6,968 research outputs found
Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets
We investigate antiferromagnetic spin chains, which are coupled by a weak
antiferromagnetic exchange interaction on a hexagonal lattice. We particulary
study the role of the dipole-dipole interaction within the framework of a
Heisenberg model with nearest-neighbor exchange and additional dipolar
interaction. We find several commensurate and incommensurate phases depending
on the ratio of dipolar energy to interchain-exchange energy due to their
competing qualtity. The ground-state analysis is supplemented by a stability
analysis by means of a linear spin-wave theory. In comparison with experiments
(CsMnBr_3, RbMnBr_3) we obtain good agreement for the energy gaps. From this we
conclude, that the dipolar interaction is the most important source of
anisotropy in these Mn-compounds.Comment: 3 pages, 3 Postscript figure
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
Orbit of the Mercury-Manganese binary 41 Eridani
Context. Mercury-manganese (HgMn) stars are a class of slowly rotating
chemically peculiar main-sequence late B-type stars. More than two-thirds of
the HgMn stars are known to belong to spectroscopic binaries.
Aims. By determining orbital solutions for binary HgMn stars, we will be able
to obtain the masses for both components and the distance to the system.
Consequently, we can establish the position of both components in the
Hertzsprung-Russell diagram and confront the chemical peculiarities of the HgMn
stars with their age and evolutionary history.
Methods. We initiated a program to identify interferometric binaries in a
sample of HgMn stars, using the PIONIER near-infrared interferometer at the
VLTI on Cerro Paranal, Chile. For the detected systems, we intend to obtain
full orbital solutions in conjunction with spectroscopic data.
Results. The data obtained for the SB2 system 41 Eridani allowed the
determination of the orbital elements with a period of just five days and a
semi-major axis of under 2 mas. Including published radial velocity
measurements, we derived almost identical masses of 3.17 +/- 0.07 M_Sun for the
primary and 3.07 +/- 0.07 M_Sun for the secondary. The measured magnitude
difference is less than 0.1 mag. The orbital parallax is 18.05 +/- 0.17 mas,
which is in good agreement with the Hipparcos trigonometric parallax of 18.33
+/- 0.15 mas. The stellar diameters are resolved as well at 0.39 +/- 0.03 mas.
The spin rate is synchronized with the orbital rate.Comment: 5 pages, 4 figure
The First Supernovae: Source Density And Observability Of Pair Instability Supernovae
Theoretical models predict that some of the first stars ended their lives as extremely energetic Pair Instability Supernovae (PISNe). With energies approaching 10(53) erg, these supernovae are expected to be within the detection limits of the upcoming James Webb Space Telescope (JWST) allowing observational constraints to be placed on the properties of the first stars. We estimate the source density of PISNe using a semi-analytic Press-Schecter based approach informed by cosmological simulations, with an upper limit of similar to 0.2 PISNe visible per JWST field of view at any given time. We find that the main obstacle to observing PISNe is their scarcity rather than their faintness. Given this we suggest a mosaic style search strategy for detecting PISNe from the first stars.Astronom
Comparison of trace metal bioavailabilities in European coastal waters using mussels from Mytilus edulis
Mussels from Mytilus edulis complex were used as biomonitors of the trace metals Fe, Mn, Pb, Zn, and Cu at 17 sampling sites to assess the relative bioavailability of metals in coastal waters around the European continent. Because accumulated metal concentrations in a given area can differ temporally, data were corrected for the effect of season before large-scale spatial comparisons were made. The highest concentration of Fe was noted in the North Sea and of Mn in the Baltic. Increased tissue concentrations of Pb were recorded in the mussels from the Bay of Biscay and the Baltic Sea. Low concentrations of metals were determined in the mussels from the Mediterranean Sea and the Northern Baltic. Relatively low geographic variations of Cu and Zn indicate that mussels are able to partially regulate accumulated body concentrations, which means Cu and Zn are, to some extent, independent of environmental concentrations
Antigen-driven clonal proliferation of B cells within the target tissue of an autoimmune disease: the salivary glands of patients with Sjögren's syndrome
Structures resembling germinal centers are seen in the salivary glands of patients with Sjögren's syndrome, but it is not known whether the microenvironment of these cell clusters is sufficient for the induction of a germinal center response. Therefore, we cloned and sequenced rearranged Ig V genes expressed by B cells isolated from sections of labial salivary gland biopsies from two Sjögren's syndrome patients. Rearranged V genes from B cells within one cell cluster were polyclonal and most had few somatic mutations. Two adjacent clusters from another patient each contained one dominant B cell clone expressing hypermutated V genes. None of the rearranged V genes was found in both clusters, suggesting that cells are unable to migrate out into the surrounding tissue and seed new clusters. The ratios of replacement to silent mutations in the framework and complementarity determining regions suggest antigen selection of high-affinity mutants. These results show that an antigen-driven, germinal center-type B cell response is taking place within the salivary glands of Sjögren's syndrome patients. In view of the recent demonstration of a germinal center response within the rheumatoid synovial membrane and the existence of similar structures in the target tissues of other autoimmune. diseases, we propose that germinal center- type responses can be induced in the nonlymphoid target tissues of a variety of autoimmune diseases
PHP132 - Value of multi-criteria decision analysis in early assessment of medical diagnostic devices
OBJECTIVES:\ud
Multicriteria decision analytic (MCDA) techniques are a powerful tool in evaluating health care interventions where multiple, often competing, factors need to be considered. The analytic hierarchy process (AHP) is one such technique. We have applied AHP to evaluate medical diagnostic technologies as to support product development and market access. The main objective of this study is to show feasibility of AHP in terms of applicability and outcome. We therefore have studied the expected performance of the Photoacoustic Mammoscope (PAM), a new imaging device for detection of breast cancer based on photoacoustic imaging.\ud
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METHODS:\ud
Before starting the study we defined different diagnostic and screening scenarios for the PAM including criteria that are important in the acceptance of the device. We finally defined four strategies including mammography, ultrasound and MRI imaging. The Analytic Hierarchy Process (AHP) analysis was then used to identify the relative importance of evaluation criteria, and to identify the relative performance of four different breast cancer imaging techniques.\ud
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RESULTS:\ud
According to the expert panel the most important criterion in the design of a diagnostic breast imaging device is sensitivity (overall weight was 54%). Factors that mainly determine the performance on sensitivity are the visualization of mass margins, mass shape and vascularization. Second most important criterion was safety.\ud
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CONCLUSIONS:\ud
From an early assessment of photoacoustic imaging it may be concluded that the potential clinical performance justifies further development and implementation of PAM. However, further research on the use of AHP in medical product development is required. We present and discuss some ideas to further integrate clinical evidence into MCDA using Bayesian techniques as well as sensitivity analyses on the model inputs
Jet stability, dynamics and energy transport
Relativistic jets carry energy and particles from compact to very large
scales compared with their initial radius. This is possible due to their
remarkable collimation despite their intrinsic unstable nature. In this
contribution, I review the state-of-the-art of our knowledge on instabilities
growing in those jets and several stabilising mechanisms that may give an
answer to the question of the stability of jets. In particular, during the last
years we have learned that the limit imposed by the speed of light sets a
maximum amplitude to the instabilities, contrary to the case of classical jets.
On top of this stabilising mechanism, the fast growth of unstable modes with
small wavelengths prevents the total disruption and entrainment of jets. I also
review several non-linear processes that can have an effect on the collimation
of extragalactic and microquasar jets. Within those, I remark possible causes
for the decollimation and decelleration of FRI jets, as opposed to the
collimated FRII's. Finally, I give a summary of the main reasons why jets can
propagate through such long distances.Comment: For the proceedings of High Energy Phenomena in Relativistic Outflows
III (HEPRO III, IJMPD, accepted). 12 page
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