2,578 research outputs found
A dynamical transition from atomic to molecular intermediate-velocity clouds
Towards the high galactic latitude sky, the far-infrared (FIR) intensity is
tightly correlated to the total hydrogen column density which is made up of
atomic (HI) and molecular hydrogen (H. Above a certain column density
threshold, atomic hydrogen turns molecular. We analyse gas and dust properties
of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore
their transition from the atomic to the molecular phase. Driven by
observations, we investigate the physical processes that transform a purely
atomic IVC into a molecular one. Data from the Effelsberg-Bonn HI-Survey
(EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS.
Modified black-body emission spectra are fitted to deduce dust optical depths
and grain temperatures. We remove the contribution of atomic hydrogen to the
FIR intensity to estimate molecular hydrogen column densities. Two IVCs show
different FIR properties, despite their similarity in HI, such as narrow
spectral lines and large column densities. One FIR bright IVC is associated
with H, confirmed by CO emission; the other IVC
is FIR dim and shows no FIR excess, which indicates the absence of molecular
hydrogen. We propose that the FIR dim and bright IVCs probe the transition
between the atomic and molecular gas phase. Triggered by dynamical processes,
this transition happens during the descent of IVCs onto the galactic disk. The
most natural driver is ram pressure exerted onto the cloud by the increasing
halo density. Because of the enhanced pressure, the formation timescale of
H is reduced, allowing the formation of large amounts of H within a
few Myr.Comment: 13 pages, 14 figures, accepted for publication by A&
Non-destructive testing of composite plates by holographic vibrometry
We report on a wide-field optical monitoring method for revealing local
delaminations in sandwich-type composite plates at video-rate by holographic
vibrometry. Non-contact measurements of low frequency flexural waves is
performed with time-averaged heterodyne holography. It enables narrowband
imaging of local out-of-plane nanometric vibration amplitudes under sinusoidal
excitation, and reveals delamination defects, which cause local resonances of
flexural waves. The size of the defect can be estimated from the first
resonance frequency of the flexural wave and the mechanical parameters of the
observed layer of the composite plate
Two-dimensional flows of foam: drag exerted on circular obstacles and dissipation
A Stokes experiment for foams is proposed. It consists in a two-dimensional
flow of a foam, confined between a water subphase and a top plate, around a
fixed circular obstacle. We present systematic measurements of the drag exerted
by the flowing foam on the obstacle, \emph{versus} various separately
controlled parameters: flow rate, bubble volume, solution viscosity, obstacle
size and boundary conditions. We separate the drag into two contributions, an
elastic one (yield drag) at vanishing flow rate, and a fluid one (viscous
coefficient) increasing with flow rate. We quantify the influence of each
control parameter on the drag. The results exhibit in particular a power-law
dependence of the drag as a function of the solution viscosity and the flow
rate with two different exponents. Moreover, we show that the drag decreases
with bubble size, increases with obstacle size, and that the effect of boundary
conditions is small. Measurements of the streamwise pressure gradient,
associated to the dissipation along the flow of foam, are also presented: they
show no dependence on the presence of an obstacle, and pressure gradient
depends on flow rate, bubble volume and solution viscosity with three
independent power laws.Comment: 23 pages, 13 figures, proceeding of Eufoam 2004 conferenc
Acoustic measurements of boundary layer flux profiles over a sandy rippled bed under regular waves
The study of boundary layer sediment transport processes requires contemporaneous measurements of the bedforms, the flow and the sediment movement. Obtaining these three parameters, at the required temporal-spatial resolutions, has been traditionally difficult, especially within a few centimetres of the bed. To circumvent some of the deployment of an acoustic backscatter system, ABS, an acoustic ripple profiler, ARP, and an acoustic Doppler velocity profiler, ADVP, to measure sediment entrainment processes above a rippled bed under regular waves. High resolution acoustic observations of the suspend sediment concentration, flow and bedforms have been collected. Here we report on some of the initial results obtained from this study
Four decades of transmission of a multidrug-resistant Mycobacterium tuberculosis outbreak strain
The rise of drug-resistant strains is a major challenge to containing the tuberculosis (TB) pandemic. Yet, little is known about the extent of resistance in early years of chemotherapy and when transmission of resistant strains on a larger scale became a major public health issue. Here we reconstruct the timeline of the acquisition of antimicrobial resistance during a major ongoing outbreak of multidrug-resistant TB in Argentina. We estimate that the progenitor of the outbreak strain acquired resistance to isoniazid, streptomycin and rifampicin by around 1973, indicating continuous circulation of a multidrug-resistant TB strain for four decades. By around 1979 the strain had acquired additional resistance to three more drugs. Our results indicate that Mycobacterium tuberculosis (Mtb) with extensive resistance profiles circulated 15 years before the outbreak was detected, and about one decade before the earliest documented transmission of Mtb strains with such extensive resistance profiles globally.Fil: Eldholm, Vegard. Norwegian Institute of Public Health; NoruegaFil: Monteserin, Johana. Dirección Nacional de Institutos de Investigación. Administración Nacional de Laboratorios e Institutos de Salud. Instituto Nacional de Enfermedades Infecciosas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rieux, Adrien. Colegio Universitario de Londres; Reino UnidoFil: Lopez, Beatriz. Dirección Nacional de Institutos de Investigación. Administración Nacional de Laboratorios e Institutos de Salud. Instituto Nacional de Enfermedades Infecciosas; ArgentinaFil: Sobkowiak, Benjamin. Colegio Universitario de Londres; Reino UnidoFil: Ritacco, Gloria Viviana. Dirección Nacional de Institutos de Investigación. Administración Nacional de Laboratorios e Institutos de Salud. Instituto Nacional de Enfermedades Infecciosas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Balloux, Francois. Colegio Universitario de Londres; Reino Unid
Comparison of measurements of the outer scale of turbulence by three different techniques
We have made simultaneous and nearly simultaneous measurements of L0, the outer scale of turbulence, at the Palomar Observatory by using three techniques: angle-of-arrival covariance measurements with the Generalized Seeing Monitor (GSM), differential-image-motion measurements with the adaptive-optics system on the Hale 5-m telescope, and fringe speed measurements with the Palomar Testbed Interferometer (PTI). The three techniques give consistent results, an outer scale of approximately 10-20 m, despite the fact that the spatial scales of the three instruments vary from 1 m for the GSM to 100 m for the PTI
First performance of the gems + gmos system. Part1. Imaging
During the commissioning of the Gemini MCAO System (GeMS), we had the
opportunity to obtain data with the Gemini Multi-Object Spectrograph (GMOS),
the most utilised instrument at Gemini South Observatory, in March and May
2012. Several globular clusters were observed in imaging mode that allowed us
to study the performance of this new and untested combination. GMOS is a
visible instrument, hence pushing MCAO toward the visible.We report here on the
results with the GMOS instruments, derive photometric performance in term of
Full Width Half Maximum (FWHM) and throughput. In most of the cases, we
obtained an improvement factor of at least 2 against the natural seeing. This
result also depends on the Natural Guide Star constellation selected for the
observations and we then study the impact of the guide star selection on the
FWHM performance.We also derive a first astrometric analysis showing that the
GeMS+GMOS system provide an absolute astrometric precision better than 8mas and
a relative astrometric precision lower than 50 mas.Comment: 13 pages, 11 figures, accepted for publication in MNRAS on March 23rd
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