482 research outputs found
La lutte contre les feux de forêts
A l'occasion de la publication du n°100 de la revue Forêt Méditerranéenne, il a été demandé aux grandes institutions nationales qu'elles nous présentent leur vision des espaces forestiers méditerranéens "depuis Paris". Cet article nous donne le regard porté par le Ministère de l'Intérieur sur la forêt méditerranéenne
Link between laboratory and astrophysical radiative shocks
This work provides analytical solutions describing the post-shock structure
of radiative shocks growing in astrophysics and in laboratory. The equations
including a cooling function are solved for any values of the exponents , and
. This modeling is appropriate to astrophysics as the observed
radiative shocks arise in optically thin media. In contrast, in laboratory,
radiative shocks performed using high-power lasers present a radiative
precursor because the plasma is more or less optically thick. We study the
post-shock region in the laboratory case and compare with astrophysical shock
structure. In addition, we attempt to use the same equations to describe the
radiative precursor, but the cooling function is slightly modified. In future
experiments we will probe the PSR using X-ray diagnostics. These new
experimental results will allow to validate our astrophysical numerical codes
Wavelength-Dependent UV Photodesorption of Pure and Ices
Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: and are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption.
Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure and thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic and isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: photodesorption mainly occurs through excitation of the state and subsequent desorption of surface molecules. The observed vibronic structure in the photodesorption spectrum, together with the absence of formation, supports that the photodesorption mechanism of is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of and integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between and photodesorbed molecules per incoming photon.Astronom
X-ray photodesorption of complex organic molecules in protoplanetary disks -- I. Acetonitrile CH3CN
X-rays emitted from pre-main-sequence stars at the center of protoplanetary
disks can induce nonthermal desorption from interstellar ices populating the
cold regions. This X-ray photodesorption needs to be quantified for complex
organic molecules (COMs), including acetonitrile CH3CN, which has been detected
in several disks. We experimentally estimate the X-ray photodesorption yields
of neutral species from pure CH3CN ices and from interstellar ice analogs for
which CH3CN is mixed either in a CO- or H2O-dominated ice. The ices were
irradiated at 15 K by soft X-rays (400-600 eV) from synchrotron light (SOLEIL
synchrotron). X-ray photodesorption was probed in the gas phase via quadrupole
mass spectrometry. X-ray photodesorption yields were derived from the mass
signals and were extrapolated to higher X-ray energies for astrochemical
models. X-ray photodesorption of the intact CH3CN is detected from pure CH3CN
ices and from mixed 13CO:CH3CN ices, with a yield of about 5x10^(-4)
molecules/photon at 560 eV. When mixed in H2O-dominated ices, X-ray
photodesorption of the intact CH3CN at 560 eV is below its detection limit,
which is 10^(-4) molecules/photon. Yields associated with the desorption of
HCN, CH4 , and CH3 are also provided. The derived astrophysical yields
significantly depend on the local conditions expected in protoplanetary disks.
They vary from 10^(-4) to 10(-6) molecules/photon for the X-ray photodesorption
of intact CH3CN from CO-dominated ices. Only upper limits varying from
5x10^(-5) to 5x10^(-7) molecules/photon could be derived for the X-ray
photodesorption of intact CH3CN from H2O-dominated ices. X-ray photodesorption
of intact CH3CN from interstellar ices might in part explain the abundances of
CH3CN observed in protoplanetary disks. The desorption efficiency is expected
to vary with the local physical conditions, hence with the disk region
Highlights from the 6th International Society for Computational Biology Student Council Symposium at the 18th Annual International Conference on Intelligent Systems for Molecular Biology
This meeting report gives an overview of the keynote lectures and a selection of the student oral and poster presentations at the 6th International Society for Computational Biology Student Council Symposium that was held as a precursor event to the annual international conference on Intelligent Systems for Molecular Biology (ISMB). The symposium was held in Boston, MA, USA on July 9th, 2010
Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
We present results from new experiments to study the dynamics of radiative
shocks, reverse shocks and radiative precursors. Laser ablation of a solid
piston by the Orion high-power laser at AWE Aldermaston UK was used to drive
radiative shocks into a gas cell initially pressurised between and $1.0 \
bar with different noble gases. Shocks propagated at {80 \pm 10 \ km/s} and
experienced strong radiative cooling resulting in post-shock compressions of {
\times 25 \pm 2}. A combination of X-ray backlighting, optical self-emission
streak imaging and interferometry (multi-frame and streak imaging) were used to
simultaneously study both the shock front and the radiative precursor. These
experiments present a new configuration to produce counter-propagating
radiative shocks, allowing for the study of reverse shocks and providing a
unique platform for numerical validation. In addition, the radiative shocks
were able to expand freely into a large gas volume without being confined by
the walls of the gas cell. This allows for 3-D effects of the shocks to be
studied which, in principle, could lead to a more direct comparison to
astrophysical phenomena. By maintaining a constant mass density between
different gas fills the shocks evolved with similar hydrodynamics but the
radiative precursor was found to extend significantly further in higher atomic
number gases (\sim4$ times further in xenon than neon). Finally, 1-D and 2-D
radiative-hydrodynamic simulations are presented showing good agreement with
the experimental data.Comment: HEDLA 2016 conference proceeding
Hydrodynamic instabilities in a highly radiative environment
In this paper, we present the effects of a radiative shock (RS) on the morphology of jet-like objects subjected to hydrodynamic instabilities. To this end, we used an experimental platform developed to create RSs on high energy laser facilities such as LULI2000 and GEKKO XII. Here, we employed modulated targets to initiate Richtmyer–Meshkov and Rayleigh–Taylor instability (RTI) growth in the presence of an RS. The RS is obtained by generating a strong shock in a dense pusher that expands into a low-density xenon gas. With our design, only a limited RTI growth occurs in the absence of radiative effects. A strongly radiative shock has opposite effects on RTI growth. While its deceleration enhances the instability growth, the produced radiations tend to stabilize the interfaces. Our indirect experimental observations suggest a lower instability growth despite the interface deceleration. In addition, the jets, produced during the experiment, are relevant to astrophysical structures such as Herbig–Haro objects or other radiatively cooling jets
Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
Context. Methane is among the main components of the ice mantles of
insterstellar dust grains, where it is at the start of a rich solid-phase
chemical network. Quantification of the photon-induced desorption yield of
these frozen molecules and understanding of the underlying processes is
necessary to accurately model the observations and the chemical evolution of
various regions of the interstellar medium. Aims. This study aims at
experimentally determining absolute photodesorption yields for the CH4 molecule
as a function of photon energy. The influence of the ice composition is also
investigated. By studying the methane desorption from layered CH4:CO ice,
indirect desorption processes triggered by the excitation of the CO molecules
is monitored and quantified. Methods. Tunable monochromatic VUV light from the
DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91
nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice
samples. The release of species in the gas phase is monitored by quadrupole
mass spectrometry and absolute photodesorption yields of intact CH4 are
deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV
(~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4,
which confirms a desorption mechanism mediated by electronic transitions in the
ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a
pattern characteristic of CO absorption, indicating desorption induced by
energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from
the pure ice in various interstellar environments is around 2.0 x 10^-3
molecules per incident photon. Results on CO-induced indirect desorption of CH4
provide useful insights for the generalization of this process to other
molecules co-existing with CO in ice mantles
Muscle fiber conduction velocity is more affected after eccentric than concentric exercise
It has been shown that mean muscle fiber conduction velocity (CV) can be acutely impaired after eccentric exercise. However, it is not known whether this applies to other exercise modes. Therefore, the purpose of this experiment was to compare the effects of eccentric and concentric exercises on CV, and amplitude and frequency content of surface electromyography (sEMG) signals up to 24 h post-exercise. Multichannel sEMG signals were recorded from biceps brachii muscle of the exercised arm during isometric maximal voluntary contraction (MVC) and electrically evoked contractions induced by motor-point stimulation before, immediately after and 2 h after maximal eccentric (ECC group, N = 12) and concentric (CON group, N = 12) elbow flexor exercises. Isometric MVC decreased in CON by 21.7 ± 12.0% (± SD, p < 0.01) and by 30.0 ± 17.7% (p < 0.001) in ECC immediately post-exercise when compared to baseline. At 2 h post-exercise, ECC showed a reduction in isometric MVC by 24.7 ± 13.7% (p < 0.01) when compared to baseline, while no significant reduction (by 8.0 ± 17.0%, ns) was observed in CON. Similarly, reduction in CV was observed only in ECC both during the isometric MVC (from baseline of 4.16 ± 0.3 to 3.43 ± 0.4 m/s, p < 0.001) and the electrically evoked contractions (from baseline of 4.33 ± 0.4 to 3.82 ± 0.3 m/s, p < 0.001). In conclusion, eccentric exercise can induce a greater and more prolonged reduction in muscle force production capability and CV than concentric exercis
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