291,765 research outputs found
Noncollinear magnetic order in quasicrystals
Based on Monte-Carlo simulations, the stable magnetization configurations of
an antiferromagnet on a quasiperiodic tiling are derived theoretically. The
exchange coupling is assumed to decrease exponentially with the distance
between magnetic moments. It is demonstrated that the superposition of
geometric frustration with the quasiperiodic ordering leads to a
three-dimensional noncollinear antiferromagnetic spin structure. The structure
can be divided into several ordered interpenetrating magnetic supertilings of
different energy and characteristic wave vector. The number and the symmetry of
subtilings depend on the quasiperiodic ordering of atoms.Comment: RevTeX, 4 pages, 5 low-resolution color figures (due to size
restrictions); to appear in Physical Review Letter
Radio continuum, far infrared and star formation
A very tight correlation was found between the radio emission and the far infrared emission from galaxies. This has been found for various samples of galaxies and is explained in terms of recent star formation. The tight correlation would imply that the total radio emission is a good tracer of star formation. The correlation between the radio power at 5 GHz and the far infrared luminosity is shown. The galaxies are of various morphological types and were selected from the various IRAS circulars, hence the sample is an infrared selected sample. The far infrared luminosities were corrected for the dust temperature. This is significant because it decreases the dispersion in the correlation
Searching for Stars in Compact High-Velocity Clouds. II
We address the hypothesis that High Velocity Clouds correspond to the
"missing" dwarf galaxies of the Local Group predicted by cosmological
simulations. To this end, we present optical and near-infrared photometry of
five additional High Velocity Clouds, one of which produces Lyman series
absorption on the sight line towards the Quasar Ton S210, with sufficient
resolution and sensitivity to enable the detection of an associated stellar
content. We do not detect significant stellar populations intrinsic to any of
the five clouds. In combination with the results from our paper I, which had
yielded non detections of stellar content in another five cases, we find that
there is a 50% chance of getting a null result in ten trials if fewer than 7%
of all High Velocity Clouds contain stars. We conclude that the population of
High Velocity Clouds is an unlikely repository for the "missing" dwarfs of the
Local Group.Comment: 6 pages, 3 figures. submitted to MNRA
Kepler super-flare stars: what are they?
The Kepler mission has led to the serendipitous discovery of a significant
number of `super flares' - white light flares with energies between 10^33 erg
and 10^36 erg - on solar-type stars. It has been speculated that these could be
`freak' events that might happen on the Sun, too. We have started a programme
to study the nature of the stars on which these super flares have been
observed. Here we present high-resolution spectroscopy of 11 of these stars and
discuss our results. We find that several of these stars are very young,
fast-rotating stars where high levels of stellar activity can be expected, but
for some other stars we do not find a straightforward explanation for the
occurrence of super flares.Comment: Accepted for publication in A&A, 9 pages, 4 figure
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