Aristaeus

Prose by Miriam Berne. Second place in the 2019 Manuscripts Prose Contest

The Kelvin-Helmholtz instability in Orion: a source of turbulence and chemical mixing

Hydrodynamical instabilities are believed to power some of the small scale (0.1-10 pc) turbulence and chemical mixing in the interstellar medium. Identifying such instabilities has always been difficult but recent observations of a wavelike structure (the Ripples) in the Orion nebula have been interpreted as a signature of the Kelvin-Helmholtz instability (KHI), occurring at the interface between the HII region and the molecular cloud. However, this has not been verified theoretically. In this letter, we investigate theoretically the stability of this interface using observational constraints for the local physical conditions. A linear analysis shows that the HII/molecular cloud interface is indeed KH unstable for a certain range of magnetic field orientation. We find that the maximal growth-rates correspond to typical timescales of a few 1e4 years and instability wavelengths of 0.06 to 0.6 pc. We predict that after 2e5 years the KHI saturates and forms a turbulent layer of thickness ~0.5 pc. The KHI can remain in linear phase over a maximum distance of 0.75 pc. These spatial and time scales are compatible with the Ripples representing the linear phase of the KHI. These results suggest that the KHI may be crucial to generate turbulence and to bring heavy elements injected by the winds of massive stars in HII regions to colder regions where planetary systems around low mass stars are being formed. This could apply to the transport of 26Al injected by a massive star in an HII region to the nascent solar-system.Comment: Accepted for publication in ApJ Letter

Privatization Experiences in France

France lived through a long period of nationalization until the early 1980s, when nearly all the largest industrials firms as well as all the banks and public services were state-owned. Since then, a series of privatization moves have caused the state sector to shrink. Public services present a complex situation. For example, after its IPO, the national telecommunications operator France Telecom was simultaneously subjected to the deregulation of the telecom market in the world and to a new type of governance. EDF, the national electricity producer is next in line and should experience the same problems and opportunities.privatisation, competition, regulation, utilities

Geostatistical simulation of two-dimensional fields of raindrop size distributions at the meso-¿ scale

The large variability of the raindrop size distribution (DSD) in space and time must be taken into account to improve remote sensing of precipitation. The ability to simulate a large number of 2-D fields of DSDs sharing the same statistical properties provides a very useful simulation framework that nicely complements experimental approaches based on DSD ground measurements. These simulations can be used to investigate radar beam propagation through rain and to evaluate different radar retrieval techniques. The proposed approach uses geostatistical methods to provide structural analysis and stochastic simulation of DSD fields. First, the DSD is assumed to follow a Gamma distribution with three parameters. As a consequence, 2-D fields of DSDs can be described as a multivariate random function. The parameters are normalized using a Gaussian anamorphosis and simulated by taking advantage of fast Gaussian simulation algorithms. Variograms are used to characterize the spatial structure of the DSD fields. The generated fields have identical spatial structure and are consistent with the observations. Because intermittency cannot be simulated using this technique, the size of the simulation domain is limited to the meso-¿ scale (2-20 km). To assess the proposed approach, the method is applied to data collected during intense Mediterranean rainfall. Taylor's hypothesis is invoked to convert time series into 1-D range profiles. The anisotropy of the fields is derived from radar measurements. Simulated and measured reflectivity fields are in good agreement with respect to the mean, the standard deviation, and the spatial structure, demonstrating the promising potential of the proposed stochastic model of DSD field

Blind Signal Separation Methods for the Identification of Interstellar Carbonaceous Nanoparticles

The use of Blind Signal Separation methods (ICA and other approaches) for the analysis of astrophysical data remains quite unexplored. In this paper, we present a new approach for analyzing the infrared emission spectra of interstellar dust, obtained with NASA's Spitzer Space Telescope, using FastICA and Non-negative Matrix Factorization (NMF). Using these two methods, we were able to unveil the source spectra of three different types of carbonaceous nanoparticles present in interstellar space. These spectra can then constitute a basis for the interpretation of the mid-infrared emission spectra of interstellar dust in the Milky Way and nearby galaxies. We also show how to use these extracted spectra to derive the spatial distribution of these nanoparticles