32,582 research outputs found
Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae
We present the first temperature surface map of EK Dra from
very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and
Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in
spectral line profiles are inverted to a stellar surface temperature map using
our Map code. The long-term photometric record is employed to compare our
map with previously published maps. Four cool spots were reconstructed, but no
polar spot was seen. The temperature difference to the photosphere of the spots
is between 990 and 280K. Two spots are reconstructed with a typical solar
morphology with an umbra and a penumbra. For the one isolated and relatively
round spot (A), we determine an umbral temperature of 990K and a penumbral
temperature of 180K below photospheric temperature. The umbra to photosphere
intensity ratio of EK Dra is approximately only half of that of a comparison
sunspot. A test inversion from degraded line profiles showed that the higher
spectral resolution of PEPSI reconstructs the surface with a temperature
difference that is on average 10% higher than before and with smaller surface
areas by 10-20%. PEPSI is therefore better suited to detecting and
characterising temperature inhomogeneities. With ten more years of photometry,
we also refine the spot cycle period of EK Dra to 8.90.2 years with a
continuing long-term fading trend. The temperature morphology of spot A so far
appears to show the best evidence for the existence of a solar-like penumbra
for a starspot. We emphasise that it is more the non-capture of the true umbral
contrast rather than the detection of the weak penumbra that is the limiting
factor. The relatively small line broadening of EK Dra, together with the only
moderately high spectral resolutions previously available, appear to be the
main contributors to the lower-than-expected spot contrasts when comparing to
the Sun.Comment: Accepted for A&
Non-destructive interferometric characterization of an optical dipole trap
A method for non-destructive characterization of a dipole trapped atomic
sample is presented. It relies on a measurement of the phase-shift imposed by
cold atoms on an optical pulse that propagates through a free space
Mach-Zehnder interferometer. Using this technique we are able to determine,
with very good accuracy, relevant trap parameters such as the atomic sample
temperature, trap oscillation frequencies and loss rates. Another important
feature is that our method is faster than conventional absorption or
fluorescence techniques, allowing the combination of high-dynamical range
measurements and a reduced number of spontaneous emission events per atom.Comment: 9 pages, 6 figures, submitted to PR
A steerable UV laser system for the calibration of liquid argon time projection chambers
A number of liquid argon time projection chambers (LAr TPC's) are being build
or are proposed for neutrino experiments on long- and short baseline beams. For
these detectors a distortion in the drift field due to geometrical or physics
reasons can affect the reconstruction of the events. Depending on the TPC
geometry and electric drift field intensity this distortion could be of the
same magnitude as the drift field itself. Recently, we presented a method to
calibrate the drift field and correct for these possible distortions. While
straight cosmic ray muon tracks could be used for calibration, multiple coulomb
scattering and momentum uncertainties allow only a limited resolution. A UV
laser instead can create straight ionization tracks in liquid argon, and allows
one to map the drift field along different paths in the TPC inner volume. Here
we present a UV laser feed-through design with a steerable UV mirror immersed
in liquid argon that can point the laser beam at many locations through the
TPC. The straight ionization paths are sensitive to drift field distortions, a
fit of these distortion to the linear optical path allows to extract the drift
field, by using these laser tracks along the whole TPC volume one can obtain a
3D drift field map. The UV laser feed-through assembly is a prototype of the
system that will be used for the MicroBooNE experiment at the Fermi National
Accelerator Laboratory (FNAL)
A method to suppress dielectric breakdowns in liquid argon ionization detectors for cathode to ground distances of several millimeters
We present a method to reach electric field intensity as high as 400 kV/cm in
liquid argon for cathode-ground distances of several millimeters. This can be
achieved by suppressing field emission from the cathode, overcoming limitations
that we reported earlier
Long-Range Ordering of Vibrated Polar Disks
Vibrated polar disks have been used experimentally to investigate collective
motion of driven particles, where fully-ordered asymptotic regimes could not be
reached. Here we present a model reproducing quantitatively the single, binary
and collective properties of this granular system. Using system sizes not
accessible in the laboratory, we show in silico that true long-range order is
possible in the experimental system. Exploring the model's parameter space, we
find a phase diagram qualitatively different from that of dilute or point-like
particle systems.Comment: 5 pages, 4 figure
Exponential beams of electromagnetic radiation
We show that in addition to well known Bessel, Hermite-Gauss, and
Laguerre-Gauss beams of electromagnetic radiation, one may also construct
exponential beams. These beams are characterized by a fall-off in the
transverse direction described by an exponential function of rho. Exponential
beams, like Bessel beams, carry definite angular momentum and are periodic
along the direction of propagation, but unlike Bessel beams they have a finite
energy per unit beam length. The analysis of these beams is greatly simplified
by an extensive use of the Riemann-Silberstein vector and the Whittaker
representation of the solutions of the Maxwell equations in terms of just one
complex function. The connection between the Bessel beams and the exponential
beams is made explicit by constructing the exponential beams as wave packets of
Bessel beams.Comment: Dedicated to the memory of Edwin Powe
Proton Fall or Bicarbonate Rise: Glycolytic Rate In Mouse Astrocytes Is Paved By Intracellular Alkalinization
Glycolysis is the primary step for major energy production in the cell. There is strong evidence suggesting that glucose consumption and rate of glycolysis are highly modulated by cytosolic pH/[H(+)], but those can also be stimulated by an increase in the intracellular [HCO3 (-)]. Because proton and bicarbonate shift concomitantly, it remained unclear whether enhanced glucose consumption and glycolytic rate were mediated by the changes in intracellular [H(+)] or [HCO3 (-)]. We have asked whether glucose metabolism is enhanced by either a fall in intracellular [H(+)] or a rise in intracellular [HCO3 (-)], or by both, in mammalian astrocytes. We have recorded intracellular glucose in mouse astrocytes using a FRET-based nanosensor, while imposing different intracellular [H(+)] and [CO2]/[HCO3 (-)]. Glucose consumption and glycolytic rate were augmented by a fall in intracellular [H(+)], irrespective of a concomitant rise or fall in intracellular [HCO3 (-)]. Transport of HCO3 (-) into and out of astrocytes by the electrogenic sodium bicarbonate cotransporter (NBCe1) played a crucial role in causing changes in intracellular pH and [HCO3 (-)], but was not obligatory for the pH-dependent changes in glucose metabolism. Our results clearly show that it is the cytosolic pH that modulates glucose metabolism in cortical astrocytes, and possibly also in other cell types
Measurement of the drift field in the ARGONTUBE LAr TPC with 266~nm pulsed laser beams
ARGONTUBE is a liquid argon time projection chamber (LAr TPC) with a drift
field generated in-situ by a Greinacher voltage multiplier circuit. We present
results on the measurement of the drift-field distribution inside ARGONTUBE
using straight ionization tracks generated by an intense UV laser beam. Our
analysis is based on a simplified model of the charging of a multi-stage
Greinacher circuit to describe the voltages on the field cage rings
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