Observation comparative du déplacement ionique dans les couches minces de PbF2 β et de CaF2 par diffusion Rutherford

Des couches minces de PbF2 β et de CaF2, dont les conductivités ioniques sont très différentes, ont été analysées par rétro-diffusion de particules α. On a pu observer, dans le cas de PbF2, une variation importante du rapport des concentrations fluor/plomb dans l'épaisseur de la couche, correspondant à une accumulation de fluor du côté du faisceau incident. Cet effet est atténué dans les couches de CaF2. L'interprétation des résultats est basée sur l'existence d'un nombre important de défauts créés par le faisceau, et sur leur déplacement sous l'effet de la charge superficielle due à l'émission secondaire d'électrons

Global well-posedness for a Smoluchowski equation coupled with Navier-Stokes equations in 2D

We prove global existence for a nonlinear Smoluchowski equation (a nonlinear Fokker-Planck equation) coupled with Navier-Stokes equations in two dimensions. The proof uses a deteriorating regularity estimate and the tensorial structure of the main nonlinear terms

Asymmetries in random motions of neutral Hydrogen gas in spiral galaxies

(Abridged). It has been recently shown that random motions of the neutral Hydrogen gas of the Triangulum galaxy (M33) exhibit a bisymmetric perturbation which is aligned with the minor axis of the galaxy, suggesting a projection effect. To investigate if perturbations in the velocity dispersion of nearby discs are comparable to those of M33, the sample is extended to 32 galaxies from The HI Nearby Galaxy Survey and the Westerbork HI Survey of Spiral and Irregular Galaxies. We study velocity asymmetries in the disc planes by performing Fourier transforms of high-resolution HI velocity dispersion maps corrected for beam smearing effects, and measure the amplitudes and phase angles of the Fourier harmonics. We find strong perturbations of first, second and fourth orders. The strongest asymmetry is the bisymmetry, which is predominantly associated with the presence of spiral arms. The first order asymmetry is generally oriented close to the disc major axis, and the second and fourth order asymmetries are preferentially oriented along intermediate directions between the major and minor axes of the discs. These results are evidence that strong projection effects shape the HI velocity dispersion maps. The most likely source of systematic orientations is the anisotropy of velocities, through the projection of streaming motions stronger along one of the planar directions in the discs. Moreover, systematic phase angles of asymmetries in the HI velocity dispersion could arise from tilted velocity ellipsoids. We expect a larger incidence of correlation between the radial and tangential velocities of HI gas. Our methodology is a powerful tool to constrain the dominant direction of streaming motions and thus the shape of the velocity ellipsoid of HI gas, which is de facto anisotropic at the angular scales probed by the observations.Comment: 40 pages, 33 figures. Accepted for publication in Astronomy & Astrophysics. Full resolution version available upon reques

Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438

Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Halpha emission are closely linked. The kinematics of the northern extraplanar Halpha emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Halpha measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 mum emission are due to this difference in stripping efficiency.Comment: 8 pages, 6 figures, A&A, accepted for publicatio

On the flow map for 2D Euler equations with unbounded vorticity

In Part I, we construct a class of examples of initial velocities for which the unique solution to the Euler equations in the plane has an associated flow map that lies in no Holder space of positive exponent for any positive time. In Part II, we explore inverse problems that arise in attempting to construct an example of an initial velocity producing an arbitrarily poor modulus of continuity of the flow map.Comment: http://iopscience.iop.org/0951-7715/24/9/013/ for published versio

Global well-posedness issues for the inviscid Boussinesq system with Yudovich's type data

The present paper is dedicated to the study of the global existence for the inviscid two-dimensional Boussinesq system. We focus on finite energy data with bounded vorticity and we find out that, under quite a natural additional assumption on the initial temperature, there exists a global unique solution. None smallness conditions are imposed on the data. The global existence issues for infinite energy initial velocity, and for the B\'enard system are also discussed.Comment: 12 page

Existence of global strong solutions in critical spaces for barotropic viscous fluids

This paper is dedicated to the study of viscous compressible barotropic fluids in dimension $N\geq2$. We address the question of the global existence of strong solutions for initial data close from a constant state having critical Besov regularity. In a first time, this article show the recent results of \cite{CD} and \cite{CMZ} with a new proof. Our result relies on a new a priori estimate for the velocity, where we introduce a new structure to \textit{kill} the coupling between the density and the velocity as in \cite{H2}. We study so a new variable that we call effective velocity. In a second time we improve the results of \cite{CD} and \cite{CMZ} by adding some regularity on the initial data in particular $\rho_{0}$ is in $H^{1}$. In this case we obtain global strong solutions for a class of large initial data on the density and the velocity which in particular improve the results of D. Hoff in \cite{5H4}. We conclude by generalizing these results for general viscosity coefficients

The Triangulum Extended (TREX) Survey: The Stellar Disk Dynamics of M33 as a Function of Stellar Age

Triangulum, M33, is a low mass, relatively undisturbed spiral galaxy that offers a new regime in which to test models of dynamical heating. In spite of its proximity, the dynamical heating history of M33 has not yet been well constrained. In this work, we present the TREX Survey, the largest stellar spectroscopic survey across the disk of M33. We present the stellar disk kinematics as a function of age to study the past and ongoing dynamical heating of M33. We measure line of sight velocities for ~4,500 disk stars. Using a subset, we divide the stars into broad age bins using Hubble Space Telescope and Canada-France-Hawaii-Telescope photometric catalogs: massive main sequence stars and helium burning stars (~80 Myr), intermediate mass asymptotic branch stars (~1 Gyr), and low mass red giant branch stars (~4 Gyr). We compare the stellar disk dynamics to that of the gas using existing HI, CO, and Halpha kinematics. We find that the disk of M33 has relatively low velocity dispersion (~16 km/s), and unlike in the Milky Way and Andromeda galaxies, there is no strong trend in velocity dispersion as a function of stellar age. The youngest disk stars are as dynamically hot as the oldest disk stars and are dynamically hotter than predicted by most M33 like low mass simulated analogs in Illustris. The velocity dispersion of the young stars is highly structured, with the large velocity dispersion fairly localized. The cause of this high velocity dispersion is not evident from the observations and simulated analogs presented here.Comment: 27 pages, 15 figures, 6 table

Development of singularities for the compressible Euler equations with external force in several dimensions

We consider solutions to the Euler equations in the whole space from a certain class, which can be characterized, in particular, by finiteness of mass, total energy and momentum. We prove that for a large class of right-hand sides, including the viscous term, such solutions, no matter how smooth initially, develop a singularity within a finite time. We find a sufficient condition for the singularity formation, "the best sufficient condition", in the sense that one can explicitly construct a global in time smooth solution for which this condition is not satisfied "arbitrary little". Also compactly supported perturbation of nontrivial constant state is considered. We generalize the known theorem by Sideris on initial data resulting in singularities. Finally, we investigate the influence of frictional damping and rotation on the singularity formation.Comment: 23 page

The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies

[Abridged]We compare both the Milky Way and M31 galaxies to local external disk galaxies within the same mass range, using their relative locations in the planes formed by V_flat versus M_K, j_disk, and the average Fe abundance of stars in the galaxy outskirts. We find, for all relationships, that the MW is systematically offset by ~ 1 sigma, showing a significant deficiency in stellar mass, in angular momentum, in disk radius and [Fe/H] in the stars in its outskirts at a given V_flat. On the basis of their location in the M_K, V_flat, and R_d volume, the fraction of spirals like the MW is 7+/-1%, while M31 appears to be a "typical'' spiral. Our Galaxy appears to have escaped any significant merger over the last ~10 Gyrs which may explain why it is deficient by a factor 2 to 3 in stellar mass, angular momentum and outskirts metallicity and then, unrepresentative of the typical spiral. As with M31, most local spirals show evidence for a history shaped mainly by relatively recent merging. We conclude that the standard scenario of secular evolution is generally unable to reproduce the properties of most (if not all) spiral galaxies. However, the so-called "spiral rebuilding'' scenario proposed by Hammer et al. 2005 is consistent with the properties of both distant galaxies and of their descendants - the local spirals.Comment: 14 pages, 6 figures, to appear in Ap