2,592 research outputs found
Coordinate representation of particle dynamics in AdS and in generic static spacetimes
We discuss the quantum dynamics of a particle in static curved spacetimes in
a coordinate representation. The scheme is based on the analysis of the squared
energy operator E^2, which is quadratic in momenta and contains a scalar
curvature term. Our main emphasis is on AdS spaces, where this term is fixed by
the isometry group. As a byproduct the isometry generators are constructed and
the energy spectrum is reproduced. In the massless case the conformal symmetry
is realized as well. We show the equivalence between this quantization and the
covariant quantization, based on the Klein-Gordon type equation in AdS. We
further demonstrate that the two quantization methods in an arbitrary
(N+1)-dimensional static spacetime are equivalent to each other if the scalar
curvature terms both in the operator E^2 and in the Klein-Gordon type equation
have the same coefficient equal to (N-1)/(4N).Comment: 14 pages, no figures, typos correcte
Impact of the measured parameters of exoplanets on the inferred internal structure
Exoplanet characterization is one of the main foci of current exoplanetary
science. For super-Earths and sub-Neptunes, we mostly rely on mass and radius
measurements, which allow to derive the body's mean density and give a rough
estimate of the planet's bulk composition. However, the determination of
planetary interiors is a very challenging task. In addition to the uncertainty
in the observed fundamental parameters, theoretical models are limited due to
the degeneracy in determining the planetary composition. We aim to study
several aspects that affect internal characterization of super-Earths and
sub-Neptunes: observational uncertainties, location on the M-R diagram, impact
of additional constraints as bulk abundances or irradiation, and model
assumptions. We use a full probabilistic Bayesian inference analysis that
accounts for observational and model uncertainties. We employ a Nested Sampling
scheme to efficiently produce the posterior probability distributions for all
the planetary structural parameter of interest. We include a structural model
based on self-consistent thermodynamics of core, mantle, high-pressure ice,
liquid water, and H-He envelope. Regarding the effect of mass and radius
uncertainties on the determination of the internal structure, we find three
different regimes: below the Earth-like composition line and above the
pure-water composition line smaller observational uncertainties lead to better
determination of the core and atmosphere mass respectively, and between them
structure characterization only weakly depends on the observational
uncertainties. We show that small variations in the temperature or entropy
profiles lead to radius variations that are comparable to the observational
uncertainty, suggesting that uncertainties linked to model assumptions can
become more relevant to determine the internal structure than observational
uncertainties.Comment: 12 pages, 12 figure
Comparative study of gp130 cytokine effects on corticotroph AtT-20 cells - Redundancy or specificity of neuroimmunoendocrine modulators?
Objective: This comparative in vitro study examined the effects of all known gp130 cytokines on murine corticotroph AtT-20 cell function. Methods: Cytokines were tested at equimolar concentrations from 0.078 to 10 nM. Tyrosine phosphorylation of the signal transducer and activator of transcription ( STAT) 3 and STAT1, the STAT-dependent suppressor of cytokine signaling (SOCS)-3 promoter activity, SOCS-3 gene expression, STAT-dependent POMC promoter activity and adrenocorticotropic hormone ( ACTH) secretion were determined. Results: Leukemia inhibitory factor (LIF), human oncostatin M (OSM) and cardiotrophin (CT)-1 (LIFR/gp130 ligands), as well as ciliary neurotrophic factor ( CNTF) and novel neurotrophin1/B-cell stimulating factor-3 (CNTFRalpha/LIFR/gp130 ligands) are potent stimuli of corticotroph cells in vitro. In comparison, interleukin (IL)-6 (IL-6R/gp130 ligand) and IL-11 (IL-11R/gp130 ligand) exhibited only modest direct effects on corticotrophs, while murine OSM (OSMR/gp130 ligand) showed no effect. Conclusion: (i) CNTFR complex ligands are potent stimuli of corticotroph function, comparable to LIFR complex ligands; (ii) IL-6 and IL-11 are relatively weak direct stimuli of corticotroph function; (iii) differential effects of human and murine OSM suggest that LIFR/gp130 (OSMR type I) but not OSMR/gp130 (OSMR type II) are involved in corticotroph signaling. (iv) CT-1 has the hitherto unknown ability to stimulate corticotroph function, and (v) despite redundant immuno-neuroendocrine effects of different gp130 cytokines, corticotroph cells are preferably activated through the LIFR and CNTFR complexes. Copyright (C) 2004 S. Karger AG, Basel
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
Parameterized Directed -Chinese Postman Problem and Arc-Disjoint Cycles Problem on Euler Digraphs
In the Directed -Chinese Postman Problem (-DCPP), we are given a
connected weighted digraph and asked to find non-empty closed directed
walks covering all arcs of such that the total weight of the walks is
minimum. Gutin, Muciaccia and Yeo (Theor. Comput. Sci. 513 (2013) 124--128)
asked for the parameterized complexity of -DCPP when is the parameter.
We prove that the -DCPP is fixed-parameter tractable.
We also consider a related problem of finding arc-disjoint directed
cycles in an Euler digraph, parameterized by . Slivkins (ESA 2003) showed
that this problem is W[1]-hard for general digraphs. Generalizing another
result by Slivkins, we prove that the problem is fixed-parameter tractable for
Euler digraphs. The corresponding problem on vertex-disjoint cycles in Euler
digraphs remains W[1]-hard even for Euler digraphs
Evaluating the performance of ionic liquid coatings for mitigation of spacecraft surface charges
To reduce the impact of charging effects on satellites, cheap and lightweight
conductive coatings are desirable. We mimic space-like charging environments in
ultra-high vacuum (UHV) chambers during deposition of charges via the electron
beam of a scanning electron microscope (SEM). We use the charge induced
signatures in SEM images of a thin ionic liquid (IL) film on insulating
surfaces such as glass, to assess the general performance of such coatings. In
order to get a reference structure in SEM, the samples were structured by
nanosphere lithography and coated with IL. The IL film (we choose BMP DCA, due
to its beneficial physical properties) was applied ex situ and a thickness of
10 to 30 nm was determined by reflectometry. Such an IL film is stable under
vacuum conditions. It would also only lead to additional mass of below 20
mg/m. At about 5 A/m e/(sm), a typical
sample charging rate in SEM, imaging is possible with no noticeable contrast
changes over many hours; this electron current density is already 6 orders of
magnitudes higher than "worst case geosynchronous environments" of
A/m. Measurements of the surface potential are used for
further insights in the reaction of IL films to the electron beam of a SEM.
Participating mechanisms such as polarization or reorientation will are
discussed.Comment: Submitted to Proceedings of the 14th IAA Symposium on Small
Satellites for Earth System Observatio
Loop Equation in Two-dimensional Noncommutative Yang-Mills Theory
The classical analysis of Kazakov and Kostov of the Makeenko-Migdal loop
equation in two-dimensional gauge theory leads to usual partial differential
equations with respect to the areas of windows formed by the loop. We extend
this treatment to the case of U(N) Yang-Mills defined on the noncommutative
plane. We deal with all the subtleties which arise in their two-dimensional
geometric procedure, using where needed results from the perturbative
computations of the noncommutative Wilson loop available in the literature. The
open Wilson line contribution present in the non-commutative version of the
loop equation drops out in the resulting usual differential equations. These
equations for all N have the same form as in the commutative case for N to
infinity. However, the additional supplementary input from factorization
properties allowing to solve the equations in the commutative case is no longer
valid.Comment: 20 pages, 3 figures, references added, small clarifications adde
Quantum-Gravitational Diffusion and Stochastic Fluctuations in the Velocity of Light
We argue that quantum-gravitational fluctuations in the space-time background
give the vacuum non-trivial optical properties that include diffusion and
consequent uncertainties in the arrival times of photons, causing stochastic
fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated
within a Liouville string formulation of quantum gravity that also suggests a
frequency-dependent refractive index of the particle vacuum. We construct an
explicit realization by treating photon propagation through quantum excitations
of -brane fluctuations in the space-time foam. These are described by
higher-genus string effects, that lead to stochastic fluctuations in couplings,
and hence in the velocity of light. We discuss the possibilities of
constraining or measuring photon diffusion ``in vacuo'' via -ray
observations of distant astrophysical sources.Comment: 17 pages LATEX, uses axodraw style fil
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