2,640 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
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
V-like formations in flocks of artificial birds
We consider flocks of artificial birds and study the emergence of V-like
formations during flight. We introduce a small set of fully distributed
positioning rules to guide the birds' movements and demonstrate, by means of
simulations, that they tend to lead to stabilization into several of the
well-known V-like formations that have been observed in nature. We also provide
quantitative indicators that we believe are closely related to achieving V-like
formations, and study their behavior over a large set of independent
simulations
Ultralong-range Rydberg molecules
We review ultralong-range Rydberg molecules (ULRM), which are bound states
between a Rydberg atom and one or more ground-state atoms with bond lengths on
the order of thousands of Bohr radii. The binding originates from multiple
electron-atom scattering and leads to exotic oscillatory potential energy
surfaces that reflect the probability density of the Rydberg electron. This
unconventional binding mechanism opens fascinating possibilities to tune
molecular properties via weak external fields, to study spin-resolved
low-energy electron-atom scattering as well as to control and to probe
interatomic forces in few- and many-body systems. Here, we provide an overview
on recent theoretical and experimental progress in the field with an emphasis
on polyatomic ULRMs, field control and spin interactions.Comment: 14 pages, 8 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
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
A purely geometric distance to the binary star Atlas, a member of the Pleiades
We present radial velocity and new interferometric measurements of the double
star Atlas, which permit, with the addition of published interferometric data,
to precisely derive the orbital parameters of the binary system and the masses
of the components. The derived semi-major axis, compared with its measured
angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely
geometrical way. Under the assumption that the location of Atlas is
representative of the average distance of the cluster, we confirm the distance
value generally obtained through main sequence fitting, in contradiction with
the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
Observational Evidence of Accretion Disk-Caused Jet Precession in Galactic Nuclei
We show that the observational data of extragalactic radio sources tend to
support the theoretical relationship between the jet precession period and the
optical luminosity of the sources, as predicted by the model in which an
accretion disk causes the central black hole to precess.Comment: 13 pages, 1 figure, accepted for publication in ApJ Letter
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