates the negative and positive aspects of the racial and cultural schizophrenia of the Caribbean, for both writers in their different ways are preoccupied with (and therefore have embodied in their work) the juxtaposition and, contrasting of apparently irreconcilable emotional and intellectual qualities - the Caribbean Syzygy
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