1,968 research outputs found
Transport Phenomena and Structuring in Shear Flow of Suspensions near Solid Walls
In this paper we apply the lattice-Boltzmann method and an extension to
particle suspensions as introduced by Ladd et al. to study transport phenomena
and structuring effects of particles suspended in a fluid near sheared solid
walls. We find that a particle free region arises near walls, which has a width
depending on the shear rate and the particle concentration. The wall causes the
formation of parallel particle layers at low concentrations, where the number
of particles per layer decreases with increasing distance to the wall.Comment: 14 pages, 14 figure
Simultaneous sub-second hyperpolarization of the nuclear and electron spins of phosphorus in silicon
We demonstrate a method which can hyperpolarize both the electron and nuclear
spins of 31P donors in Si at low field, where both would be essentially
unpolarized in equilibrium. It is based on the selective ionization of donors
in a specific hyperfine state by optically pumping donor bound exciton
hyperfine transitions, which can be spectrally resolved in 28Si. Electron and
nuclear polarizations of 90% and 76%, respectively, are obtained in less than a
second, providing an initialization mechanism for qubits based on these spins,
and enabling further ESR and NMR studies on dilute 31P in 28Si.Comment: 4 pages, 3 figure
Molecular line mapping of the giant molecular cloud associated with RCW 106 - II. Column density and dynamical state of the clumps
We present a fully sampled C^{18}O (1-0) map towards the southern giant
molecular cloud (GMC) associated with the HII region RCW 106, and use it in
combination with previous ^{13}CO (1-0) mapping to estimate the gas column
density as a function of position and velocity. We find localized regions of
significant ^{13}CO optical depth in the northern part of the cloud, with
several of the high-opacity clouds in this region likely associated with a
limb-brightened shell around the HII region G333.6-0.2. Optical depth
corrections broaden the distribution of column densities in the cloud, yielding
a log-normal distribution as predicted by simulations of turbulence.
Decomposing the ^{13}CO and C^{18}O data cubes into clumps, we find relatively
weak correlations between size and linewidth, and a more sensitive dependence
of luminosity on size than would be predicted by a constant average column
density. The clump mass spectrum has a slope near -1.7, consistent with
previous studies. The most massive clumps appear to have gravitational binding
energies well in excess of virial equilibrium; we discuss possible
explanations, which include magnetic support and neglect of time-varying
surface terms in the virial theorem. Unlike molecular clouds as a whole, the
clumps within the RCW 106 GMC, while elongated, appear to show random
orientations with respect to the Galactic plane.Comment: 17 pages, to appear in MNRA
Expression of a Dominant Negative CELF Protein In Vivo Leads to Altered Muscle Organization, Fiber Size, and Subtype
CUG-BP and ETR-3-like factor (CELF) proteins regulate tissue- and developmental stage-specific alternative splicing in striated muscle. We previously demonstrated that heart muscle-specific expression of a nuclear dominant negative CELF protein in transgenic mice (MHC-CELFΔ) effectively disrupts endogenous CELF activity in the heart in vivo, resulting in impaired cardiac function. In this study, transgenic mice that express the dominant negative protein under a skeletal muscle-specific promoter (Myo-CELFΔ) were generated to investigate the role of CELF-mediated alternative splicing programs in normal skeletal muscle.Myo-CELFΔ mice exhibit modest changes in CELF-mediated alternative splicing in skeletal muscle, accompanied by a reduction of endomysial and perimysial spaces, an increase in fiber size variability, and an increase in slow twitch muscle fibers. Weight gain and mean body weight, total number of muscle fibers, and overall muscle strength were not affected.Although these findings demonstrate that CELF activity contributes to the normal alternative splicing of a subset of muscle transcripts in vivo, the mildness of the effects in Myo-CELFΔ muscles compared to those in MHC-CELFΔ hearts suggests CELF activity may be less determinative for alternative splicing in skeletal muscle than in heart muscle. Nonetheless, even these small changes in CELF-mediated splicing regulation were sufficient to alter muscle organization and muscle fiber properties affected in myotonic dystrophy. This lends further evidence to the hypothesis that dysregulation of CELF-mediated alternative splicing programs may be responsible for the disruption of these properties during muscle pathogenesis
Noiseless nonreciprocity in a parametric active device
Nonreciprocal devices such as circulators and isolators belong to an
important class of microwave components employed in applications like the
measurement of mesoscopic circuits at cryogenic temperatures. The measurement
protocols usually involve an amplification chain which relies on circulators to
separate input and output channels and to suppress backaction from different
stages on the sample under test. In these devices the usual reciprocal symmetry
of circuits is broken by the phenomenon of Faraday rotation based on magnetic
materials and fields. However, magnets are averse to on-chip integration, and
magnetic fields are deleterious to delicate superconducting devices. Here we
present a new proposal combining two stages of parametric modulation emulating
the action of a circulator. It is devoid of magnetic components and suitable
for on-chip integration. As the design is free of any dissipative elements and
based on reversible operation, the device operates noiselessly, giving it an
important advantage over other nonreciprocal active devices for quantum
information processing applications.Comment: 17 pages, 4 figures + 12 pages Supplementary Informatio
The physical and dynamical structure of Serpens - Two very different sub-(proto)clusters
Context. The Serpens North cluster is a nearby low mass star forming region which is part of the Gould belt. It contains a range of young stars thought to correspond to two different bursts of star formation and provides the opportunity to study different stages of cluster formation.
Aims. This work aims to study the molecular gas in the Serpens North cluster to probe the origin of the most recent burst of star formation in Serpens.
Methods. Transitions of the C17O and C18O observed with the IRAM 30 m telescope and JCMT are used to study the mass and velocity structure of the region while the physical properties of the gas are derived using LTE and non-LTE analyses of the three lowest transitions of C18O.
Results. The molecular emission traces the two centres of star formation which are seen in submillimetre dust continuum emission. In the ~40 NW sub-cluster the gas and dust emission trace the same structures although there is evidence of some depletion of the gas phase C18O. The gas has a very uniform temperature (~10 K) and velocity (~8.5 km s-1) throughout the region. This is in marked contrast to the SE sub-cluster. In this region the dust and the gas trace different features, with the temperature peaking between the submillimetre continuum sources, reaching up to ~14 K. The gas in this region has double peaked line profiles which reveal the presence of a second cloud in the line of sight. The submillimetre dust continuum sources predominantly appear located in the interface region between the two clouds.
Conclusions. Even though they are at a similar stage of evolution, the two Serpens sub-clusters have very different characteristics. We propose that these differences are linked to the initial trigger of the collapse in the regions and suggest that a cloud-cloud collision could explain the observed properties
Observations and radiative transfer modelling of a massive dense cold core in G333
Cold massive cores are one of the earliest manifestations of high mass star
formation. Following the detection of SiO emission from G333.125-0.562, a cold
massive core, further investigations of the physics, chemistry and dynamics of
this object has been carried out. Mopra and NANTEN2 molecular line profile
observations, Australia Telescope Compact Array (ATCA) line and continuum
emission maps, and Spitzer 24 and 70 \mum images were obtained. These new data
further constrain the properties of this prime example of the very early stages
of high mass star formation. A model for the source was constructed and
compared directly with the molecular line data using a 3D molecular line
transfer code - MOLLIE. The ATCA data reveal that G333.125-0.562 is composed of
two sources. One of the sources is responsible for the previously detected
molecular outflow and is detected in the Spitzer 24 and 70 \mum band data.
Turbulent velocity widths are lower than other more active regions of G333
which reflects the younger evolutionary stage and/or lower mass of this core.
The molecular line modelling requires abundances of the CO isotopes that
strongly imply heavy depletion due to freeze-out of this species onto dust
grains. The principal cloud is cold, moderately turbulent and possesses an
outflow which indicates the presence of a central driving source. The secondary
source could be an even less evolved object as no apparent associations with
continuum emissions at (far-)infrared wavelengths.Comment: 10 pages, accepted to MNRA
Simulating Three-Dimensional Hydrodynamics on a Cellular-Automata Machine
We demonstrate how three-dimensional fluid flow simulations can be carried
out on the Cellular Automata Machine 8 (CAM-8), a special-purpose computer for
cellular-automata computations. The principal algorithmic innovation is the use
of a lattice-gas model with a 16-bit collision operator that is specially
adapted to the machine architecture. It is shown how the collision rules can be
optimized to obtain a low viscosity of the fluid. Predictions of the viscosity
based on a Boltzmann approximation agree well with measurements of the
viscosity made on CAM-8. Several test simulations of flows in simple geometries
-- channels, pipes, and a cubic array of spheres -- are carried out.
Measurements of average flux in these geometries compare well with theoretical
predictions.Comment: 19 pages, REVTeX and epsf macros require
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Plastic dislocation motion via nonequilibrium molecular and continuum dynamics
The classical two-dimensional close-packed triangular lattice, with nearest-neighbor spring forces, is a convenient standard material for the investigation of dislocation motion and plastic flow. Two kinds of calculations, based on this standard material, are described here: (1) Molecular Dynamics simulations, incorporating adiabatic strains described with the help of Doll's Tensor, and (2) Continuum Dynamics simulations, incorporating periodic boundaries and dislocation interaction through stress-field superposition
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