4,758 research outputs found
Transient Oscillation of Currents in Quantum Hall Effect of Bloch Electrons
We consider the quantum Hall effect of two-dimensional electrons with a
periodic potential and study the time dependence of the Hall and longitudinal
currents when the electric field is applied abruptly. We find that the currents
oscillate in time with very large frequencies because of quantum fluctuation
and the oscillations eventually vanish, for their amplitudes decay as 1/t.Comment: 16 pages and 8 figure
A New Giant Branch Clump Structure In the Large Magellanic Cloud
We present Washington C, T1 CCD photometry of 21 fields located in the
northern part of the Large Magellanic Cloud (LMC), and spread over a region of
more than 2.52 degrees approximately 6 degrees from the bar. The surveyed areas
were chosen on the basis of their proximity to SL 388 and SL 509, whose fields
showed the presence of a secondary giant clump, observationally detected by
Bica et al. (1998, AJ, 116, 723). From the collected data we found that most of
the observed field CMDs do not show a separate secondary clump, but rather a
continuous vertical structure (VS), which is clearly seen for the first time.
Its position and size are nearly the same throughout the surveyed regions: it
lies below the Red Giant Clump (RGC) and extends from the bottom of the RGC to
approximately 0.45 mag fainter, spanning the bluest color range of the RGC. The
more numerous the VS stars in a field, the larger the number of LMC giants in
the same zone. Our analysis demonstrate that VS stars belong to the LMC and are
most likely the consequence of some kind of evolutionary process in the LMC,
particularly in those LMC regions with a noticeable large giant population. Our
results suggest that in order to trigger the formation of VS stars, there
should be other conditions besides the appropriate age, metallicity, and the
necessary red giant star density. Indeed, stars satisfying the requisites
mentioned above are commonly found throughout the LMC, but the VS phenomenon is
only clearly seen in some isolated regions. Finally, the fact that clump stars
have an intrinsic luminosity dispersion further constrains the use of the clump
magnitude as a reliable distance indicator.Comment: 25 pages, 9 figures, 3 tables; to be published in AJ, Dec. issu
Optimizing individual benefits of pulmonary rehabilitation including a multifaceted dietary intervention – A single-arm feasibility study
IntroductionFew studies have examined the effectiveness of nutrition interventions in municipal COPD rehabilitation programs. The objectives of this study were to examine the feasibility of implementation in practice (primary) and the feasibility of study methods and potential effectiveness (secondary) of a multidisciplinary dietary intervention offered to patients who start municipal rehabilitations program.MethodsThis study was a single arm intervention study with a pre-post design. Participant were recruited from five different municipal rehabilitation centers and received three individualized dietary counselling's. The primary outcome was retention, compliance to the intervention and complement of data collection. Secondary outcomes included changes in dietary intake, body composition and physical function. Analysis of the primary and secondary outcomes was primarily based on descriptive statistics.ResultsIn total, 111 (77%) of 145 eligible patients from five different municipalities consented to participate. Of them 99 (89%) completed. Before the intervention 67 (63%) of the participants had 75% or more of their requirement of energy covered, and 48 (45%) had 75% or more of their protein requirements covered. At the end of the intervention, 76 (77%) of the participants had 75% or more coverage of energy requirements and 80 (83%) had 75% or more coverage of their protein requirements. In general, the level of completeness was high.ConclusionWe found recruitment to be feasible, a high rate of retention, a high compliance to the intervention and high completeness of the data collection
Statistics of Stellar Populations of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud
A comparative analysis of Washington color-magnitude diagrams (CMDs) for 14
star clusters and respective surrounding fields in the Large Magellanic Cloud
(LMC) outer disk is presented. Each CCD frame including field and respective
cluster covers an area of 185 arcmin^2. The stellar population sampled is of
intermediate age and metallicity. CMD radial analysis involving star count
ratios, morphology and integrated light properties are carried out. Luminosity
functions (LFs) are also presented. Two main results are: (i) Within the range
4<R(kpc)<8, the distance from the LMC center is well correlated with the
average age in the sense that inner fields are younger and; (ii) Beyond
approximately 8kpc the outer fields do not show evidence of a significant
intermediate-age component in their stellar populations, as inferred from red
giant clump star counts.Comment: 27 pages, 4 tables, 11 figures; accepted by the A
The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample
The mass-loss process from evolved stars is a key ingredient for our
understanding of many fields of astrophysics, including stellar evolution and
the chemical enrichment of the interstellar medium via stellar yields. One the
main unsolved questions is the geometry of the mass-loss process. Taking
advantage of the results from the Herschel Mass loss of Evolved StarS (MESS)
programme, we initiated a coordinated effort to characterise the geometry of
mass loss from evolved red giants at various spatial scales. For this purpose
we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner
envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this
contribution we present an overview of the interferometric data collected
within the frame of our Large Programme, and we also add archive data for
completeness. We studied the geometry of the inner atmosphere by comparing the
observations with predictions from different geometric models. Asymmetries are
detected for five O-rich and S-type, suggesting that asymmetries in the N band
are more common among stars with such chemistry. We speculate that this fact is
related to the characteristics of the dust grains. Except for one star, no
interferometric variability is detected, i.e. the changes in size of the shells
of non-mira stars correspond to changes of the visibility of less than 10%. The
observed spectral variability confirms previous findings from the literature.
The detection of dust in our sample follows the location of the AGBs in the
IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in
region II and in the carbon stars in region VII. The SiC dust feature does not
appear in the visibility spectrum of UAnt and SSct, which are two carbon stars
with detached shells. This finding has implications for the theory of SiC dust
formation.Comment: 43 pages, 31 figures; accepted for publication in Astronomy &
Astrophysics. Abstract shortened for compilation reasons. Metadata correcte
Exploring Halo Substructure with Giant Stars III: First Results from the Grid Giant Star Survey and Discovery of a Possible Nearby Sagittarius Tidal Structure in Virgo
We describe first results of a spectroscopic probe of selected fields from
the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in
a strip of eleven fields along delta approximately -17^{circ}, in the range 12
<~ alpha <~ 17 hours, reveals a group of 8 giants that have kinematical
characteristics differing from the main field population, but that as a group
maintain coherent, smoothly varying distances and radial velocities with
position across the fields. Moreover, these stars have roughly the same
abundance, according to their MgH+Mgb absorption line strengths. Photometric
parallaxes place these stars in a semi-loop structure, arcing in a contiguous
distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial,
kinematical, and abundance coherence of these stars suggests that they are part
of a diffuse stream of tidal debris, and one roughly consistent with a wrapped,
leading tidal arm of the Sagittarius dwarf spheroidal galaxy.Comment: 8 pages including 4 figures. Accepted for publication in ApJ
Monorail/Foxa2 regulates floorplate differentiation and specification of oligodendrocytes, serotonergic raphe neurones and cranial motoneurones
In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol(-/-) embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol(-/-) mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphe nucleus and the trochlear motor nucleus are absent in mol(-/-) embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins
Quantum Hall Effect on the Hofstadter Butterfly
Motivated by recent experimental attempts to detect the Hofstadter butterfly,
we numerically calculate the Hall conductivity in a modulated two-dimensional
electron system with disorder in the quantum Hall regime. We identify the
critical energies where the states are extended for each of butterfly subbands,
and obtain the trajectory as a function of the disorder. Remarkably, we find
that when the modulation becomes anisotropic, the critical energy branches
accompanying a change of the Hall conductivity.Comment: 4 pages, 6 figure
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The counter-propagating Rossby-wave perspective on baroclinic instability. II: Application to the Charney model
The constant-density Charney model describes the simplest unstable basic state with a planetary-vorticity gradient, which is uniform and positive, and baroclinicity that is manifest as a negative contribution to the potential-vorticity (PV) gradient at the ground and positive vertical wind shear. Together, these ingredients satisfy the necessary conditions for baroclinic instability. In Part I it was shown how baroclinic growth on a general zonal basic state can be viewed as the interaction of pairs of ‘counter-propagating Rossby waves’ (CRWs) that can be constructed from a growing normal mode and its decaying complex conjugate. In this paper the normal-mode solutions for the Charney model are studied from the CRW perspective.
Clear parallels can be drawn between the most unstable modes of the Charney model and the Eady model, in which the CRWs can be derived independently of the normal modes. However, the dispersion curves for the two models are very different; the Eady model has a short-wave cut-off, while the Charney model is unstable at short wavelengths. Beyond its maximum growth rate the Charney model has a neutral point at finite wavelength (r=1). Thereafter follows a succession of unstable branches, each with weaker growth than the last, separated by neutral points at integer r—the so-called ‘Green branches’. A separate branch of westward-propagating neutral modes also originates from each neutral point. By approximating the lower CRW as a Rossby edge wave and the upper CRW structure as a single PV peak with a spread proportional to the Rossby scale height, the main features of the ‘Charney branch’ (0<r<1) can be deduced without prior calculation of the normal modes. Furthermore, CRWs from this branch are seen to make a smooth transition into the boundary and interior PV structure of the neutral modes appearing at r=1. The behaviour of the other branches and neutral points is essentially the same when viewed from the CRW perspective, but with cancelling interior PV structures reducing the self and mutual interaction of the CRWs. The underlying dynamics determining the nature of all the solutions is the difference in the scale-dependence of PV inversion for boundary and interior PV anomalies, the Rossby-wave propagation mechanism and the CRW interaction. The behaviour of the Charney modes and the first neutral branch, which rely on tropospheric PV gradients, are arguably more applicable to the atmosphere than modes of the Eady model where the positive PV gradient exists only at the tropopaus
History and phylogeny of intermediate filaments: Now in insects
Intermediate filaments include the nuclear lamins, which are universal in metazoans, and the cytoplasmic intermediate filaments, which are much more varied and form cell type-specific networks in animal cells. Until now, it has been thought that insects harbor lamins only. This view is fundamentally challenged by the discovery, reported in BMC Biology, of an intermediate filament-like cytoplasmic protein, isomin, in the hexapod Isotomurus maculatus. Here we briefly review the history of research on intermediate filaments, and discuss the implications of this latest finding in the context of what is known of their structure and functions
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