24,273 research outputs found
Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes
The optical identification of large number of X-ray sources such as those
from the ROSAT All-Sky Survey is challenging with conventional spectroscopic
follow-up observations. We investigate two ROSAT All-Sky Survey fields of size
10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b
= -5 deg (Sge), in order to optically identify the majority of sources. We used
optical variability, among other more standard methods, as a means of
identifying a large number of ROSAT All-Sky Survey sources. All objects fainter
than about 12 mag and brighter than about 17 mag, in or near the error circle
of the ROSAT positions, were tested for optical variability on hundreds of
archival plates of the Sonneberg field patrol.
The present paper contains probable optical identifications of altogether 256
of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some of
them may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptive
double stars (mostly CVs), 43 chromospherically active stars, 65 stars brighter
than about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variable
stars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnant
and 1 planetary nebula.
X-ray emission is, expectedly, tightly correlated with optical variability,
and thus our new method for optically identifying X-ray sources is demonstrated
to be feasible.Comment: 92 pages, 521 figures, A&A (accepted
Green's function theory of quasi-two-dimensional spin-half Heisenberg ferromagnets: stacked square versus stacked kagom\'e lattice
We consider the thermodynamic properties of the quasi-two-dimensional
spin-half Heisenberg ferromagnet on the stacked square and the stacked kagom\'e
lattices by using the spin-rotation-invariant Green's function method. We
calculate the critical temperature , the uniform static susceptibility
, the correlation lengths and the magnetization and
investigate the short-range order above . We find that and at
are smaller for the stacked kagom\'e lattice which we attribute to
frustration effects becoming relevant at finite temperatures.Comment: shortened version as published in PR
Local physics of magnetization plateaux in the Shastry-Sutherland model
We address the physical mechanism responsible for the emergence of
magnetization plateaux in the Shastry-Sutherland model. By using a hierarchical
mean-field approach we demonstrate that a plateau is stabilized in a certain
{\it spin pattern}, satisfying {\it local} commensurability conditions derived
from our formalism. Our results provide evidence in favor of a robust local
physics nature of the plateaux states, and are in agreement with recent NMR
experiments on \scbo.Comment: 4 pages, LaTeX 2
The UH-1H helicopter icing flight test program: An overview
An ongoing joint NASA/Army program to study the effects of ice accretion on unprotected helicopter rotor aerodynamic performance is discussed. This program integrates flight testing, wind tunnel testing, and analytical modeling. Results are discussed for helicopter flight testing in the Canadian NRC hover spray rig facility to measure rotor aero performance degradation and document rotor ice accretion characteristics. The results of dry wind tunnel testing of airfoil sections with artificial ice accretions and predictions of rotor performance degradation using available rotor performance codes and the wind tunnel data are presented. An alternative approach to conducting future helicopter icing flight programs is discussed
Emergent Ising degrees of freedom in frustrated two-leg ladder and bilayer Heisenberg antiferromagnets
Based on exact diagonalization data for finite quantum Heisenberg
antiferromagnets on two frustrated lattices (two-leg ladder and bilayer) and
analytical arguments we map low-energy degrees of freedom of the spin models in
a magnetic field on classical lattice-gas models. Further we use
transfer-matrix calculations and classical Monte Carlo simulations to give a
quantitative description of low-temperature thermodynamics of the quantum spin
models. The classical lattice-gas model yields an excellent description of the
quantum spin models up to quite large temperatures. The main peculiarity of the
considered frustrated bilayer is a phase transition which occurs at low
temperatures for a wide range of magnetic fields below the saturation magnetic
field and belongs to the two-dimensional Ising model universality class.Comment: 17 pages, 8 figure
The structure and possible origins of stacking faults in gamma-yttrium disilicate
Parallel stacking faults on (010) planes are frequently observed in hot-pressed Y2Si2O7. A combination of conventional dark-field imaging and high-resolution transmission electron microscopy was used to investigate the structure of these faults and it was found that they consist of the repeat of one layer of the two layer γ-Y2Si2O7 structure with an associated in-plane rigid body displacement. The resulting structure was confirmed by image simulation of high-resolution images from two perpendicular projections. A model for the formation of the stacking faults is proposed as a consequence of a transformation from β-Y2Si2O7 to γ-Y2Si2O7 in the hot pressing
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
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