3,354 research outputs found
Study of temperature dependent atomic correlations in MgB
We have studied the evolution with temperature of the local as well as the
average crystal structure of MgB using the real-space atomic pair
distribution function (PDF) measured by high resolution neutron powder
diffraction. We have investigated the correlations of the B-B and B-Mg nearest
neighbor pair motion by comparing, in the wide temperature range from T=10 K up
to T=600 K, the mean-square displacements (MSD) of single atoms with the
mean-square relative displacements (MSRD) obtained from the PDF peak
linewidths. The results show that the single atom B and Mg vibrations are
mostly decoupled from each other, with a small predominance of positive (in
phase) correlation factor for both the B-B and B-Mg pairs. The small positive
correlation is almost temperature independent, in contrast with our theoretical
calculations; this can be a direct consequence of the strong decay processes of
the anharmonic phonons
On the temperature dependence of the symmetry energy
We perform large-scale shell model Monte Carlo (SMMC) calculations for many
nuclei in the mass range A=56-65 in the complete pfg_{9/2}d_{5/2} model space
using an effective quadrupole-quadrupole+pairing residual interaction. Our
calculations are performed at finite temperatures between T=0.33-2 MeV. Our
main focus is the temperature dependence of the symmetry energy which we
determine from the energy differences between various isobaric pairs with the
same pairing structure and at different temperatures. Our SMMC studies are
consistent with an increase of the symmetry energy with temperature. We also
investigate possible consequences for core-collapse supernovae events
Conditions for Shock Revival by Neutrino Heating in Core-Collapse Supernovae
Energy deposition by neutrinos can rejuvenate the stalled bounce shock and
can provide the energy for the supernova explosion of a massive star. This
neutrino-heating mechanism, however, is not finally accepted or proven as the
trigger of the explosion. Part of the problem is that the complexity of the
hydrodynamic models often hampers a clear and simple interpretation of the
results. This demands a deeper theoretical understanding of the requirements of
a successful shock revival. A toy model is presented here for discussing the
neutrino heating phase analytically by a time-dependent treatment, which allows
one to calculate the radius and velocity of the supernova shock from global
properties of the gain layer as solutions of an initial value problem. A
criterion is derived for the requirements of shock revival. It confirms the
existence of a minimum neutrino luminosity needed for shock expansion, but also
demonstrates the importance of a sufficiently large mass infall rate to the
shock. The possibility of very energetic neutrino-driven explosions seems
excluded because the total specific energy transferred to nucleons is limited
by about 1e52 ergs per solar mass (about 5 MeV per nucleon) and the total mass
in the gain layer is typically only around 0.1 solar masses. Energy transport
by convection from the region of maximum heating to radii closer behind the
shock is found to support the explosion by reducing the energy loss associated
with the inward advection of neutrino-heated matter through the gain radius.
(abridged)Comment: 36 pages, A&A LaTeX, 14 eps figures, major extensions due to referee
comments; accepted by Astronomy & Astrophysic
Probing impulsive strain propagation with x-ray pulses
Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden
reflections in InSb is used to follow the propagation of a coherent acoustic
pulse generated by ultrafast laser-excitation. The surface and bulk components
of the strain could be simultaneously measured due to the large x-ray
penetration depth. Comparison of the experimental data with dynamical
diffraction simulations suggests that the conventional model for impulsively
generated strain underestimates the partitioning of energy into coherent modes.Comment: 4 pages, 2 figures, LaTeX, eps. Accepted for publication in Phys.
Rev. Lett. http://prl.aps.or
Dust, pulsation, chromospheres and their role in driving mass loss from red giants in Galactic globular clusters
Context: Mass loss from red giants in old globular clusters affects the
horizontal branch (HB) morphology and post-HB stellar evolution including the
production of ultraviolet-bright stars, dredge up of nucleosynthesis products
and replenishment of the intra-cluster medium. Studies of mass loss in globular
clusters also allows one to investigate the metallicity dependence of the mass
loss from cool, low-mass stars down to very low metallicities.
Aims: We present an analysis of new VLT/UVES spectra of 47 red giants in the
Galactic globular clusters 47 Tuc (NGC 104), NGC 362, omega Cen (NGC 5139), NGC
6388, M54 (NGC 6715) and M15 (NGC 7078). The spectra cover the wavelength
region 6100-9900A at a resolving power of R = 110,000. Some of these stars are
known to exhibit mid-infrared excess emission indicative of circumstellar dust.
Our aim is to detect signatures of mass loss, identify the mechanism(s)
responsible for such outflows, and measure the mass-loss rates.
Methods: We determine for each star its effective temperature, luminosity,
radius and escape velocity. We analyse the H-alpha and near-infrared calcium
triplet lines for evidence of outflows, pulsation and chromospheric activity,
and present a simple model for estimating mass-loss rates from the H-alpha line
profile. We compare our results with a variety of other, independent methods.
Results: We argue that a chromosphere persists in Galactic globular cluster
giants and controls the mass-loss rate to late-K/early-M spectral types, where
pulsation becomes strong enough to drive shock waves at luminosities above the
RGB tip. This transition may be metallicity-dependent. We find mass-loss rates
of ~10^-7 to 10^-5 solar masses per year, largely independent of metallicity.Comment: 23 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Long- and short-type double-balloon enteroscopy-assisted therapeutic ERCP for intact papilla in patients with a Roux-en-Y anastomosis
Transrectal drainage of a diverticular abscess using a pigtail catheter without radiological guidance: a case report
<p>Abstract</p> <p>Introduction</p> <p>Percutaneous or endocavitory drainage of a diverticular abscess under radiological guidance often enables one to perform a one-staged resection and anastomosis (without stoma formation) instead of a two-staged procedure. It reduces the significant postoperative morbidity and mortality associated with the conventional emergency surgical management. However, radiological guidance is not always available due to limited resources during out-of-hours.</p> <p>Case presentation</p> <p>A 78-year-old Caucasian woman underwent transrectal drainage of a diverticular abscess performed with a pigtail catheter without radiological guidance. Technical details of the procedure are described and alternative options discussed.</p> <p>Conclusion</p> <p>In carefully selected patients, per-rectal drainage using a pigtail catheter can be performed without radiological guidance and the procedure offers a simple and effective way of controlling sepsis.</p
Exploring the relativistic regime with Newtonian hydrodynamics: An improved effective gravitational potential for supernova simulations
We investigate the possibility to approximate relativistic effects in
hydrodynamical simulations of stellar core collapse and post-bounce evolution
by using a modified gravitational potential in an otherwise standard Newtonian
hydrodynamic code. Different modifications of the effective relativistic
potential introduced by Rampp & Janka (2002) are discussed. Corresponding
hydrostatic solutions are compared with solutions of the TOV equations, and
hydrodynamic simulations with two different codes are compared with fully
relativistic results. One code is applied for one- and two-dimensional
calculations with a simple equation of state, and employs either the modified
effective relativistic potential in a Newtonian framework or solves the general
relativistic field equations under the assumption of the conformal flatness
condition (CFC) for the three-metric. The second code allows for full-scale
supernova runs including a microphysical equation of state and neutrino
transport based on the solution of the Boltzmann equation and its moments
equations. We present prescriptions for the effective relativistic potential
for self-gravitating fluids to be used in Newtonian codes, which produce
excellent agreement with fully relativistic solutions in spherical symmetry,
leading to significant improvements compared to previously published
approximations. Moreover, they also approximate qualitatively well relativistic
solutions for models with rotation.Comment: 20 pages, 13 figures; corrected minor inaccuracies and added two
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