10,720 research outputs found
Adapting to variable prismatic displacement
In each of two studies, subjects were exposed to a continuously changing prismatic displacement with a mean value of 19 prism diopters (variable displacement) and to a fixed 19-diopter displacement (fixed displacement). In Experiment 1, significant adaptation (post-pre shifts in hand-eye coordination) was found for fixed, but not for variable, displacement. Experiment 2 demonstrated that adaptation was obtained for variable displacement, but it was very fragile and is lost if the measures of adaptation are preceded by even a very brief exposure of the hand to normal or near-normal vision. Contrary to the results of some previous studies, an increase in within-S dispersion was not found of target pointing responses as a result of exposure to variable displacement
Radiation trapping in coherent media
We show that the effective decay rate of Zeeman coherence, generated in a
Rb87 vapor by linearly polarized laser light, increases significantly with the
atomic density. We explain this phenomenon as the result of radiation trapping.
Our study shows that radiation trapping must be taken into account to fully
understand many electromagnetically induced transparency experiments with
optically thick media
On the Interpretation of Supernova Light Echo Profiles and Spectra
The light echo systems of historical supernovae in the Milky Way and local
group galaxies provide an unprecedented opportunity to reveal the effects of
asymmetry on observables, particularly optical spectra. Scattering dust at
different locations on the light echo ellipsoid witnesses the supernova from
different perspectives and the light consequently scattered towards Earth
preserves the shape of line profile variations introduced by asymmetries in the
supernova photosphere. However, the interpretation of supernova light echo
spectra to date has not involved a detailed consideration of the effects of
outburst duration and geometrical scattering modifications due to finite
scattering dust filament dimension, inclination, and image point-spread
function and spectrograph slit width. In this paper, we explore the
implications of these factors and present a framework for future resolved
supernova light echo spectra interpretation, and test it against Cas A and SN
1987A light echo spectra. We conclude that the full modeling of the dimensions
and orientation of the scattering dust using the observed light echoes at two
or more epochs is critical for the correct interpretation of light echo
spectra. Indeed, without doing so one might falsely conclude that differences
exist when none are actually present.Comment: 18 pages, 22 figures, accepted for publication in Ap
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
BIMA and Keck Imaging of the Radio Ring PKS 1830-211
We discuss BIMA (Berkeley Illinois Maryland Association) data and present new
high quality optical and near-IR Keck images of the bright radio ring PKS
1830-211. Applying a powerful new deconvolution algorithm we have been able to
identify both images of the radio source. In addition we recover an extended
source in the optical, consistent with the expected location of the lensing
galaxy. The source counterparts are very red, I-K=7, suggesting strong Galactic
absorption with additional absorption by the lensing galaxy at z=0.885, and
consistent with the detection of high redshift molecules in the lens.Comment: To be published in the ASP Conference Proceedings, 'Highly Redshifted
Radio Lines', Greenbank, W
Nonlinear Magneto-Optical Rotation of Elliptically Polarized Light
We predict theoretically and demonstrate experimentally an ellipticity-dependent nonlinear magneto-optic rotation of elliptically-polarized light propagating in a coherent atomic medium. We show that this effect results from a hexadecapole and higher order momenta of atomic coherence, and is associated with an enhancement of Kerr and higher orders nonlinearities accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. These nonlinearities might be useful for quantum signal processing. In particular, we report an observation of an enhancement the polarization rotation of elliptically polarized light resonant with the 5S_{1/2} F=2 -> 5P_{1/2} F=1 transition of Rb87
Effect of Magnetization Inhomogeneity on Magnetic Microtraps for Atoms
We report on the origin of fragmentation of ultracold atoms observed on a
permanent magnetic film atom chip. A novel technique is used to characterize
small spatial variations of the magnetic field near the film surface using
radio frequency spectroscopy of the trapped atoms. Direct observations indicate
the fragmentation is due to a corrugation of the magnetic potential caused by
long range inhomogeneity in the film magnetization. A model which takes into
account two-dimensional variations of the film magnetization is consistent with
the observations.Comment: 4 pages, 4 figure
Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers
The nematic twist-bend (NTB) phase, exhibited by certain thermotropic liquid crystalline (LC) dimers, represents a new orientationally ordered mesophase -- the first distinct nematic variant discovered in many years. The NTB phase is distinguished by a heliconical winding of the average molecular long axis (director) with a remarkably short (nanoscale) pitch and, in systems of achiral dimers, with an equal probability to form right- and left-handed domains. The NTB structure thus provides another fascinating example of spontaneous chiral symmetry breaking in nature. The order parameter driving the formation of the heliconical state has been theoretically conjectured to be a polarization field, deriving from the bent conformation of the dimers, that rotates helically with the same nanoscale pitch as the director field. It therefore presents a significant challenge for experimental detection. Here we report a second harmonic light scattering (SHLS) study on two achiral, NTB-forming LCs, which is sensitive to the polarization field due to micron-scale distortion of the helical structure associated with naturally-occurring textural defects. These defects are parabolic focal conics of smectic-like ``pseudo-layers", defined by planes of equivalent phase in a coarse-grained description of the NTB state. Our SHLS data are explained by a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a linear coupling between the two
- …