351 research outputs found
Search for Solar Axions Produced in the Reaction
A search for the axioelectric absorption of 5.5-MeV solar axions produced in
the reaction was performed with
two BGO detectors placed inside a low-background setup. A model independent
limit on axion-photon and axion-nucleon couplings was obtained: . Constraints on the axion-electron
coupling constant were obtained for axions with masses in the MeV
range: . The solar positron flux from
decay was determined for axions with masses . Using the existing experimental data on the interplanetary positron
flux, a new constraint on the axion-electron coupling constant for axions with
masses in the MeV range was obtained: .Comment: 6 pages, 5 figure
Phase Diagram of a Spin Ladder with Cyclic Four Spin Exchange
We present the phase diagram of the Heisenberg model on the two leg
ladder with cyclic four spin exchange, determined by a combination of Exact
Diagonalization and Density Matrix Renormalization Group techniques. We find
six different phases and regimes: the rung singlet phase, a ferromagnetic
phase, two symmetry broken phases with staggered dimers and staggered scalar
chiralities, and a gapped region with dominant vector chirality or collinear
spin correlations. We localize the phase transitions and investigate their
nature.Comment: 4 pages, 6 figures, REVTeX 4, published versio
Long slit Spectropolarimetry of Jupiter and Saturn
We present ground-based limb polarization measurements of Jupiter and Saturn
consisting of full disk imaging polarimetry for the wavelength 7300 A and
spatially resolved (long slit) spectropolarimetry covering the wavelength range
5200 to 9350 A.
For the polar region of Jupiter we find for wl=6000 A a very strong radial
polarization with a seeing corrected maximum of about +11.5% in the South and
+10.0% in the North. Our model calculations demonstrate that the high limb
polarization can be explained by strongly polarizing (p~1.0), high albedo
(omega~0.98) haze particles with a scattering asymmetry parameter of g~0.6 as
expected for aggregate particles. The deduced particle parameters are
distinctively different when compared to lower latitude regions.
The spectropolarimetry of Jupiter shows a decrease in the polar limb
polarization towards longer wavelengths and a significantly enhanced
polarization in strong methane bands when compared to the adjacent continuum.
For lower latitudes the fractional polarization is small, negative, and it
depends only little on wavelength except for the strong CH4-band at 8870 A.
The South pole of Saturn shows a lower polarization (p~1.0-1.5%) than the
poles of Jupiter. The spectropolarimetric signal for Saturn decrease rapidly
with wavelength and shows no significant enhancements in the fractional
polarization in the absorption bands. These properties can be explained by a
vertically extended stratospheric haze region composed of small particles
<100nm.
In addition we find a previously not observed strong polarization feature
(p=1.5-2.0%) near the equator of Saturn. The origin of this polarization signal
is unclear but it could be related to a seasonal effect.Comment: Accepted for publication in ICARU
High-contrast imaging constraints on gas giant planet formation - The Herbig Ae/Be star opportunity
Planet formation studies are often focused on solar-type stars, implicitly
considering our Sun as reference point. This approach overlooks, however, that
Herbig Ae/Be stars are in some sense much better targets to study planet
formation processes empirically, with their disks generally being larger,
brighter and simply easier to observe across a large wavelength range. In
addition, massive gas giant planets have been found on wide orbits around early
type stars, triggering the question if these objects did indeed form there and,
if so, by what process. In the following I briefly review what we currently
know about the occurrence rate of planets around intermediate mass stars,
before discussing recent results from Herbig Ae/Be stars in the context of
planet formation. The main emphasis is put on spatially resolved polarized
light images of potentially planet forming disks and how these images - in
combination with other data - can be used to empirically constrain (parts of)
the planet formation process. Of particular interest are two objects, HD100546
and HD169142, where, in addition to intriguing morphological structures in the
disks, direct observational evidence for (very) young planets has been
reported. I conclude with an outlook, what further progress we can expect in
the very near future with the next generation of high-contrast imagers at 8-m
class telescopes and their synergies with ALMA.Comment: Accepted by Astrophysics and Space Science as invited short review in
special issue about Herbig Ae/Be stars; 12 pages incl. 5 figures, 2 tables
and reference
Effects of Population Based Screening for Chlamydia Infections in The Netherlands Limited by Declining Participation Rates
Background: A large trial to investigate the effectiveness of population based screening for chlamydia infections was conducted in the Netherlands in 2008-2012. The trial was register based and consisted of four rounds of screening of women and men in the age groups 16-29 years in three regions in the Netherlands. Data were collected on participation rates and positivity rates per round. A modeling study was conducted to project screening effects for various screening strategies into the future. Methods and Findings: We used a stochastic network simulation model incorporating partnership formation and dissolution, aging and a sexual life course perspective. Trends in baseline rates of chlamydia testing and treatment were used to describe the epidemiological situation before the start of the screening program. Data on participation rates was used to describe screening uptake in rural and urban areas. Simulations were used to project the effectiveness of screening on chlamydia prevalence for a time period of 10 years. In addition, we tested alternative screening strategies, such as including only women, targeting different age groups, and biennial screening. Screening reduced prevalence by about 1% in the first two screening rounds and leveled off after that. Extrapolating observed participation rates into the future indicated very low participation in the long run. Alternative strategies only marginally changed the effectiveness of screening. Higher participation rates as originally foreseen in the program would have succeeded in reducing chlamydia prevalence to very low levels in the long run. Conclusions: Decreasing participation rates over time profoundly impact the effectiveness of population based screening for chlamydia infections. Using data from several consecutive rounds of screening in a simulation model enabled us to assess the future effectiveness of screening on prevalence. If participation rates cannot be kept at a sufficient level, the effectiveness of screening on prevalence will remain limited
HD 142527: quantitative disk polarimetry with SPHERE
We present high-precision photometry and polarimetry for the protoplanetary
disk around HD142527, with a focus on determining the light scattering
parameters of the dust. We re-reduced polarimetric differential imaging data of
HD142527 in the VBB (735 nm) and H-band (1625 nm) from the ZIMPOL and IRDIS
subinstruments of SPHERE/VLT. With polarimetry and photometry based on
reference star differential imaging, we were able to measure the linearly
polarized intensity and the total intensity of the light scattered by the
circumstellar disk with high precision. We used simple Monte Carlo simulations
of multiple light scattering by the disk surface to derive constraints for
three scattering parameters of the dust: the maximum polarization of , the asymmetry parameter , and the single-scattering albedo .
We measure a reflected total intensity of mJy and mJy
and a polarized intensity of mJy and mJy in the VBB
and H-band, respectively. We also find in the visual range a degree of
polarization that varies between on the far side of the disk and
on the near side. The disk shows a red color for the scattered light intensity
and the polarized intensity, which are about twice as high in the near-infrared
when compared to the visual. We determine with model calculations the
scattering properties of the dust particles and find evidence for strong
forward scattering (), relatively low single-scattering
albedo (), and high maximum polarization () at the surface on the far side of the disk for both observed
wavelengths. The optical parameters indicate the presence of large aggregate
dust particles, which are necessary to explain the high maximum polarization,
the strong forward-scattering nature of the dust, and the observed red disk
color.Comment: 20 pages, 14 figure
Linear and Second-order Optical Response of the III-V Mono-layer Superlattices
We report the first fully self-consistent calculations of the nonlinear
optical properties of superlattices. The materials investigated are mono-layer
superlattices with GaP grown on the the top of InP, AlP and GaAs (110)
substrates. We use the full-potential linearized augmented plane wave method
within the generalized gradient approximation to obtain the frequency dependent
dielectric tensor and the second-harmonic-generation susceptibility. The effect
of lattice relaxations on the linear optical properties are studied. Our
calculations show that the major anisotropy in the optical properties is the
result of strain in GaP. This anisotropy is maximum for the superlattice with
maximum lattice mismatch between the constituent materials. In order to
differentiate the superlattice features from the bulk-like transitions an
improvement over the existing effective medium model is proposed. The
superlattice features are found to be more pronounced for the second-order than
the linear optical response indicating the need for full supercell calculations
in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.
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