748 research outputs found
Supernova Remnants in the Magellanic Clouds III: An X-ray Atlas of LMC Supernova Remnants
We have used archival ROSAT data to present X-ray images of thirty-one
supernova remnants (SNRs) in the Large Magellanic Cloud (LMC). We have
classified these remnants according to their X-ray morphologies, into the
categories of Shell-Type, Diffuse Face, Centrally Brightened, Point-Source
Dominated, and Irregular. We suggest possible causes of the X-ray emission for
each category, and for individual features of some of the SNRs.Comment: 27 pages, 6 figures (9 figure files). To appear in the Supplement
Series of the Astrophysical Journal, August 1999 Vol. 123 #
Quasilinear theory of the 2D Euler equation
We develop a quasilinear theory of the 2D Euler equation and derive an
integro-differential equation for the evolution of the coarse-grained
vorticity. This equation respects all the invariance properties of the Euler
equation and conserves angular momentum in a circular domain and linear impulse
in a channel. We show under which hypothesis we can derive a H-theorem for the
Fermi-Dirac entropy and make the connection with statistical theories of 2D
turbulence.Comment: 4 page
Extented ionized gas emission and kinematics of the compact group galaxies in HCG 16: Signatures of mergers
We report on kinematic observations of Ha emission line from four late-type
galaxies of Hickson Compact Group 16 (H16a,b,c and d) obtained with a scanning
Fabry-Perot interferometer and samplings of 16 km/s and 1". The velocity fields
show kinematic peculiarities for three of the four galaxies: H16b, c and d.
Misalignments between the kinematic and photometric axes of gas and stellar
components (H16b,c,d), double gas systems (H16c) and severe warping of the
kinematic major axis (H16b and c) were some of the peculiarities detected. We
conclude that major merger events have taken place in at least two of the
galaxies group. H16c and d, based on their significant kinematic peculiarities,
their double nuclei and high infrared luminosities. Their Ha gas content is
strongly spatially concentred - H16d contains a peculiar bar-like structure
confined to the inner 1 h^-1 kpc region. These observations are in
agreement with predictions of simulations, namely that the gas flows towards
the galaxy nucleus during mergers, forms bars and fuel the central activity.
Galaxy H16b, and Sb galaxy, also presents some of the kinematic evidences for
past accretion events. Its gas content, however, is very spare, limiting our
ability to find other kinematic merging indicators, if they are present. We
find that isolated mergers, i.e., they show an anormorphous morphology and no
signs of tidal tails. Tidal arms and tails formed during the mergers may have
been stripped by the group potential (Barnes & Hernquist 1992) ar alternatively
they may have never been formed. Our observations suggest that HCG 16 may be a
young compact group in formation throught the merging of close-by objects in a
dense environment.Comment: Accepted for publication in ApJ. 35 pages, 13 figures. tar file
gzipped and uuencode
Extensive characterization of a high Reynolds number decelerating boundary layer using advanced optical metrology
An experiment conducted in the framework of the EUHIT project and designed to
characterize large scale structures in an adverse pressure gradient boundary
layer flow is presented. Up to 16 sCMOS cameras were used in order to perform
large scale turbulent boundary layer PIV measurements with a large field of
view and appropriate spatial resolution. To access the span-wise / wall-normal
signature of the structures as well, stereoscopic PIV measurements in
span-wise/wall-normal planes were performed at specific stream-wise locations.
To complement these large field of view measurements, long-range micro-PIV,
time resolved near wall velocity profiles and film-based measurements were
performed in order to determine the wall-shear stress and its fluctuations at
some specific locations along the model.Comment: 50 page
Evolutionary history of tuberculosis shaped by conserved mutations in the PhoPR virulence regulator
Although the bovine tuberculosis (TB) agent, Mycobacterium bovis, may infect humans and cause disease, long-term epidemiological data indicate that humans represent a spill-over host in which infection with M. bovis is not self-maintaining. Indeed, human-to-human transmission of M. bovis strains and other members of the animal lineage of the tubercle bacilli is very rare. Here, we report on three mutations affecting the two-component virulence regulation system PhoP/PhoR (PhoPR) in M. bovis and in the closely linked Mycobacterium africanum lineage 6 (L6) that likely account for this discrepancy. Genetic transfer of these mutations into the human TB agent, Mycobacterium tuberculosis, resulted in down-regulation of the PhoP regulon, with loss of biologically active lipids, reduced secretion of the 6-kDa early antigenic target (ESAT-6), and lower virulence. Remarkably, the deleterious effects of the phoPR mutations were partly compensated by a deletion, specific to the animal-adapted and M. africanum L6 lineages, that restores ESAT-6 secretion by a PhoPR-independent mechanism. Similarly, we also observed that insertion of an IS6110 element upstream of the phoPR locus may completely revert the phoPR-bovisâassociated fitness loss, which is the case for an exceptional M. bovis human outbreak strain from Spain. Our findings ultimately explain the long-term epidemiological data, suggesting that M. bovis and related phoPR-mutated strains pose a lower risk for progression to overt human TB, with major impact on the evolutionary history of TB
Scaling laws and vortex profiles in 2D decaying turbulence
We use high resolution numerical simulations over several hundred of turnover
times to study the influence of small scale dissipation onto vortex statistics
in 2D decaying turbulence. A self-similar scaling regime is detected when the
scaling laws are expressed in units of mean vorticity and integral scale, as
predicted by Carnevale et al., and it is observed that viscous effects spoil
this scaling regime. This scaling regime shows some trends toward that of the
Kirchhoff model, for which a recent theory predicts a decay exponent .
In terms of scaled variables, the vortices have a similar profile close to a
Fermi-Dirac distribution.Comment: 4 Latex pages and 4 figures. Submitted to Phys. Rev. Let
Perpendicular momentum injection by lower hybrid wave in a tokamak
The injection of lower hybrid waves for current drive into a tokamak affects
the profile of intrinsic rotation. In this article, the momentum deposition by
the lower hybrid wave on the electrons is studied. Due to the increase in the
poloidal momentum of the wave as it propagates into the tokamak, the parallel
momentum of the wave increases considerably. The change of the perpendicular
momentum of the wave is such that the toroidal angular momentum of the wave is
conserved. If the perpendicular momentum transfer via electron Landau damping
is ignored, the transfer of the toroidal angular momentum to the plasma will be
larger than the injected toroidal angular momentum. A proper quasilinear
treatment proves that both perpendicular and parallel momentum are transferred
to the electrons. The toroidal angular momentum of the electrons is then
transferred to the ions via different mechanisms for the parallel and
perpendicular momentum. The perpendicular momentum is transferred to ions
through an outward radial electron pinch, while the parallel momentum is
transferred through collisions.Comment: 22 pages, 4 figure
Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations
Using a single coil mutual inductance technique, we measure the low
temperature dependence of the magnetic penetration depth in superconducting NbN
films prepared with similar critical temperatures around 16 K but with
different microstructures. Only (100) epitaxial and weakly granular (100)
textured films display the characteristic exponential dependence of
conventional BCS s-wave superconductors. More granular (111) textured films
exhibit a linear dependence, indicating a gapless state in spite of the s-wave
gap. This result is quantitatively explained by a model of thermal phase
fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.
Beyond scaling and locality in turbulence
An analytic perturbation theory is suggested in order to find finite-size
corrections to the scaling power laws. In the frame of this theory it is shown
that the first order finite-size correction to the scaling power laws has
following form , where
is a finite-size scale (in particular for turbulence, it can be the Kolmogorov
dissipation scale). Using data of laboratory experiments and numerical
simulations it is shown shown that a degenerate case with can
describe turbulence statistics in the near-dissipation range , where
the ordinary (power-law) scaling does not apply. For moderate Reynolds numbers
the degenerate scaling range covers almost the entire range of scales of
velocity structure functions (the log-corrections apply to finite Reynolds
number). Interplay between local and non-local regimes has been considered as a
possible hydrodynamic mechanism providing the basis for the degenerate scaling
of structure functions and extended self-similarity. These results have been
also expanded on passive scalar mixing in turbulence. Overlapping phenomenon
between local and non-local regimes and a relation between position of maximum
of the generalized energy input rate and the actual crossover scale between
these regimes are briefly discussed.Comment: extended versio
Long-time discrete particle effects versus kinetic theory in the self-consistent single-wave model
The influence of the finite number N of particles coupled to a monochromatic
wave in a collisionless plasma is investigated. For growth as well as damping
of the wave, discrete particle numerical simulations show an N-dependent long
time behavior resulting from the dynamics of individual particles. This
behavior differs from the one due to the numerical errors incurred by Vlasov
approaches. Trapping oscillations are crucial to long time dynamics, as the
wave oscillations are controlled by the particle distribution inhomogeneities
and the pulsating separatrix crossings drive the relaxation towards thermal
equilibrium.Comment: 11 pages incl. 13 figs. Phys. Rev. E, in pres
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