834 research outputs found
The twistor spinors of generic 2- and 3-distributions
Generic distributions on 5- and 6-manifolds give rise to conformal structures
that were discovered by P. Nurowski resp. R. Bryant. We describe both as
Fefferman-type constructions and show that for orientable distributions one
obtains conformal spin structures. The resulting conformal spin geometries are
then characterized by their conformal holonomy and equivalently by the
existence of a twistor spinor which satisfies a genericity condition. Moreover,
we show that given such a twistor spinor we can decompose a conformal Killing
field of the structure. We obtain explicit formulas relating conformal Killing
fields, almost Einstein structures and twistor spinors.Comment: 26 page
Synthesis and characterisation of the hollandite solid solution Ba1.2-xCsxFe2.4-xTi5.6+xO16 for partitioning and conditioning of radiocaesium
The geological disposal of high level radioactive waste requires careful budgeting of the heat load produced by radiogenic decay. Removal of high-heat generating radionuclides, such as 137 Cs, reduces the heat load in the repository allowing the remaining high level waste to be packed closer together therefore reducing demand for repository space and the cost of the disposal of the remaining wastes. Hollandites have been proposed as a possible host matrix for the long-term disposal of Cs separated from HLW raffinate. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the hollandite structure, including iron. A range of Cs containing iron hollandites were synthesised via an alkoxide-nitrate route and the structural environment of Fe in the resultant material characterised by Mössbauer and X-ray Absorption Near Edge Spectroscopy. The results of spectroscopic analysis found that Fe was present as octahedrally co-ordinated Fe (III) in all cases and acts as an effective charge compensator over a wide solid solution range
A holonomy characterisation of Fefferman spaces
We prove that Fefferman spaces, associated to non--degenerate CR structures
of hypersurface type, are characterised, up to local conformal isometry, by the
existence of a parallel orthogonal complex structure on the standard tractor
bundle. This condition can be equivalently expressed in terms of conformal
holonomy. Extracting from this picture the essential consequences at the level
of tensor bundles yields an improved, conformally invariant analogue of
Sparling's characterisation of Fefferman spaces.Comment: AMSLaTeX, 15 page
Sliding blocks with random friction and absorbing random walks
With the purpose of explaining recent experimental findings, we study the
distribution of distances traversed by a block that
slides on an inclined plane and stops due to friction. A simple model in which
the friction coefficient is a random function of position is considered.
The problem of finding is equivalent to a First-Passage-Time
problem for a one-dimensional random walk with nonzero drift, whose exact
solution is well-known. From the exact solution of this problem we conclude
that: a) for inclination angles less than \theta_c=\tan(\av{\mu})
the average traversed distance \av{\lambda} is finite, and diverges when
as \av{\lambda} \sim (\theta_c-\theta)^{-1}; b) at
the critical angle a power-law distribution of slidings is obtained:
. Our analytical results are confirmed by
numerical simulation, and are in partial agreement with the reported
experimental results. We discuss the possible reasons for the remaining
discrepancies.Comment: 8 pages, 8 figures, submitted to Phys. Rev.
Smooth metric measure spaces, quasi-Einstein metrics, and tractors
We introduce the tractor formalism from conformal geometry to the study of
smooth metric measure spaces. In particular, this gives rise to a
correspondence between quasi-Einstein metrics and parallel sections of certain
tractor bundles. We use this formulation to give a sharp upper bound on the
dimension of the vector space of quasi-Einstein metrics, providing a different
perspective on some recent results of He, Petersen and Wylie.Comment: 33 pages; final versio
Lorentz-violating effects on topological defects generated by two real scalar fields
The influence of a Lorentz-violation on soliton solutions generated by a
system of two coupled scalar fields is investigated. Lorentz violation is
induced by a fixed tensor coefficient that couples the two fields. The
Bogomol'nyi method is applied and first-order differential equations are
obtained whose solutions minimize energy and are also solutions of the
equations of motion. The analysis of the solutions in phase space shows how the
stability is modified with the Lorentz violation. It is shown explicitly that
the solutions preserve linear stability despite the presence of Lorentz
violation. Considering Lorentz violation as a small perturbation, an analytical
method is employed to yield analytical solutions.Comment: (9 pages, 11 figures
Pathway Analysis Integrating Genome-Wide and Functional Data Identifies PLCG2 as a Candidate Gene for Age-Related Macular Degeneration
PURPOSE. Age-related macular degeneration (AMD) is the worldwide leading cause of blindness
among the elderly. Although genome-wide association studies (GWAS) have identified AMD
risk variants, their roles in disease etiology are not well-characterized, and they only explain a
portion of AMD heritability.
METHODS. We performed pathway analyses using summary statistics from the International
AMD Genomics Consortium’s 2016 GWAS and multiple pathway databases to identify
biological pathways wherein genetic association signals for AMD may be aggregating. We
determined which genes contributed most to significant pathway signals across the databases.
We characterized these genes by constructing protein-protein interaction networks and
performing motif analysis.
RESULTS. We determined that eight genes (C2, C3, LIPC, MICA, NOTCH4, PLCG2, PPARA, and
RAD51B) ‘‘drive’’ the statistical signals observed across pathways curated in the Kyoto
Encyclopedia of Genes and Genomes (KEGG), Reactome, and Gene Ontology (GO) databases.
We further refined our definition of statistical driver gene to identify PLCG2 as a candidate
gene for AMD due to its significant gene-level signals (P < 0.0001) across KEGG, Reactome,
GO, and NetPath pathways.
CONCLUSIONS. We performed pathway analyses on the largest available collection of advanced
AMD cases and controls in the world. Eight genes strongly contributed to significant pathways
from the three larger databases, and one gene (PLCG2) was central to significant pathways from
all four databases. This is, to our knowledge, the first study to identify PLCG2 as a candidate
gene for AMD based solely on genetic burden. Our findings reinforce the utility of integrating in
silico genetic and biological pathway data to investigate the genetic architecture of AMD
Synthesis and characterisation of HIP Ca0.80Ce0.20ZrTi1.60Cr0.40O7 zirconolite and observations of the ceramic–canister interface
A sample of zirconolite with nominal composition Ca0.80Ce0.20ZrTi1.60Cr0.40O7 was processed via Hot Isostatic Pressing (HIP), with a dwell temperature and pressure of 1320 °C/100 MPa maintained for 4 h. The produced wasteform was characterised by powder XRD, SEM–EDS, Ce L3 and Cr K-edge XANES. A significant portion of the Ce inventory did not fully partition within the zirconolite phase, instead remaining as CeO2 within the microstructure. Inspection of the stainless steel–ceramic interface detailed the presence of an interaction region dominated by a Cr-rich oxide layer. No significant Cr or Fe migration was observed, although a greater concentration of perovskite was observed at the periphery, relative to the bulk ceramic matrix. The X-ray absorption features of Cr remained analogous with Cr3+ accommodation within TiO6 octahedra in the zirconolite matrix. The absorption edge of Ce was comprised of contributions from zirconolite-2M and unincorporated CeO2, with an average oxidation state of Ce3.9+. As zirconolite-2M accounted for > 92 wt% of the overall phase assemblage, it is clear that the dominant oxidation state of Ce in this phase was Ce4+
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
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