1,476 research outputs found
Mud Volcanoes - A New Class of Sites for Geological and Astrobiological Exploration of Mars
Mud volcanoes provide a unique low-temperature window into the Earth s subsurface - including the deep biosphere - and may prove to be significant sources of atmospheric methane. The identification of analogous features on Mars would provide an important new class of sites for geological and astrobiological exploration. We report new work suggesting that features in Acidalia Planitia are most consistent with their being mud volcanoes
Consistent deformations method applied to a topological coupling of antisymmetric gauge fields in D=3
In this work we use the method of consistent deformations of the master
equation by Barnich and Henneaux in order to prove that an abelian topological
coupling between a zero and a two form fields in D=3 has no nonabelian
generalization. We conclude that a topologically massive model involving the
Kalb-Ramond two-form field does not admit a nonabelian generalization. The
introduction of a connection-type one form field keeps the previous result.Comment: 8 pages. To appear in Physics Letters
A concise stereoselective synthesis of pterosin B
Pterosin B is a naturally occurring indanone found in bracken fern (Pteridium aquilinum) that displays a variety of interesting pharmacological properties, but for which few stereoselective syntheses exist. Herein we describe a 7-step stereoselective synthesis of (2R)-pterosin B via 6-bromo-5,7-dimethylindan-1-one whose structure was confirmed by NOE analysis and structure determination by X-ray crystallography. The hydroxyethyl chain was introduced via a Suzuki-Miyaura cross-coupling reaction. The 2-methyl group was introduced stereoselectively by methylation of a SAMP [(S)-1-amino-2-methoxymethyl)pyrrolidine] hydrazone and the chiral auxiliary was removed to produce (2R)-pterosin B. The structure of pterosin B was confirmed by specific rotation and structural determination by X-ray crystallography
Detection methods for non-Gaussian gravitational wave stochastic backgrounds
We address the issue of finding an optimal detection method for a
discontinuous or intermittent gravitational wave stochastic background. Such a
signal might sound something like popcorn popping. We derive an appropriate
version of the maximum likelihood detection statistic, and compare its
performance to that of the standard cross-correlation statistic both
analytically and with Monte Carlo simulations. The maximum likelihood statistic
performs better than the cross-correlation statistic when the background is
sufficiently non-Gaussian. For both ground and space based detectors, this
results in a gain factor, ranging roughly from 1 to 3, in the minimum
gravitational-wave energy density necessary for detection, depending on the
duty cycle of the background. Our analysis is exploratory, as we assume that
the time structure of the events cannot be resolved, and we assume white,
Gaussian noise in two collocated, aligned detectors. Before this detection
method can be used in practice with real detector data, further work is
required to generalize our analysis to accommodate separated, misaligned
detectors with realistic, colored, non-Gaussian noise.Comment: 25 pages, 12 figures, submitted to physical review D, added revisions
in response to reviewers comment
The Gaseous Environments of Radio Galaxies
X-ray emission traces the gaseous environments of radio sources. The medium
must be present for jet confinement, but what are its influence on jet
fuelling, dynamics, propagation, and disruption? The observational situation is
both complicated and enriched by radio sources being multi-component X-ray
emitters, with several possible regions of non-thermal emission. Recent work,
primarily based on sensitive ROSAT pointings, is used to contrast the X-ray
emission and environments of radio sources with (a) low power, (b) high power
at high redshift, (c) high power at lower redshift, and (d) GHz peaked spectrum
emission. The trends in external gas density and pressure near extended radio
structures are reviewed. Imminently-available X-ray measurements with vastly
improved resolution and sensitivity have great potential for resolving many
open issues.Comment: 20 pages, including 11 figures, using elsart.sty to appear in `Life
Cycles of Radio Galaxies' ed. J Biretta et al., New Astronomy Reviews
(Elsevier Science
Density Functional Study of Cubic to Rhombohedral Transition in -AlF
Under heating, -AlF undergoes a structural phase transition from
rhombohedral to cubic at temperature around 730 K. The density functional
method is used to examine the =0 energy surface in the structural parameter
space, and finds the minimum in good agreement with the observed rhombohedral
structure. The energy surface and electronic wave-functions at the minimum are
then used to calculate properties including density of states, -point
phonon modes, and the dielectric function. The dipole formed at each fluorine
ion in the low temperature phase is also calculated, and is used in a classical
electrostatic picture to examine possible antiferroelectric aspects of this
phase transition.Comment: A 6-page manuscript with 4 figures and 4 table
Targeting Conservation Investments in Heterogeneous Landscapes: A distance function approach and application to watershed management
To achieve a given level of an environmental amenity at least cost, decision-makers must integrate information about spatially variable biophysical and economic conditions. Although the biophysical attributes that contribute to supplying an environmental amenity are often known, the way in which these attributes interact to produce the amenity is often unknown. Given the difficulty in converting multiple attributes into a unidimensional physical measure of an environmental amenity (e.g., habitat quality), analyses in the academic literature tend to use a single biophysical attribute as a proxy for the environmental amenity (e.g., species richness). A narrow focus on a single attribute, however, fails to consider the full range of biophysical attributes that are critical to the supply of an environmental amenity. Drawing on the production efficiency literature, we introduce an alternative conservation targeting approach that relies on distance functions to cost-efficiently allocate conservation funds across a spatially heterogeneous landscape. An approach based on distance functions has the advantage of not requiring a parametric specification of the amenity function (or cost function), but rather only requiring that the decision-maker identify important biophysical and economic attributes. We apply the distance-function approach empirically to an increasingly common, but little studied, conservation initiative: conservation contracting for water quality objectives. The contract portfolios derived from the distance-function application have many desirable properties, including intuitive appeal, robust performance across plausible parametric amenity measures, and the generation of ranking measures that can be easily used by field practitioners in complex decision-making environments that cannot be completely modeled. Working Paper # 2002-01
Two-level systems: exact solutions and underlying pseudo-supersymmetry
Chains of first-order SUSY transformations for the spin equation are studied
in detail. It is shown that the transformation chains are related with a
olynomial pseudo-supersymmetry of the system. Simple determinant formulas for
the final Hamiltonian of a chain and for solutions of the spin equation are
derived. Applications are intended for a two-level atom in an electromagnetic
field with a possible time-dependence of the field frequency. For a specific
form of this dependence, the time oscillations of the probability to populate
the excited level disappear. Under certain conditions this probability becomes
a function tending monotonously to a constant value which can exceed 1/2.Comment: to be published in Ann. Phys. (NY), 6 figures, 17 page
Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia
Yttria-stabilized zirconia powders, containing different levels of SiO2 and Al2O3, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<_50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes
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