Similarity Properties and Scaling Laws of Radiation Hydrodynamic Flows in Laboratory Astrophysics

The spectacular recent development of modern high-energy density laboratory facilities which concentrate more and more energy in millimetric volumes allows the astrophysical community to reproduce and to explore, in millimeter-scale targets and during very short times, astrophysical phenomena where radiation and matter are strongly coupled. The astrophysical relevance of these experiments can be checked from the similarity properties and especially scaling laws establishment, which constitutes the keystone of laboratory astrophysics. From the radiating optically thin regime to the so-called optically thick radiative pressure regime, we present in this paper, for the first time, a complete analysis of the main radiating regimes that we encountered in laboratory astrophysics with the same formalism based on the Lie-group theory. The use of the Lie group method appears as systematic which allows to construct easily and orderly the scaling laws of a given problem. This powerful tool permits to unify the recent major advances on scaling laws and to identify new similarity concepts that we discuss in this paper and which opens important applications for the present and the future laboratory astrophysics experiments. All these results enable to demonstrate theoretically that astrophysical phenomena in such radiating regimes can be explored experimentally thanks to powerful facilities. Consequently the results presented here are a fundamental tool for the high-energy density laboratory astrophysics community in order to quantify the astrophysics relevance and justify laser experiments. Moreover, relying on the Lie-group theory, this paper constitutes the starting point of any analysis of the self-similar dynamics of radiating fluids.Comment: Astrophys. J. accepte

Radiation Hydrodynamics Scaling Laws in High Energy Density Physics and Laboratory Astrophysics

accepted paperInternational audienceIn this paper, radiating fluids scaling laws are studied. We focus on optically thin and optically thick regimes which are relevant for both astrophysics and laboratory experiments. By using homothetic Lie groups, we obtain the scaling laws, the similarity properties and the number of free parameters which allow to rescale experiments in the two astrophyscial situations

Analytical solutions of specific classes of astrophysical radiating shocks

In this paper we study specific classes of radiating shocks which are widely spread in astrophysical environments. We present more general solutions of their structure and proceed to the analytical determination of physical quantities

Un organisme de recherche s'approprie la veille stratégique

L'agriculture est au cœur d'enjeux majeurs (sécurité sanitaire et alimentaire, préservation des ressources naturelles, changement climatique, agro-écologie…). L'INRA, premier organisme européen de recherche agronomique, a défini de nouvelles orientations pour la période 2010-2020 autour de quatre chantiers prioritairesi : l'amélioration de toutes les composantes de l'agriculture, des systèmes alimentaires sains et durables, la valorisation de la biomasse et l'atténuation et l'adaptation au réchauffement climatique. Cette approche, au-delà des missions traditionnelles de production et diffusion des connaissances, impose de repenser le pilotage de la recherche (par exemple la création de métaprogrammes), de mobiliser des approches transdisciplinaires, d'initier des partenariats et des collaborations nationales et internationales pour contribuer à l'expertise et à l'innovation. Dans ce paysage changeant et fortement évolutif, le besoin d'informations très actualisées et la nécessité d'anticiper s'imposent aux différents acteurs : la veille stratégique y trouve assez naturellement sa place. C’est dans les domaines les plus associés à l’innovation qu’émergent des dispositifs de veille organisés et collaboratifs : biotechnologies vertes, biotechnologies blanches, … Le besoin de veille stratégique s’exprime également sur des thématiques transversales, au cœur du dialogue avec les partenaires professionnels ou la société : bien-être animal, sécurité alimentaire, impact des pesticides et des intrants… Au-delà de l’acquisition de connaissances dans les nouveaux champs investis, la veille est utilisée principalement pour identifier les acteurs d’une thématique, se positionner, suivre l’évolution des politiques publiques et de la réglementation. Elle se révèle également comme un mode d’organisation structurant pour des collectifs hétérogènes, et, pour certains projets, comme un outil de communication entre l’Inra et ses partenaires ou le grand public. Cette arrivée de la veille stratégique dans le paysage de l'institut est favorisée par une restructuration interne de l'Information Scientifique et Technique (IST) en 2010 avec la création d'un Pôle Veille. Ce pôle adapte l'organisation de la veille à l'organisation matricielle de l'Inra avec une structure flexible, adaptable au plus près des porteurs de projets. La parution en 2012 du "Guide de l'Intelligence Economique en rechercheii" est un élément sur lequel le pôle peut s'appuyer pour une sensibilisation accrue à l'intelligence économique. Alors que la veille scientifique est bien implantée à l’échelle du laboratoire, au sein de l'institut, la culture industrielle sur les questions de concurrence, sur le potentiel économique des recherches est faible. Au travers de nouveaux services, le pôle Veille permet le développement de l'intelligence économique grâce à des professionnels formés à la veille, le développement d'outils méthodologiques, un appui aux projets, une plateforme de veille professionnelle. Beaucoup de projets, programmes ou directions ont clairement identifié la veille stratégique comme un élément favorisant la réflexion et l'innovation mais, la prise en compte de l'IE reste encore insuffisante. Il subsiste des freins d’ordre culturel (notion de service public, open science...) Partant de ce constat, le pôle veille a identifié plusieurs leviers : sensibilisation très ciblée des responsables lors de séminaire d'introduction à la veille stratégique, formation et pédagogie, exemples de déploiement réussi de dispositifs de veille, renforcement des collaborations internes (ex avec les services de partenariat et de valorisation…)

Quasi-periodic oscillations in accreting magnetic white dwarfs II. The asset of numerical modelling for interpreting observations

Magnetic cataclysmic variables are close binary systems containing a strongly magnetized white dwarf that accretes matter coming from an M-dwarf companion. High-energy radiation coming from those objects is emitted from the accretion column close to the white dwarf photosphere at the impact region. Its properties depend on the characteristics of the white dwarf and an accurate accretion column model allows the properties of the binary system to be inferred, such as the white dwarf mass, its magnetic field, and the accretion rate. We study the temporal and spectral behaviour of the accretion region and use the tools we developed to accurately connect the simulation results to the X-ray and optical astronomical observations. The radiation hydrodynamics code Hades was adapted to simulate this specific accretion phenomena. Classical approaches were used to model the radiative losses of the two main radiative processes: bremsstrahlung and cyclotron. The oscillation frequencies and amplitudes in the X-ray and optical domains are studied to compare those numerical results to observational ones. Different dimensional formulae were developed to complete the numerical evaluations. The complete characterization of the emitting region is described for the two main radiative regimes: when only the bremsstrahlung losses and when both cyclotron and bremsstrahlung losses are considered. The effect of the non-linear cooling in- stability regime on the accretion column behaviour is analysed. Variation in luminosity on short timescales (~ 1 s quasi-periodic oscillations) is an expected consequence of this specific dynamic. The importance of secondary shock instability on the quasi-periodic oscillation phenomenon is discussed. The stabilization effect of the cyclotron process is confirmed by our numerical simulations, as well as the power distribution in the various modes of oscillation.Comment: 13 pages, 13 figures, 2 tables. Accepted for publication in A&

Quasi-periodic oscillations in accreting magnetic white dwarfs: I. Observational constraints in X-ray and optical

International audienceQuasi-periodic oscillations (QPOs) are observed in the optical flux of some polars with typical periods of 1 to 3 s but none have been observed yet in X-rays where a significant part of the accreting energy is released. QPOs are expected and predicted from shock oscillations. Most of the polars have been observed by the XMM-Newton satellite. We made use of the homogeneous set of observations of the polars by XMM-Newton to search for the presence of QPOs in the (0.5–10 keV) energy range and to set significant upper limits for the brightest X-ray polars. We extracted high time-resolution X-ray light curves by taking advantage of the 0.07 s resolution of the EPIC-PN camera. Among the 65 polars observed with XMM-Newton from 1998 to 2012, a sample of 24 sources was selected on the basis of their counting rate in the PN instrument to secure significant limits. We searched for QPOs using Fast Fourier Transform (FFT) methods and defined limits of detection using statistical tools. Among the sample surveyed, none shows QPOs at a significant level. Upper limits to the fractional flux in QPOs range from 7% to 71%. These negative results are compared to the detailed theoretical predictions of numerical simulations based on a 2D hydrodynamical code presented in Paper II. Cooling instabilities in the accretion column are expected to produce shock quasi-oscillations with a maximum amplitude reaching ~40% in the bremsstrahlung (0.5–10 keV) X-ray emission and ~20% in the optical cyclotron emission. The absence of X-ray QPOs imposes an upper limit of ~(5–10) g cm-2 s-1 on the specific accretion rate but this condition is found inconsistent with the value required to account for the amplitudes and frequencies of the observed optical QPOs. This contradiction outlines probable shortcomings with the shock instability model

Link between laboratory and astrophysical radiative shocks

This work provides analytical solutions describing the post-shock structure of radiative shocks growing in astrophysics and in laboratory. The equations including a cooling function $\Lambda \propto \rho^{\epsilon} P^{\zeta} x^{\theta}$ are solved for any values of the exponents $\epsilon$, $\zeta$ and $\theta$. This modeling is appropriate to astrophysics as the observed radiative shocks arise in optically thin media. In contrast, in laboratory, radiative shocks performed using high-power lasers present a radiative precursor because the plasma is more or less optically thick. We study the post-shock region in the laboratory case and compare with astrophysical shock structure. In addition, we attempt to use the same equations to describe the radiative precursor, but the cooling function is slightly modified. In future experiments we will probe the PSR using X-ray diagnostics. These new experimental results will allow to validate our astrophysical numerical codes

Fast quasi-periodic oscillations in the eclipsing polar VV Puppis from VLT and XMM-Newton observations

International audienceWe present high time resolution optical photometric data of the polar VV Puppis obtained simultaneously in three filters (u′, HeII λ4686, r′) with the ULTRACAM camera mounted at the ESO-VLT telescope. An analysis of a long 50 ks XMM-Newton observation of the source, retrieved from the database, is also provided. Quasi-periodic oscillations (QPOs) are clearly detected in the optical during the source bright phase intervals when the accreting pole is visible, confirming the association of the QPOs with the basis of the accretion column. QPOs are detected in the three filters at a mean frequency of ∼0.7 Hz with a similar amplitude ∼1%. Mean orbitally-averaged power spectra during the bright phase show a rather broad excess with a quality factor Q = ν/Δν = 5−7 but smaller data segments commonly show a much higher coherency with Q up to 30. The X-ray Multi-mirror Mission XMM (0.5–10 keV) observation provides the first accurate estimation of the hard X-ray component with a high kT ∼ 40 keV temperature and confirms the high extreme ultraviolet (EUV)-soft/hard ratio in the range of 4−15 for VV Pup. The detailed X-ray orbital light curve displays a short Δϕ ≃ 0.05 ingress into self-eclipse of the active pole, indicative of an accretion shock height of ∼75 km. No significant X-ray QPOs are detected with an amplitude upper limit of ∼30% in the range 0.1–5 Hz. Detailed hydrodynamical numerical simulations of the post-shock accretion region with parameters consistent with VV Pup demonstrate that the expected frequencies from radiative instability are identical for X-rays and optical regime at values ν ∼ 40–70 Hz, more than one order magnitude higher than observed. This confirms previous statements suggesting that present instability models are unable to explain the full QPO characteristics within the parameters commonly known for polars