144 research outputs found
Homogenization of the one-dimensional wave equation
We present a method for two-scale model derivation of the periodic
homogenization of the one-dimensional wave equation in a bounded domain. It
allows for analyzing the oscillations occurring on both microscopic and
macroscopic scales. The novelty reported here is on the asymptotic behavior of
high frequency waves and especially on the boundary conditions of the
homogenized equation. Numerical simulations are reported
Relativistic tidal compressions of a star by a massive black hole
Aims: We investigate the stellar pancake mechanism during which a solar-type
star is tidally flattened within its orbital plane passing close to a 10^6
solar masses black hole. We simulate the relativistic orthogonal compression
process and follow the associated shock waves formation.
Methods: We consider a one-dimensional hydrodynamical stellar model moving in
the relativistic gravitational field of a non-rotating black hole. The model is
numerically solved using a Godunov-type shock-capturing source-splitting method
in order to correctly reproduce the shock waves profiles.
Results: Simulations confirm that the space-time curvature can induce several
successive orthogonal compressions of the star which give rise to several
strong shock waves. The shock waves finally escape from the star and repeatedly
heat up the stellar surface to high energy values. Such a shock-heating could
interestingly provide a direct observational signature of strongly disruptive
star - black hole encounters through the emission of hard X or soft gamma-ray
bursts. Timescales and energies of such a process are consistent with some
observed events such as GRB 970815.Comment: 8 pages, 11 figures, submitted to Astron. Astrophy
Shock waves in tidally compressed stars by massive black holes
We interest in the case of a main-sequence star deeply penetrating within the
tidal radius of a massive black hole. We focus on the compression phase leading
to a so-called pancake configuration of the star at the instant of maximal
compression. The aim is to study the tidal compression process paying
particular attention to the development of shock waves;to deduce reliable
estimates of the thermodynamical quantities involved in the pancake star; and
to solve a controversy about whether or not thermonuclear reactions can be
triggered in the core of a tidally compressed star. We have set up a
one-dimensional hydrodynamical model well-adapted to the geometry of the
problem. Based on the high-resolution shock-capturing Godunov-type approach, it
allows to study the compression phase undergone by the star in the direction
orthogonal to its orbital plane. We show the existence of two regimes depending
on whether shock waves develop before or after the instant of maximal core
compression. In both cases we confirm high compression and heating factors in
the stellar core able to trigger a thermonuclear explosion. Moreover, we show
that the shock waves carry outwards a brief but very high peak of temperature
from the centre to the surface of the star. We tentatively conclude that the
phenomenon could give rise to hard electromagnetic radiation, to be compared to
some X-ray flares already observed in some galactic nuclei harbouring massive
black holes. Finally, we estimate that the rate of pancake stars should be
about per galaxy per year. If generated in hard X- or -ray
band, several events of this kind per year should be detectable within the full
observable universe.Comment: 19 pages, 38 figures, 7 tables; v2 : corrected to match version
accepted in Astron. Astrophys. Tables and references added, new simulations
also performed for adiabatic index 4/
Terre et agriculture sous la Révolution et l’Empire
Laurent BrassartDes travaux récents sur la base d’analyses transnationales ont montré que beaucoup des événements agraires imputables à la Révolution s’étaient déjà produits auparavant : c’est le cas de la sécularisation des biens du clergé (notamment avec le Joséphisme) ou de la remise en cause de certains droits de propriété. La Révolution peut-elle encore être tenue pour un moment déterminant dans l’histoire des campagnes européennes ? Jean-Pierre JessenneLa Révolution doit être inscrite d..
Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole
Massive black holes are believed to reside at the centres of most galaxies.
They can be- come detectable by accretion of matter, either continuously from a
large gas reservoir or impulsively from the tidal disruption of a passing star,
and conversion of the gravitational energy of the infalling matter to light.
Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be
variable but have never been observed to turn on or off. Tidal disruption of
stars by dormant massive black holes has been inferred indirectly but the on-
set of a tidal disruption event has never been observed. Here we report the
first discovery of the onset of a relativistic accretion-powered jet in the new
extragalactic transient, Swift J164449.3+573451. The behaviour of this new
source differs from both theoretical models of tidal disruption events and
observations of the jet-dominated AGN known as blazars. These differences may
stem from transient effects associated with the onset of a powerful jet. Such
an event in the massive black hole at the centre of our Milky Way galaxy could
strongly ionize the upper atmosphere of the Earth, if beamed towards us.Comment: Submitted to Nature. 4 pages, 3 figures (main paper). 26 pages, 13
figures (supplementary information
Prognostic factors to succeed in surgical treatment of chronic acromioclavicular dislocations
AbstractIntroductionTreatment of chronic acromioclavicular joint dislocation (ACJD) remains a poorly known and controversial subject. Given the many surgical options, it is not always easy to determine which steps are indispensable.MethodsThis article reports a multicenter prospective study. The clinical and radiological follow-up involved a comparative analysis of the preoperative and postoperative data at 1 year, including pain (visual analogue scale), subjective functional incapacity (QuickDASH), and the objective Constant score, as well as a comparative analysis of vertical and horizontal movements measured on simple x-rays.ResultsBased on a series of 140 operated ACJDs, we included 24 chronic ACJDs. The mean time to surgery was 46 weeks (range, 1 month to 4 years). The patients’ mean age was 41 years, with a majority of males (75%), 72% of whom participated in recreational sports. Professionally, 40% of the subjects had jobs involving manual labor. We noted 40% grade III, 24% grade IV, and 36% grade V injury according to the Rockwood classification. In 92% of cases, coracoclavicular stabilization was provided by a double button implant, reinforced with a biological graft in 88% of the cases. In 29%, millimeters to centimeters of the distal clavicle were resected and acromioclavicular stabilization was associated in 54%. We observed complications in 33% of the cases. At 1 year postoperative, 21 patients underwent clinical and radiological follow-up (87.5%). Only 35% of the patients were satisfied or very satisfied, whereas 100% of them would recommend the operation. Full-time work was resumed in 91% of the cases and all sports could be resumed in 86%. The pre- and postoperative values at 1 year changed as follows: the mean Constant score improved from 61 to 87 (p=0.00002); the subjective QuickDASH score decreased from 41 to 9 (p=0.00002); and radiologically significant reduction of the initial displacement was observed in the vertical plane (p<10−3) and the horizontal plane (p=0.022).ConclusionIn this study, the favorable prognostic factors found were: time to surgery less than 3 months (p=0.02), associated acromioclavicular stabilization, and postoperative immobilization with a sling extended to 6 weeks. However, resection of the distal clavicle did not influence the final result.Level of proofLevel II prospective non-randomized comparative study
Double-J ureteral stent under local anesthesia for women
INTRODUCTION: Ureteral stent placement is a key urologic procedure used to manage ureteral obstructions. It is usually performed under general anesthesia (GA) with its inherent risks. The objective was to evaluate safety, feasibility and tolerance of ureteral stent placement under local anesthesia (LA) in women. MATERIALS AND METHODS: From January 2010 to January 2013, we prospectively and consecutively reviewed all female patients who had an urgent retrograde ureteral stent placement under LA. Only primary stent placements were included in the study. Pain was assessed after surgery by Visual Analog Scale (VAS) and pain and comfort assessment during stent placement were reported. We compared outcomes and tolerance with patients under general anesthesia (GA) matched by age and operatives indications during the same period. RESULTS: We included 36 patients (18 under LA and 18 under GA) with a mean age of 59.4 +/- 22.4 years. The mean operative time was 24.4 +/- 12.9 min and 18.8 +/- 6.5 min in LA group and GA group (p = 0.110), respectively. One patient needed GA due to a poor tolerance. The mean perioperative VAS scores under LA and GA were 5.89 +/-2.95 and 2.06 +/- 2.67 (p < 0.0001), respectively. There were no intraoperative complications in either group. The procedure was painful for 16 (88.8%) patients from the LA group and 9 (50%) patients would not accept to undergo this intervention under LA again. CONCLUSION: Ureteral stent placement under LA in women can be performed safely and effectively. However, this procedure is painful and should be proposed only to selected cases
Elastin Peptides Signaling Relies on Neuraminidase-1-Dependent Lactosylceramide Generation
The sialidase activity of neuraminidase-1 (Neu-1) is responsible for ERK 1/2 pathway activation following binding of elastin peptide on the elastin receptor complex. In this work, we demonstrate that the receptor and lipid rafts colocalize at the plasma membrane. We also show that the disruption of these microdomains as well as their depletion in glycolipids blocks the receptor signaling. Following elastin peptide treatment, the cellular GM3 level decreases while lactosylceramide (LacCer) content increases consistently with a GM3/LacCer conversion. The use of lactose or Neu-1 siRNA blocks this process suggesting that the elastin receptor complex is responsible for this lipid conversion. Flow cytometry analysis confirms this elastin peptide-driven LacCer generation. Further, the use of a monoclonal anti-GM3 blocking antibody shows that GM3 is required for signaling. In conclusion, our data strongly suggest that Neu-1-dependent GM3/LacCer conversion is the key event leading to signaling by the elastin receptor complex. As a consequence, we propose that LacCer is an early messenger for this receptor
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