23,068 research outputs found
Pulsed Adiabatic Photoassociation via Scattering Resonances
We develop the theory for the Adiabatic Raman Photoassociation (ARPA) of
ultracold atoms to form ultracold molecules in the presence of scattering
resonances. Based on a computational method in which we replace the continuum
with a discrete set of "effective modes", we show that the existence of
resonances greatly aids in the formation of deeply bound molecular states. We
illustrate our general theory by computationally studying the formation of
Rb molecules from pairs of colliding ultracold Rb atoms. The
single-event transfer yield is shown to have a near-unity value for wide
resonances, while the ensemble-averaged transfer yield is shown to be higher
for narrow resonances. The ARPA yields are compared with that of (the
experimentally measured) "Feshbach molecule" magneto-association. Our findings
suggest that an experimental investigation of ARPA at sub-K temperatures
is warranted.Comment: 20 pages, 11 figure
Forward modelling of brightness variations in Sun-like stars I. Emergence and surface transport of magnetic flux
The latitudinal distribution of starspots deviates from the solar pattern
with increasing rotation rate. Numerical simulations of magnetic flux emergence
and transport can help model the observed stellar activity patterns and the
associated brightness variations. We set up a composite model for the processes
of flux emergence and transport on Sun-like stars, to simulate stellar
brightness variations for various levels of magnetic activity and rotation
rates. Assuming that the distribution of magnetic flux at the base of the
convection zone follows solar scaling relations, we calculate the emergence
latitudes and tilt angles of bipolar regions at the surface for various
rotation rates, using thin-flux-tube simulations. Taking these two quantities
as input to a surface flux transport SFT model, we simulate the
diffusive-advective evolution of the radial field at the stellar surface,
including effects of active region nesting. As the rotation rate increases, (1)
magnetic flux emerges at higher latitudes and an inactive gap opens around the
equator, reaching a half-width of for , (2) the tilt
angles of freshly emerged bipolar regions show stronger variations with
latitude. Polar spots can form at by accumulation of
follower-polarity flux from decaying bipolar regions. From to
, the maximum spot coverage changes from 3 to 20%, respectively,
compared to 0.4% for the solar model. Nesting of activity can lead to strongly
non-axisymmetric spot distributions. On Sun-like stars rotating at
( days), polar spots can form, owing to
higher levels of flux emergence rate and tilt angles. Defining spots by a
threshold field strength yields global spot coverages that are roughly
consistent with stellar observations.Comment: 16 pages, 13 figures. Astron. & Astrophys. (in press); minor language
corrections mad
The nature of solar brightness variations
The solar brightness varies on timescales from minutes to decades.
Determining the sources of such variations, often referred to as solar noise,
is of importance for multiple reasons: a) it is the background that limits the
detection of solar oscillations, b) variability in solar brightness is one of
the drivers of the Earth's climate system, c) it is a prototype of stellar
variability which is an important limiting factor for the detection of
extra-solar planets. Here we show that recent progress in simulations and
observations of the Sun makes it finally possible to pinpoint the source of the
solar noise. We utilise high-cadence observations from the Solar Dynamic
Observatory and the SATIRE model to calculate the magnetically-driven
variations of solar brightness. The brightness variations caused by the
constantly evolving cellular granulation pattern on the solar surface are
computed with the MURAM code. We find that surface magnetic field and
granulation can together precisely explain solar noise on timescales from
minutes to decades, i.e. ranging over more than six orders of magnitude in the
period. This accounts for all timescales that have so far been resolved or
covered by irradiance measurements. We demonstrate that no other sources of
variability are required to explain the data. Recent measurements of Sun-like
stars by CoRoT and Kepler uncovered brightness variations similar to that of
the Sun but with much wider variety of patterns. Our finding that solar
brightness variations can be replicated in detail with just two well-known
sources will greatly simplify future modelling of existing CoRoT and Kepler as
well as anticipated TESS and PLATO data.Comment: This is the submitted version of the paper published in Nature
Astronom
Scaling of the conductance distribution near the Anderson transition
The single parameter scaling hypothesis is the foundation of our
understanding of the Anderson transition. However, the conductance of a
disordered system is a fluctuating quantity which does not obey a one parameter
scaling law. It is essential to investigate the scaling of the full conductance
distribution to establish the scaling hypothesis. We present a clear cut
numerical demonstration that the conductance distribution indeed obeys one
parameter scaling near the Anderson transition
Warm and dense stellar matter under strong magnetic fields
We investigate the effects of strong magnetic fields on the equation of state
of warm stellar matter as it may occur in a protoneutron star. Both neutrino
free and neutrino trapped matter at a fixed entropy per baryon are analyzed. A
relativistic mean field nuclear model, including the possibility of hyperon
formation, is considered. A density dependent magnetic field with the magnitude
G at the surface and not more than G at the center
is considered. The magnetic field gives rise to a neutrino suppression, mainly
at low densities, in matter with trapped neutrinos. It is shown that an hybrid
protoneutron star will not evolve to a low mass blackhole if the magnetic field
is strong enough and the magnetic field does not decay. However, the decay of
the magnetic field after cooling may give rise to the formation of a low mass
blackhole.Comment: 17 pages, 10 figures, 3 tables, submitted to Phys. Rev.
Asymmetric Drift and the Stellar Velocity Ellipsoid
We present the decomposition of the stellar velocity ellipsoid using stellar
velocity dispersions within a 40 deg wedge about the major-axis (sigma_maj),
the epicycle approximation, and the asymmetric drift equation. Thus, we employ
no fitted forms for sigma_maj and escape interpolation errors resulting from
comparisons of the major and minor axes. We apply the theoretical construction
of the method to integral field data taken for NGC 3949 and NGC 3982. We derive
the vertical-to-radial velocity dispersion ratio (sigma_z / sigma_R) and find
(1) our decomposition method is accurate and reasonable, (2) NGC 3982 appears
to be rather typical of an Sb type galaxy with sigma_z / sigma_R = 0.73
(+0.13/-0.11) despite its high surface brightness and small size, and (3) NGC
3949 has a hot disk with sigma_z / sigma_R = 1.18 (+0.36/-0.28).Comment: 4 pages including 3 figures, to appear in "Island Universes:
Structure and Evolution of Disk Galaxies", Terschelling, Netherlands, July
3-8, 200
Clostridium difficile colitis in patients after kidney and pancreas-kidney transplantation
Limited data exist about Clostridium difficile colitis (CDC) in solid organ transplant patients. Between 1/1/99 and 12/31/02, 600 kidney and 102 pancreas–kidney allograft recipients were transplanted. Thirty-nine (5.5%) of these patients had CDC on the basis of clinical and laboratory findings. Of these 39 patients, 35 have information available for review. CDC developed at a median of 30 days after transplantation, and the patients undergoing pancreas–kidney transplantation had a slightly higher incidence of CDC than recipients of kidney alone (7.8% vs. 4.5%, P> 0.05). All but one patient presented with diarrhea. Twenty-four patients (64.9%) were diagnosed in the hospital, and CDC occurred during first hospitalization in 14 patients (40%). Treatment was with oral metronidazole (M) in 33 patients (94%)and M + oral vancomycin (M + V) in 2 patients. Eight patients had recurrent CDC, which occurred at a median of 30 days (range 15–314) after the first episode. Two patients (5.7%) developed fulminant CDC, presented with toxic megacolon, and underwent colectomy. One of them died; the other patient survived after colectomy. CDC should be considered as a diagnosis in transplant patients with history of diarrhea after antibiotic use, and should be treated aggressively before the infection becomes complicated
Gravitational time advancement and its possible detection
The gravitational time advancement is a natural but a consequence of curve
space-time geometry. In the present work the expressions of gravitational time
advancement have been obtained for geodesic motions. The situation when the
distance of signal travel is small in comparison to the distance of closest
approach has also been considered. The possibility of experimental detection of
time advancement effect has been explored.Comment: 5 pages, 4 figures, a part of the work has been changed in the
revised versio
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