788 research outputs found
Using UAV acquired photography and structure from motion techniques for studying glacier landforms: application to the glacial flutes at IsfallsglaciÀren
Glacier and ice sheet retreat exposes freshly deglaciated terrain which often contains small-scale fragile geomorphological features which could provide insight into subglacial or submarginal processes. Subaerial exposure results in potentially rapid landscape modification or even disappearance of the minor-relief landforms as wind, weather, water and vegetation impact on the newly exposed surface. Ongoing retreat of many ice masses means there is a growing opportunity to obtain high resolution geospatial data from glacier forelands to aid in the understanding of recent subglacial and submarginal processes. Here we used an unmanned aerial vehicle to capture close-range aerial photography of the foreland of IsfallsglaciĂ€ren, a small polythermal glacier situated in Swedish Lapland. An orthophoto and a digital elevation model with ~2âcm horizontal resolution were created from this photography using structure from motion software. These geospatial data was used to create a geomorphological map of the foreland, documenting moraines, fans, channels and flutes. The unprecedented resolution of the data enabled us to derive morphological metrics (length, width and relief) of the smallest flutes, which is not possible with other data products normally used for glacial landform metrics mapping. The map and flute metrics compare well with previous studies, highlighting the potential of this technique for rapidly documenting glacier foreland geomorphology at an unprecedented scale and resolution. The vast majority of flutes were found to have an associated stoss-side boulder, with the remainder having a likely explanation for boulder absence (burial or erosion). Furthermore, the size of this boulder was found to strongly correlate with the width and relief of the lee-side flute. This is consistent with the lee-side cavity infill model of flute formation. Whether this model is applicable to all flutes, or multiple mechanisms are required, awaits further study
Atomic Bose Gas with Negative Scattering Length
We derive the equation of state of a dilute atomic Bose gas with an
interatomic interaction that has a negative scattering length and argue that
two continuous phase transitions, occuring in the gas due to quantum degeneracy
effects, are preempted by a first-order gas-liquid or gas-solid transition
depending on the details of the interaction potential. We also discuss the
consequences of this result for future experiments with magnetically trapped
spin-polarized atomic gasses such as lithium and cesium.Comment: 16 PAGES, REVTEX 3.0, ACCEPTED FOR PUBLICATION IN PHYS. REV.
Image resonance in the many-body density of states at a metal surface
The electronic properties of a semi-infinite metal surface without a bulk gap are studied by a formalism that is able to account for the continuous spectrum of the system. The density of states at the surface is calculated within the GW approximation of many-body perturbation theory. We demonstrate the presence of an unoccupied surface resonance peaked at the position of the first image state. The resonance encompasses the whole Rydberg series of image states and cannot be resolved into individual peaks. Its origin is the shift in spectral weight when many-body correlation effects are taken into account
Self-Similar Factor Approximants
The problem of reconstructing functions from their asymptotic expansions in
powers of a small variable is addressed by deriving a novel type of
approximants. The derivation is based on the self-similar approximation theory,
which presents the passage from one approximant to another as the motion
realized by a dynamical system with the property of group self-similarity. The
derived approximants, because of their form, are named the self-similar factor
approximants. These complement the obtained earlier self-similar exponential
approximants and self-similar root approximants. The specific feature of the
self-similar factor approximants is that their control functions, providing
convergence of the computational algorithm, are completely defined from the
accuracy-through-order conditions. These approximants contain the Pade
approximants as a particular case, and in some limit they can be reduced to the
self-similar exponential approximants previously introduced by two of us. It is
proved that the self-similar factor approximants are able to reproduce exactly
a wide class of functions which include a variety of transcendental functions.
For other functions, not pertaining to this exactly reproducible class, the
factor approximants provide very accurate approximations, whose accuracy
surpasses significantly that of the most accurate Pade approximants. This is
illustrated by a number of examples showing the generality and accuracy of the
factor approximants even when conventional techniques meet serious
difficulties.Comment: 22 pages + 11 ps figure
The VLT-FLAMES Tarantula Survey
A spectroscopic analysis has been undertaken for the B-type multiple systems (excluding those with supergiant primaries) in the VLT-FLAMES Tarantula Survey (VFTS). Projected rotational velocities, vesini, for the primaries have been estimated using a Fourier Transform technique and confirmed by fitting rotationally broadened profiles. A subset of 33 systems with vesini †80 kmâs-1 have been analysed using a TLUSTY grid of model atmospheres to estimate stellar parameters and surface abundances for the primaries. The effects of a potential flux contribution from an unseen secondary have also been considered. For 20 targets it was possible to reliably estimate their effective temperatures (Teff) but for the other 13 objects it was only possible to provide a constraint of 20 000 †Teff †26 000 K â the other parameters estimated for these targets will be consequently less reliable. The estimated stellar properties are compared with evolutionary models and are generally consistent with their membership of 30 Doradus, while the nature of the secondaries of 3 SB2 system is discussed. A comparison with a sample of single stars with vesini †80 kmâs-1 obtained from the VFTS and analysed with the same techniques implies that the atmospheric parameters and nitrogen abundances of the two samples are similar. However, the binary sample may have a lack of primaries with significant nitrogen enhancements, which would be consistent with them having low rotational velocities and having effectively evolved as single stars without significant rotational mixing. This result, which may be actually a consequence of the limitations of the pathfinder investigation presented in this paper, should be considered as a motivation for spectroscopic abundance analysis of large samples of binary stars, with high quality observational data
Duality Theorems in Ergodic Transport
We analyze several problems of Optimal Transport Theory in the setting of
Ergodic Theory. In a certain class of problems we consider questions in Ergodic
Transport which are generalizations of the ones in Ergodic Optimization.
Another class of problems is the following: suppose is the shift
acting on Bernoulli space , and, consider a fixed
continuous cost function . Denote by the set
of all Borel probabilities on , such that, both its and
marginal are -invariant probabilities. We are interested in the
optimal plan which minimizes among the probabilities on
.
We show, among other things, the analogous Kantorovich Duality Theorem. We
also analyze uniqueness of the optimal plan under generic assumptions on .
We investigate the existence of a dual pair of Lipschitz functions which
realizes the present dual Kantorovich problem under the assumption that the
cost is Lipschitz continuous. For continuous costs the corresponding
results in the Classical Transport Theory and in Ergodic Transport Theory can
be, eventually, different.
We also consider the problem of approximating the optimal plan by
convex combinations of plans such that the support projects in periodic orbits
The Similarity Hypothesis in General Relativity
Self-similar models are important in general relativity and other fundamental
theories. In this paper we shall discuss the ``similarity hypothesis'', which
asserts that under a variety of physical circumstances solutions of these
theories will naturally evolve to a self-similar form. We will find there is
good evidence for this in the context of both spatially homogenous and
inhomogeneous cosmological models, although in some cases the self-similar
model is only an intermediate attractor. There are also a wide variety of
situations, including critical pheneomena, in which spherically symmetric
models tend towards self-similarity. However, this does not happen in all cases
and it is it is important to understand the prerequisites for the conjecture.Comment: to be submitted to Gen. Rel. Gra
Stigma and Fear: the 'Psy Professional' in Cultural Artifacts
This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The loss of reason called madness provokes perhaps the greatest human fear, for it is reason that
dignifies humanity and separates us from beasts. The âpsy professionalsâ - those who prescribe and
administer treatments for madness - are frequently portrayed in fiction, film, comics, computer
games and entertainments, along with the mad themselves and the asylums that confine them.
Overall, these depictions are malign: the reader/watcher/player is encouraged to fear the mad, the
madhouse and the mad-doctor. Choosing to use less abrasive vocabulary to name the condition of
madness makes no difference to the terror the condition arouses, for the content of many books and
games aims to inspire fear. In spite of considerable efforts over many years, the stigma which
attaches to mental illness remains firmly in place for patients, while psy professionals also carry their
share of âsome of the discredit of the stigmatizedâ (Goffman 1968, p 43) and join patients in a
stigmatized group. Popular belief often equates the psy professions with madness (Walter, 1989).
This paper explores ways in which the fear of madness, and the stigma which clings to sufferers and
their professional carers, is perpetuated by a constant stream of popular cultural artifacts
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