542 research outputs found
Tiny scale opacity fluctuations from VLBA, MERLIN and VLA observations of HI absorption toward 3C 138
The structure function of opacity fluctuations is a useful statistical tool
to study tiny scale structures of neutral hydrogen. Here we present high
resolution observation of HI absorption towards 3C 138, and estimate the
structure function of opacity fluctuations from the combined VLA, MERLIN and
VLBA data. The angular scales probed in this work are ~ 10-200 milliarcsec
(about 5-100 AU). The structure function in this range is found to be well
represented by a power law S_tau(x) ~ x^{beta} with index beta ~ 0.33 +/- 0.07
corresponding to a power spectrum P_tau(U) ~ U^{-2.33}. This is slightly
shallower than the earlier reported power law index of ~ 2.5-3.0 at ~ 1000 AU
to few pc scales. The amplitude of the derived structure function is a factor
of ~ 20-60 times higher than the extrapolated amplitude from observation of Cas
A at larger scales. On the other hand, extrapolating the AU scale structure
function for 3C 138 predicts the observed structure function for Cas A at the
pc scale correctly. These results clearly establish that the atomic gas has
significantly more structures in AU scales than expected from earlier pc scale
observations. Some plausible reasons are identified and discussed here to
explain these results. The observational evidence of a shallower slope and the
presence of rich small scale structures may have implications for the current
understanding of the interstellar turbulence.Comment: 6 pages, 5 figures. Accepted for publication in ApJ. The definitive
version will be available at http://iopscience.iop.org
Using Abrupt Changes in Magnetic Susceptibility within Type-II Superconductors to Explore Global Decoherence Phenomena
A phenomenon of a periodic staircase of macroscopic jumps in the admitted
magnetic field has been observed, as the magnitude of an externally applied
magnetic field is smoothly increased or decreased upon a superconducting (SC)
loop of type II niobium-titanium wire which is coated with a
non-superconducting layer of copper. Large temperature spikes were observed to
occur simultaneously with the jumps, suggesting brief transitions to the normal
state, caused by en masse motions of Abrikosov vortices. An experiment that
exploits this phenomenon to explore the global decoherence of a large
superconducting system will be discussed, and preliminary data will be
presented. Though further experimentation is required to determine the actual
decoherence rate across the superconducting system, multiple classical
processes are ruled out, suggesting that jumps in magnetic flux are fully
quantum mechanical processes which may correspond to large group velocities
within the global Cooper pair wavefunction.Comment: 13 pages, 4 figures, part of proceedings for FQMT 2011 conference in
Prague, Czech Republi
Can a charged ring levitate a neutral, polarizable object? Can Earnshaw's Theorem be extended to such objects?
Stable electrostatic levitation and trapping of a neutral, polarizable object
by a charged ring is shown to be theoretically impossible. Earnshaw's Theorem
precludes the existence of such a stable, neutral particle trap.Comment: 11 pages, 1 figur
Simulated SKA maps from Galactic 3D-emission models
(Abridged) We present maps for various Galactic longitudes and latitudes at
1.4 GHz, which is the frequency where deep SKA surveys are proposed. The maps
are about 1.5 deg in size and have an angular resolution of about 1.6 arcsec.
We analyse the maps in terms of their probability density functions (PDFs) and
structure functions. Total intensity emission is more smooth in the plane than
at high latitudes due to the different contributions from the regular and
random magnetic field. The high latitude fields show more extended polarized
emission and RM structures than those in the plane, where patchy emission
structures on very small scales dominate. The RM PDFs in the plane are close to
Gaussians, but clearly deviate from that at high latitudes. The RM structure
functions show smaller amplitudes and steeper slopes towards high latitudes.
These results emerge from the fact that much more turbulent cells are passed
through by the line-of-sights in the plane. Although the simulated random
magnetic field components distribute in 3D, the magnetic field spectrum
extracted from the structure functions of RMs conforms to 2D in the plane and
approaches 3D at high latitudes. This is partly related to the outer scale of
the turbulent magnetic field, but mainly to the different lengths of the
line-of-sights.Comment: 19 pages, 15 figures, accepted for publication in Astronomy and
Astrophysics. The sizes of figures have been significantly reduced. For the
version with full resolution, see
ftp://ftp.mpifr-bonn.mpg.de/outgoing/p098wre/sun-reich.pd
G28.17+0.05: An unusual giant HI cloud in the inner Galaxy
New 21 cm HI observations have revealed a giant HI cloud in the Galactic
plane that has unusual properties. It is quite well defined, about 150 pc in
diameter at a distance of 5 kpc, and contains as much as 100,000 Solar Masses
of atomic hydrogen. The outer parts of the cloud appear in HI emission above
the HI background, while the central regions show HI self-absorption. Models
which reproduce the observations have a core with a temperature <40 K and an
outer envelope as much as an order of magnitude hotter. The cold core is
elongated along the Galactic plane, whereas the overall outline of the cloud is
approximately spherical. The warm and cold parts of the HI cloud have a
similar, and relatively large, line width of approximately 7 km/s. The cloud
core is a source of weak, anomalously-excited 1720 MHz OH emission, also with a
relatively large line width, which delineates the region of HI self-absorption
but is slightly blue-shifted in velocity. The intensity of the 1720 MHz OH
emission is correlated with N(H) derived from models of the cold core. There is
12CO emission associated with the cloud core. Most of the cloud mass is in
molecules, and the total mass is > 200,000 Solar Masses. In the cold core the
HI mass fraction may be 10 percent. The cloud has only a few sites of current
star formation. There may be about 100 more objects like this in the inner
Galaxy; every line of sight through the Galactic plane within 50 degrees of the
Galactic center probably intersects at least one. We suggest that G28.17+0.05
is a cloud being observed as it enters a spiral arm and that it is in the
transition from the atomic to the molecular state.Comment: 35 pages, inludes 12 figure
The Microarcsecond Sky and Cosmic Turbulence
Radio waves are imprinted with propagation effects from ionized media through
which they pass. Owing to electron density fluctuations, compact sources
(pulsars, masers, and compact extragalactic sources) can display a wide variety
of scattering effects. These scattering effects, particularly interstellar
scintillation, can be exploited to provide *superresolution*, with achievable
angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained
by very long baseline interferometry on terrestrial baselines. Scattering
effects also provide a powerful sub-AU probe of the microphysics of the
interstellar medium, potentially to spatial scales smaller than 100 km, as well
as a tracer of the Galactic distribution of energy input into the interstellar
medium through a variety of integrated measures. Coupled with future gamma-ray
observations, SKA observations also may provide a means of detecting fainter
compact gamma-ray sources. Though it is not yet clear that propagation effects
due to the intergalactic medium are significant, the SKA will either detect or
place stringent constraints on intergalactic scattering.Comment: 20 pages, 8 figures in 8 PostScript files, to appear in "Science with
the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy
Reviews (Elsevier: Amsterdam
On Pulsar Distance Measurements and their Uncertainties
Accurate distances to pulsars can be used for a variety of studies of the
Galaxy and its electron content. However, most distance measures to pulsars
have been derived from the absorption (or lack thereof) of pulsar emission by
Galactic HI gas, which typically implies that only upper or lower limits on the
pulsar distance are available. We present a critical analysis of all measured
HI distance limits to pulsars and other neutron stars, and translate these
limits into actual distance estimates through a likelihood analysis that
simultaneously corrects for statistical biases. We also apply this analysis to
parallax measurements of pulsars in order to obtain accurate distance estimates
and find that the parallax and HI distance measurements are biased in different
ways, because of differences in the sampled populations. Parallax measurements
typically underestimate a pulsar's distance because of the limited distance to
which this technique works and the consequential strong effect of the Galactic
pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance
limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading
to overestimated distances because the bright pulsars on which this technique
is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the
Astrophysical Journa
An automatic micro-injection system and its use in the microcalorimetry of cation-exchange sorption
Details are given of a remote-control device which can deliver known microlite volumes of solutions simultaneously to the 'control' and 'reaction' cells in the LKB calorimeter, and can do so accurately, automatically and at preset intervals. The device incorporates Hamilton microsyringes. The facility for injection at preset intervals is designed to allow the heat change after each addition of reacting solution to be recorded completely before the next addition, the interval for a reaction being predetermined in a pilot scale experiment.
A method is described for measuring different enthalpies of potassium-calcium exchange in soils and clays using an LKB microcalorimeter incorporating this device. This achieves a considerable economy in time over the method of deriving information from the thermodynamic treatments of cation-exchange isotherms. The degree of cation-saturation is interpolated from a parallel 'isotherm' experiment on a macroscale in which several consecutive additions in the 'calorimeter' experiment are telescoped into single steps
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