44,638 research outputs found
Transport in ultradilute solutions of He in superfluid He
We calculate the effect of a heat current on transporting He dissolved in
superfluid He at ultralow concentration, as will be utilized in a proposed
experimental search for the electric dipole moment of the neutron (nEDM). In
this experiment, a phonon wind will generated to drive (partly depolarized)
He down a long pipe. In the regime of He concentrations and temperatures K, the phonons comprising the heat current
are kept in a flowing local equilibrium by small angle phonon-phonon
scattering, while they transfer momentum to the walls via the He first
viscosity. On the other hand, the phonon wind drives the He out of local
equilibrium via phonon-He scattering. For temperatures below K, both
the phonon and He mean free paths can reach the centimeter scale, and we
calculate the effects on the transport coefficients. We derive the relevant
transport coefficients, the phonon thermal conductivity and the He
diffusion constants from the Boltzmann equation. We calculate the effect of
scattering from the walls of the pipe and show that it may be characterized by
the average distance from points inside the pipe to the walls. The temporal
evolution of the spatial distribution of the He atoms is determined by the
time dependent He diffusion equation, which describes the competition
between advection by the phonon wind and He diffusion. As a consequence of
the thermal diffusivity being small compared with the He diffusivity, the
scale height of the final He distribution is much smaller than that of the
temperature gradient. We present exact solutions of the time dependent
temperature and He distributions in terms of a complete set of normal
modes.Comment: NORDITA PREPRINT 2015-37, 9 pages, 6 figure
Transport in very dilute solutions of He in superfluid He
Motivated by a proposed experimental search for the electric dipole moment of
the neutron (nEDM) utilizing neutron-He capture in a dilute solution of
He in superfluid He, we derive the transport properties of dilute
solutions in the regime where the He are classically distributed and rapid
He-He scatterings keep the He in equilibrium. Our microscopic
framework takes into account phonon-phonon, phonon-He, and He-He
scatterings. We then apply these calculations to measurements by Rosenbaum et
al. [J.Low Temp.Phys. {\bf 16}, 131 (1974)] and by Lamoreaux et al.
[Europhys.Lett. {\bf 58}, 718 (2002)] of dilute solutions in the presence of a
heat flow. We find satisfactory agreement of theory with the data, serving to
confirm our understanding of the microscopics of the helium in the future nEDM
experiment.Comment: 10 pages, 5 figures, v
Low Temperature Transport Properties of Very Dilute Classical Solutions of He in Superfluid He
We report microscopic calculations of the thermal conductivity, diffusion
constant and thermal diffusion constant for classical solutions of He in
superfluid He at temperatures T \la 0.6~K, where phonons are the dominant
excitations of the He. We focus on solutions with He concentrations
\la \,10^{-3}, for which the main scattering mechanisms are phonon-phonon
scattering via 3-phonon Landau and Beliaev processes, which maintain the
phonons in a drifting equilibrium distribution, and the slower process of
He-phonon scattering, which is crucial for determining the He
distribution function in transport. We use the fact that the relative changes
in the energy and momentum of a He atom in a collision with a phonon are
small to derive a Fokker-Planck equation for the He distribution function,
which we show has an analytical solution in terms of Sonine polynomials. We
also calculate the corrections to the Fokker-Planck results for the transport
coefficients.Comment: 29 pages, 2 figure
The Infrared Nucleus of the Wolf-Rayet Galaxy Henize 2-10
We have obtained near-infrared images and mid-infrared spectra of the
starburst core of the dwarf Wolf-Rayet galaxy He 2-10. We find that the
infrared continuum and emission lines are concentrated in a flattened ellipse
3-4'' or 150 pc across which may show where a recent accretion event has
triggered intense star formation. The ionizing radiation from this cluster has
an effective temperature of 40,000 K, corresponding to stars, and
the starburst is years old.Comment: 17 pages Latex, 7 postscript figures, 1 postscript table, accepted to
A
Magnetic loop emergence within a granule
We investigate the temporal evolution of magnetic flux emerging within a
granule in the quiet-Sun internetwork at disk center. We combined IR
spectropolarimetry performed in two Fe I lines at 1565 nm with
speckle-reconstructed G-band imaging. We determined the magnetic field
parameters by a LTE inversion of the full Stokes vector using the SIR code, and
followed their evolution in time. To interpret the observations, we created a
geometrical model of a rising loop in 3D. The relevant parameters of the loop
were matched to the observations where possible. We then synthesized spectra
from the 3D model for a comparison to the observations. We found signatures of
magnetic flux emergence within a growing granule. In the early phases, a
horizontal magnetic field with a distinct linear polarization signal dominated
the emerging flux. Later on, two patches of opposite circular polarization
signal appeared symmetrically on either side of the linear polarization patch,
indicating a small loop-like structure. The mean magnetic flux density of this
loop was roughly 450 G, with a total magnetic flux of around 3x10^17 Mx. During
the ~12 min episode of loop occurrence, the spatial extent of the loop
increased from about 1 to 2 arcsec. The middle part of the appearing feature
was blueshifted during its occurrence, supporting the scenario of an emerging
loop. The temporal evolution of the observed spectra is reproduced to first
order by the spectra derived from the geometrical model. The observed event can
be explained as a case of flux emergence in the shape of a small-scale loop.Comment: 10 pages, 13 figures; accepted for Astronomy and Astrophysics; ps and
eps figures in full resolution are available at
http://www.astro.sk/~koza/figures/aa2009_loop
Sulphur molecules in the circumstellar envelopes of M-type AGB stars
The sulphur compounds SO and SO have not been widely studied in the
circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting
and modelling a large number of SO and SO lines in the low mass-loss rate
M-type AGB star R Dor, and modelling the available lines of those molecules in
a further four M-type AGB stars, we aim to determine their circumstellar
abundances and distributions. We use a detailed radiative transfer analysis
based on the accelerated lambda iteration method to model circumstellar SO and
SO line emission and molecular data files for both SO and SO that are
more extensive than those previously available. Using 17 SO lines and 98 SO2
lines to constrain our models for R Dor, we find an SO abundance of
6.7x10 and an SO abundance of 5x10 with both species having
high abundances close to the star. We also modelled SO and found an
abundance of 3.1x10, giving an SO/SO ratio of 21.6. We
derive similar results for the circumstellar SO and SO abundances and their
distributions for the low mass-loss rate object W Hya. For these stars, the
circumstellar SO and SO abundances are much higher than predicted by
chemical models and these two species may account for all available sulphur.
For the higher mass-loss rate stars, we find shell-like SO distributions with
peak abundances that decrease and peak abundance radii that increase with
increasing mass-loss rate. The positions of the peak SO abundance agree very
well with the photodissociation radii of HO. We find evidence that SO is
most likely through the photodissociation of HO and the subsequent reaction
between S and OH. The S-bearing parent molecule appears not to be HS. The
SO models suggest an origin close to the star for this species, also
disagreeing with current chemical models.Comment: 25 page
Phase II of the ASCE Benchmark Study on SHM
The task group on structural health monitoring of the Dynamic Committee of ASCE was formed in
1999 at the 12
th
Engineering Mechanics Conference. The task group has designed a number of analytical
studies on a benchmark structure and there are plans to follow these with an experimental program. The
first phase of the analytical studies was completed in 2001. The second phase, initiated in the summer of
2001, was formulated in the light of the experience gained on phase I and focuses on increasing realism in
the simulation of the discrepancies between the actual structure and the mathematical model used in the
analysis. This paper describes the rational that lead the SHM task group to the definition of phase II and
presents the details of the cases that are being considered
Simulations of the Galaxy Cluster CIZA J2242.8+5301 I: Thermal Model and Shock Properties
The giant radio relic in CIZA J2242.8+5301 is likely evidence of a Mpc sized
shock in a massive merging galaxy cluster. However, the exact shock properties
are still not clearly determined. In particular, the Mach number derived from
the integrated radio spectrum exceeds the Mach number derived from the X-ray
temperature jump by a factor of two. We present here a numerical study, aiming
for a model that is consistent with the majority of observations of this galaxy
cluster. We first show that in the northern shock upstream X-ray temperature
and radio data are consistent with each other. We then derive progenitor masses
for the system using standard density profiles, X-ray properties and the
assumption of hydrostatic equilibrium. We find a class of models that is
roughly consistent with weak lensing data, radio data and some of the X-ray
data. Assuming a cool-core versus non-cool-core merger, we find a fiducial
model with a total mass of , a mass ratio of 1.76
and a Mach number that is consistent with estimates from the radio spectrum. We
are not able to match X-ray derived Mach numbers, because even low mass models
over-predict the X-ray derived shock speeds. We argue that deep X-ray
observations of CIZA J2242.8+5301 will be able to test our model and
potentially reconcile X-ray and radio derived Mach numbers in relics.Comment: 19 pages, 19 figure
Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants
The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium
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