10,683 research outputs found
Casimir repulsion between metallic objects in vacuum
We give an example of a geometry in which two metallic objects in vacuum
experience a repulsive Casimir force. The geometry consists of an elongated
metal particle centered above a metal plate with a hole. We prove that this
geometry has a repulsive regime using a symmetry argument and confirm it with
numerical calculations for both perfect and realistic metals. The system does
not support stable levitation, as the particle is unstable to displacements
away from the symmetry axis.Comment: 4 pages, 4 figures; added references, replaced Fig.
Calculation of nonzero-temperature Casimir forces in the time domain
We show how to compute Casimir forces at nonzero temperatures with
time-domain electromagnetic simulations, for example using a finite-difference
time-domain (FDTD) method. Compared to our previous zero-temperature
time-domain method, only a small modification is required, but we explain that
some care is required to properly capture the zero-frequency contribution. We
validate the method against analytical and numerical frequency-domain
calculations, and show a surprising high-temperature disappearance of a
non-monotonic behavior previously demonstrated in a piston-like geometry.Comment: 5 pages, 2 figures, submitted to Physical Review A Rapid
Communicatio
Microstructure Effects for Casimir Forces in Chiral Metamaterials
We examine a recent prediction for the chirality-dependence of the Casimir
force in chiral metamaterials by numerical computation of the forces between
the exact microstructures, rather than homogeneous approximations. We compute
the exact force for a chiral bent-cross pattern, as well as forces for an
idealized "omega"-particle medium in the dilute approximation and identify the
effects of structural inhomogeneity (i.e. proximity forces and anisotropy). We
find that these microstructure effects dominate the force for separations where
chirality was predicted to have a strong influence. To get observations of
chirality free from microstructure effects, one must go to large separations
where the effect of chirality is at most of the total force.Comment: 5 pages, 4 figure
Black Hole Scattering from Monodromy
We study scattering coefficients in black hole spacetimes using analytic
properties of complexified wave equations. For a concrete example, we analyze
the singularities of the Teukolsky equation and relate the corresponding
monodromies to scattering data. These techniques, valid in full generality,
provide insights into complex-analytic properties of greybody factors and
quasinormal modes. This leads to new perturbative and numerical methods which
are in good agreement with previous results.Comment: 28 pages + appendices, 2 figures. For Mathematica calculation of
Stokes multipliers, download "StokesNotebook" from
https://sites.google.com/site/justblackholes/techy-zon
Sussing merger trees: a proposed merger tree data format
We propose a common terminology for use in describing both temporal merger trees and spatial structure trees for dark-matter halos. We specify a unified data format in HDF5 and provide example I/O routines in C, FORTRAN and PYTHON
Titan Science with the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the
successor to the Hubble Space Telescope (HST) but with a significantly larger
aperture (6.5 m) and advanced instrumentation focusing on infrared science
(0.6-28.0 m ). In this paper we examine the potential for scientific
investigation of Titan using JWST, primarily with three of the four
instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be
complementary. Five core scientific themes are identified: (i) surface (ii)
tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and
(v) stratospheric hazes. We discuss each theme in depth, including the
scientific purpose, capabilities and limitations of the instrument suite, and
suggested observing schemes. We pay particular attention to saturation, which
is a problem for all three instruments, but may be alleviated for NIRCam
through use of selecting small sub-arrays of the detectors - sufficient to
encompass Titan, but with significantly faster read-out times. We find that
JWST has very significant potential for advancing Titan science, with a
spectral resolution exceeding the Cassini instrument suite at near-infrared
wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In
particular, JWST will be valuable for time-domain monitoring of Titan, given a
five to ten year expected lifetime for the observatory, for example monitoring
the seasonal appearance of clouds. JWST observations in the post-Cassini period
will complement those of other large facilities such as HST, ALMA, SOFIA and
next-generation ground-based telescopes (TMT, GMT, EELT).Comment: 50 pages, including 22 figures and 2 table
Feeding versus Feedback in NGC 4151 probed with Gemini NIFS. I. Excitation
We have used the Gemini Near-infrared Integral Field Spectrograph (NIFS) to
map the emission-line intensity distributions and ratios in the Narrow-Line
Region (NLR) of the Seyfert galaxy NGC 4151 in the Z, J, H and K bands at a
resolving power ~ 5000, covering the inner 200 pc x 300 pc of the galaxy at a
spatial resolution of 8 pc. We present intensity distributions I(r) in 14
emission lines. (1) For the ionized gas, I(r) is extended to ~ 100 pc from the
nucleus along pos. angle PA=60/240 deg-- NE--SW), consistent with an origin in
the known biconical outflow; while for the recombination lines I(r) ~ r^-1, for
the forbidden lines I(r) is flat (r^0). (2) The H_2 emission lines intensity
distributions avoid the region of the bicone, extending to r ~ 60 pc,
perpendicular to the bicone axis, supporting an origin for the H_2-emitting gas
in the galaxy plane. (3) The coronal lines show a steep intensity profile,
described by r^-2. Using the line-ratio maps [Fe II]1.644/1.257 and Pa_b/Br_g
we obtain a reddening of E(B-V)~0.5 along the NLR and E(B-V)>1 at the nucleus.
Our line-ratio map [Fe II] 1.257/[P II] 1.189 is the first such map of an
extragalactic source. Together with the [Fe II]/Pa_b map, these line ratios
correlate with the radio intensity distribution, mapping the effects of shocks
produced by the radio jet, which probably release the Fe locked in grains and
produce the enhancement of the [Fe II] emission observed at ~ 1 arcsec from the
nucleus. At these regions, we obtain densities N_e ~4000 cm^-3 and temperatures
T_e ~ 15000K for the [Fe II]-emitting gas. For the H_2-emitting gas we obtain T
~ 2100K. The distinct intensity distributions, physical properties and
locations of the ionized and molecular gas suggest that the H_2-emitting gas
traces the AGN feeding, while the ionized gas traces its feedback.Comment: 22 pages. 14 figures, accepted for publication in MNRA
Hypoxia Reduces the Pathogenicity of Pseudomonas Aeruginosa by Decreasing the Expression of Multiple Virulence Factors
Our understanding of how the course of opportunistic bacterial infection is influenced by the microenvironment is limited. We demonstrate that the pathogenicity of Pseudomonas aeruginosa strains derived from acute clinical infections is higher than that of strains derived from chronic infections, where tissues are hypoxic. Exposure to hypoxia attenuated the pathogenicity of strains from acute (but not chronic) infections, implicating a role for hypoxia in regulating bacterial virulence. Mass spectrometric analysis of the secretome of P. aeruginosa derived from an acute infection revealed hypoxia-induced repression of multiple virulence factors independent of altered bacterial growth. Pseudomonas aeruginosa lacking the Pseudomonas prolyl-hydroxylase domain-containing protein, which has been implicated in bacterial oxygen sensing, displays reduced virulence factor expression. Furthermore, pharmacological hydroxylase inhibition reduces virulence factor expression and pathogenicity in a murine model of pneumonia. We hypothesize that hypoxia reduces P. aeruginosa virulence at least in part through the regulation of bacterial hydroxylases
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