On the kinematic detection of accreted streams in the Gaia era: a cautionary tale

The $\Lambda$CDM cosmological scenario predicts that our Galaxy should contain hundreds of stellar streams at the solar vicinity, fossil relics of the merging history of the Milky Way and more generally of the hierarchical growth of galaxies. Because of the mixing time scales in the inner Galaxy, it has been claimed that these streams should be difficult to detect in configuration space but can still be identifiable in kinematic-related spaces like the energy/angular momenta spaces, E-Lz and Lperp-Lz, or spaces of orbital/velocity parameters. By means of high-resolution, dissipationless N-body simulations, containing between 25$\times10^6$ and 35$\times10^6$ particles, we model the accretion of a series of up to four 1:10 mass ratio satellites then up to eight 1:100 satellites and we search systematically for the signature of these accretions in these spaces. In all spaces considered (1) each satellite gives origin to several independent overdensities; (2) overdensities of multiple satellites overlap; (3) satellites of different masses can produce similar substructures; (4) the overlap between the in-situ and the accreted population is considerable everywhere; (5) in-situ stars also form substructures in response to the satellite(s) accretion. These points are valid even if the search is restricted to kinematically-selected halo stars only. As we are now entering the 'Gaia era', our results warn that an extreme caution must be employed before interpreting overdensities in any of those spaces as evidence of relics of accreted satellites. Reconstructing the accretion history of our Galaxy will require a substantial amount of accurate spectroscopic data, that, complemented by the kinematic information, will possibly allow us to (chemically) identify accreted streams and measure their orbital properties. (abridged)Comment: Accepted on A&A. A high-resolution version of the paper is available at http://aramis.obspm.fr/~paola/ELZ/Elz.pd

Diffractive arrays of gold nanoparticles near an interface: critical role of the substrate

The optical properties of periodic arrays of plasmonic nanoantennas are strongly affected by coherent multiple scattering in the plane of the array, which leads to sharp spectral resonances in both transmission and reflection when the wavelength is commensurate with the period. We demonstrate that the presence of a substrate (i.e., an asymmetric refractive-index environment) can inhibit long-range coupling between the particles and suppress lattice resonances, in agreement with recent experimental results. We find the substrate-to-superstrate index contrast and the distance between the array and the interface to be critical parameters determining the strength of diffractive coupling. Our rigorous electromagnetic simulations are well reproduced by a simple analytical model. These findings are important in the design of periodic structures and in the assessment of their optical resonances for potential use in sensing and other photonic technologies

Evaluating Value-Graph Translation Validation for LLVM

Translation validators are static analyzers that attempt to verify that program transformations preserve semantics. Normalizing trans- lation validators do so by trying to match the value-graphs of an original function and its transformed counterpart. In this paper, we present the design of such a validator for LLVM’s intra-procedural optimizations, a design that does not require any instrumentation of the optimizer, nor any rewriting of the source code to compile, and needs to run only once to validate a pipeline of optimizations. We present the results of our preliminary experiments on a set of bench- marks that include GCC, a perl interpreter, SQLite3, and other C programs.Engineering and Applied Science

Discrete event front tracking simulator of a physical fire spread model

International audienceSimulation of moving interfaces, like a fire front usually requires the resolution of a large scale and detailed domain. Such computing involves the use of supercomputers to process the large amount of data and calculations. This limitation is mainly due to the fact that large scale of space and time is usually split into nodes, cells or matrices, and the solving methods often require small time steps. This paper presents a novel method that enables the simulation of large scale/high resolution systems by focusing on the interface. Unlike the conventional explicit and implicit integration schemes, it is based on the discrete-event approach, which describes time advance in terms of increments of physical quantities rather than discrete time stepping. Space as well is not split into discrete nodes or cells, but we use polygons with real coordinates. The system is described by the behaviour of its interface, and evolves by computing collision events of this interface in the simulation. As this simulation technique is suited for a class of models that can explicitly provide rate of spread for a given configuration, we developed a specific radiation based propagation model of physical wildland fire. Simulations of a real large scale fire performed with an implementation of our method provide very interesting results in less than 30 seconds with a 3 metres resolution with current personal computers

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let