13,264 research outputs found
The relation between magnetic and material arms in models for spiral galaxies
Context. Observations of polarized radio emission show that large-scale
(regular) magnetic fields in spiral galaxies are not axisymmetric, but
generally stronger in interarm regions. In some nearby galaxies such as NGC
6946 they are organized in narrow magnetic arms situated between the material
spiral arms. Aims. The phenomenon of magnetic arms and their relation to the
optical spiral arms (the material arms) call for an explanation in the
framework of galactic dynamo theory. Several possibilities have been suggested
but are not completely satisfactory; here we attempt a consistent
investigation. Methods. We use a 2D mean-field dynamo model in the no-z
approximation and add injections of small-scale magnetic field, taken to result
from supernova explosions, to represent the effects of dynamo action on smaller
scales. This injection of small scale field is situated along the spiral arms,
where star-formation mostly occurs. Results. A straightforward explanation of
magnetic arms as a result of modulation of the dynamo mechanism by material
arms struggles to produce pronounced magnetic arms, at least with realistic
parameters, without introducing new effects such as a time lag between Coriolis
force and {\alpha}-effect. In contrast, by taking into account explicitly the
small-scale magnetic field that is injected into the arms by the action of the
star forming regions that are concentrated there, we can obtain dynamo models
with magnetic structures of various forms that can be compared with magnetic
arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered
as coherent magnetic structures generated by large-scale dynamo action, and
associated with spatially modulated small-scale magnetic fluctuations, caused
by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&
Enhanced Pauli blocking of light scattering in a trapped Fermi gas
Pauli blocking of spontaneous emission by a single excited-state atom has
been predicted to be dramatic at low temperature when the Fermi energy
exceeds the recoil energy . The photon scattering
rate of a ground-state Fermi gas can also be suppressed by occupation of the
final states accessible to a recoiling atom, however suppression is diminished
by scattering events near the Fermi edge. We analyze two new approaches to
improve the visibility of Pauli blocking in a trapped Fermi gas. Focusing the
incident light to excite preferentially the high-density region of the cloud
can increase the blocking signature by 14%, and is most effective at
intermediate temperature. Spontaneous Raman scattering between imbalanced
internal states can be strongly suppressed at low temperature, and is
completely blocked for a final-state in the
high imbalance limit.Comment: 12 pages, 8 figures. v4: to appear in Journal of Physics B: Atomic,
Molecular, and Optical Physic
The Skinned Fiber Technique as a Potential Method for Study of Muscle as a Food
Skeletal, smooth and cardiac muscle cells can be skinned by physical means or a variety of chemical techniques. The skinned fibers have been used to study the molecular mechanisms of contraction and the regulation of contraction by ca++. Skinned fiber preparations are also useful for study of muscle as a food. For example, it is now possible to determine fiber type of skinned fibers following study of their physical properties
High performance photonic microwave filters based on a 50GHz optical soliton crystal Kerr micro-comb
We demonstrate a photonic radio frequency (RF) transversal filter based on an
integrated optical micro-comb source featuring a record low free spectral range
of 49 GHz yielding 80 micro-comb lines across the C-band. This record-high
number of taps, or wavelengths for the transversal filter results in
significantly increased performance including a QRF factor more than four times
higher than previous results. Further, by employing both positive and negative
taps, an improved out-of-band rejection of up to 48.9 dB is demonstrated using
Gaussian apodization, together with a tunable centre frequency covering the RF
spectra range, with a widely tunable 3-dB bandwidth and versatile dynamically
adjustable filter shapes. Our experimental results match well with theory,
showing that our transversal filter is a competitive solution to implement
advanced adaptive RF filters with broad operational bandwidths, high frequency
selectivity, high reconfigurability, and potentially reduced cost and
footprint. This approach is promising for applications in modern radar and
communications systems.Comment: 19 pages, 12 figures, 107 reference
Entropy production and curvature perturbation from dissipative curvatons
Considering the curvaton field that follows dissipative slow-roll equation,
we show that the field can lead to entropy production and generation of
curvature perturbation after reheating. Spectral index is calculated to
discriminate warm and thermal scenarios of dissipative curvatons from the
standard curvaton model. In contrast to the original curvaton model, quadratic
potential is not needed in the dissipative scenario, since the growth in the
oscillating period is not essential for the model.Comment: 29 pages, 2 figures, accepted for publication in JCA
Modulation of cognitive performance and mood by aromas of peppermint and ylang-ylang
This study provides further evidence for the impact of the aromas of plant essential oils on aspects of cognition and mood in healthy participants. One hundred and forty-four volunteers were randomly assigned to conditions of ylang-ylang aroma, peppermint aroma, or no aroma control. Cognitive performance was assessed using the Cognitive Drug Research computerized assessment battery, with mood scales completed before and after cognitive testing. The analysis of the data revealed significant differences between conditions on a number of the factors underpinning the tests that constitute the battery. Peppermint was found to enhance memory whereas ylang-ylang impaired it, and lengthened processing speed. In terms of subjective mood peppermint increased alertness and ylang-ylang decreased it, but significantly increased calmness. These results provide support for the contention that the aromas of essential oils can produce significant and idiosyncratic effects on both subjective and objective assessments of aspects of human behavior. They are discussed with reference to possible pharmacological and psychological modes of influence
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