Non equilibrium thermodynamics and cosmological pancakes formation

We investigate the influence of non equilibrium thermodynamics on cosmological structure formation. In this paper, we consider the collapse of planar perturbations usually called "Zel'dovich pancakes". We have developed for that purpose a new two fluids (gas and dark matter) hydrodynamical code, with three different thermodynamical species: electrons, ions and neutral particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of accretion shock waves. We include several relevant processes for a low density, high temperature, collisional plasma such as non-equilibrium chemical reactions, cooling, shock heating, thermal energy equipartition between electrons, ions and neutral particles and electronic conduction. We find two different regions in the pancake structure: a thermal precursor ahead of the compression front and an equipartition wave after the compression front where electrons and ions temperatures differ significantly. This complex structure may have two interesting consequences: pre-heating of unshocked regions in the vicinity of massive X-ray clusters and ions and electrons temperatures differences in the outer regions of X-rays clusters.Comment: 30 pages, including 8 figures, accepted for publication in The Astrophysical Journa

Improvements and modifications to the NASA microwave signature acquisition system

A user oriented description of the modified and upgraded Microwave Signature Acquisition System is provided. The present configuration of the sensor system and its operating characteristics are documented and a step-by-step operating procedure provides instruction for mounting the antenna truss assembly, readying the system for data acquisition, and for controlling the system during the data collection sequence. The resulting data products are also identified

Strong magnetohydrodynamic turbulence with cross helicity

Magnetohydrodynamics (MHD) provides the simplest description of magnetic plasma turbulence in a variety of astrophysical and laboratory systems. MHD turbulence with nonzero cross helicity is often called imbalanced, as it implies that the energies of Alfv\'en fluctuations propagating parallel and anti-parallel the background field are not equal. Recent analytical and numerical studies have revealed that at every scale, MHD turbulence consists of regions of positive and negative cross helicity, indicating that such turbulence is inherently locally imbalanced. In this paper, results from high resolution numerical simulations of steady-state incompressible MHD turbulence, with and without cross helicity are presented. It is argued that the inertial range scaling of the energy spectra (E^+ and E^-) of fluctuations moving in opposite directions is independent of the amount of cross-helicity. When cross helicity is nonzero, E^+ and E^- maintain the same scaling, but have differing amplitudes depending on the amount of cross-helicity.Comment: To appear in Physics of Plasma

An extremal model for amorphous media plasticity

An extremal model for the plasticity of amorphous materials is studied in a simple two-dimensional anti-plane geometry. The steady-state is analyzed through numerical simulations. Long-range spatial and temporal correlations in local slip events are shown to develop, leading to non-trivial and highly anisotropic scaling laws. In particular, the plastic strain is shown to statistically concentrate over a region which tends to align perpendicular to the displacement gradient. By construction, the model can be seen as giving rise to a depinning transition, the threshold of which (i.e. the macroscopic yield stress) also reveal scaling properties reflecting the localization of the activity.Comment: 4 pages, 5 figure

Origin of the Universal Roughness Exponent of Brittle Fracture Surfaces: Correlated Percolation in the Damage Zone

We suggest that the observed large-scale universal roughness of brittle fracture surfaces is due to the fracture process being a correlated percolation process in a self-generated quadratic damage gradient. We use the quasi-static two-dimensional fuse model as a paradigm of a fracture model. We measure for this model, that exhibits a correlated percolation process, the correlation length exponent nu approximately equal to 1.35 and conjecture it to be equal to that of uncorrelated percolation, 4/3. We then show that the roughness exponent in the fuse model is zeta = 2 nu/(1+2 nu)= 8/11. This is in accordance with the numerical value zeta=0.75. As for three-dimensional brittle fractures, a mean-field theory gives nu=2, leading to zeta=4/5 in full accordance with the universally observed value zeta =0.80.Comment: 4 pages RevTeX

Topological Entropy of Braids on the Torus

A fast method is presented for computing the topological entropy of braids on the torus. This work is motivated by the need to analyze large braids when studying two-dimensional flows via the braiding of a large number of particle trajectories. Our approach is a generalization of Moussafir's technique for braids on the sphere. Previous methods for computing topological entropies include the Bestvina--Handel train-track algorithm and matrix representations of the braid group. However, the Bestvina--Handel algorithm quickly becomes computationally intractable for large braid words, and matrix methods give only lower bounds, which are often poor for large braids. Our method is computationally fast and appears to give exponential convergence towards the exact entropy. As an illustration we apply our approach to the braiding of both periodic and aperiodic trajectories in the sine flow. The efficiency of the method allows us to explore how much extra information about flow entropy is encoded in the braid as the number of trajectories becomes large.Comment: 19 pages, 44 figures. SIAM journal styl

`c' is the speed of light, isn't it?

Theories proposing a varying speed of light have recently been widely promoted under the claim that they offer an alternative way of solving the standard cosmological problems. Recent observational hints that the fine structure constant may have varied during over cosmological scales also has given impetus to these models. In theoretical physics the speed of light, $c$, is hidden in almost all equations but with different facets that we try to distinguish. Together with a reminder on scalar-tensor theories of gravity, this sheds some light on these proposed varying speed of light theories.Comment: 14 pages, Late

A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C

The characterization of a physically-diverse set of transiting exoplanets is an important and necessary step towards establishing the physical properties linked to the production of obscuring clouds or hazes. It is those planets with identifiable spectroscopic features that can most effectively enhance our understanding of atmospheric chemistry and metallicity. The newly-commissioned LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed fringing in the red optical, thus advancing the search for the spectroscopic signature of water in exoplanetary atmospheres from the ground. Using data acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b. Our measured spectrum is best explained by the presence of water vapor, a lack of potassium, and either a high-metallicity, cloud-free atmosphere or a solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of multi-scale-height spectral features in our data suggests that future observations at higher precision could break this degeneracy and reveal the planet's atmospheric chemical abundances. We also update HAT-P-26b's transit ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p = 4.2345023(7) days.Comment: 9 pages, 8 figures, Accepted for publication in Ap

PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,

The aim of the paper is to discuss the main characteristics of a complete theoretical and numerical model for turbulent polydispersed two-phase flows, pointing out some specific issues. The theoretical details of the model have already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001 ]. Consequently, the present work is mainly focused on complementary aspects, that are often overlooked and that require particular attention. In particular, the following points are analysed : the necessity to add an extra term in the equation for the velocity of the fluid seen in the case of twoway coupling, the theoretical and numerical evaluations of particle averages and the fulfilment of the particle mass-continuity constraint. The theoretical model is developed within the PDF formalism. The important-physical choice of the state vector variables is first discussed and the model is then expressed as a stochastic differential equation (SDE) written in continuous time (Langevin equations) for the velocity of the fluid seen. The interests and limitations of Langevin equations, compared to the single-phase case, are reviewed. From the numerical point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach where the fluid and particle phases are simulated by different methods. Important aspects of the Monte Carlo particle/mesh numerical method are emphasised. Finally, the complete model is validated and its performance is assessed by simulating a bluff-body case with an important recirculation zone and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure

Tectonic significance of Late Paleozoic deformation in the Cape George Peninsula, Antigonish Highlands, Nova Scotia

Late Paleozoic deformation of the Cape George Peninsula, Antigonish Highlands, Nova Scotia, provides information on post-accretionary fault movements associated with waning stages of Appalachian orogenic activity. Anomalously intense brittle to ductile deformation of the low-grade Late Paleozoic rocks of the peninsula occurred along east-west shear zones in a ca. 4 km-wide belt bounded by the NE-trending Hollow and Greendale faults. Deformation adjacent to, and between, these two faults resulted in brecciation, folding and thrusting, the development of slickensides on major dislocation surfaces, the local development of S-C fabrics and stretching lineations defined by elongate pebbles, and/or the production of extensional fractures and veins. The data suggest dextral and subordinate thrust components of movement along the east-west shear zones. Deformation is attributed to dextrally oblique compression between the bounding Hollow and Greendale faults along which significant reverse displacements are proposed on the basis of fault geometry and kinematics. The Cape George Peninsula is interpreted as a "popup" structure between these back-to-back oblique-slip reverse faults and is considered to occupy a strongly transpressive step-over zone between them. The east-west shear zones, which record dextral transpressive motion and steepen towards the north in a positive half-flower structure configuration, are parallel to Reidel R-shears of the shear fracture array and are interpreted to be transfer faults within the step-over zone along which oblique slip with dextral and reverse components of motion was transferred from the Hollow Fault to the Greendale Fault. Development of the regional stress regime required by these fault kinematics is consistent with coeval post-accretionary dextral motion between the Meguma and Avalon composite terranes along the east-west Cobcquid-Chedabucto fault system. RÉSUMÉ La déformation du Paléozoique tandif de la péninsule dc Cap George, dans les hautes terres d'Antigonish en Nouvelle-Écosse, fournit des informations sur les mouvements de failles post-accrétionaires associés aux stades terminaux de l'activité orogénique appalachienne. Une déformation fragile à ductile anormalement intense des roches du Paléozoique supérieur de faible grade s'est produite le long de zones de cisaillement est-ouest dans une ceinture d'environ 4 km de largeur limited par les failles de Hollow et de Greendale d'orientation nord-est. La déformation adjacente et entre ces failles a résulté en de la bélchification, du plissement et du chevauchement, le développement de slickensides sur des surfaces de dislocation majeures, le développement local de fabriques C-S et de linéations d'étirement définies par des cailloux allongés, et/ou la production de fractures et de veines d'extension. Les données suggèrent des composantes de mouvement dextres et, dans une moindre mesure, de chevauchement le long des zones de cisaillement est-ouest. La déformation est attributée à une compression dextre oblique entre les failles limitrophes de Hollow et de Greendale le long desquelles des deplacements inverses importants sont proposés sur la base de la géométrie et de la cinémalique des failles. La péninsule de Cap George est interprét6e comme une structure d'extrusion verticale entre ces deux failles obliques a mouvement inverse, dos à dos, et est considérée comme occupant une zone de recouvrement fortement transpressive entre elles. Les zones de cisaillement est-ouest, qui montrent un mouvement de transpression dextre et deviennent plus abruples vers le nord en une configuration de demie "flower structure" positive, sont parallèles aux riedels synlhétiques du réseau de fractures de cisaillement et sont interprétées comme étant des failles de transfert à l'intérieur de la zone de transfert suivant lesquelles un mouvement oblique avec des composantes de mouvement dextre et inverse furent transferées de la faille de Hollow à la faille de Greendale. Le développement du régime de contrainte régional requis par ces cindmatiques de failles est en accord avec un mouvement post-accrétionaire dextre entre les terrains de Meguma et d'Avalon composite le long du systeme de faille est-ouest de Cobcquid-Chedabucto. [Traduit par la rédaction