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

CO ice photodesorption: A wavelength-dependent study

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Ly-alpha), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (singulet Pi at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.Comment: 5 pages, 2 figures, published in ApJ

UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -- 14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10$^{-5}$ molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower ($<$ 10$^{-6}$ molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH$_3$OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH$_3$, OH, H$_2$CO and CH$_3$O/CH$_2$OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH$_3$O, therefore opening new gas phase chemical routes for the formation of complex molecules.Comment: 13 pages, 2 figures, 1 tabl

Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process

UV ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e. the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the subsurface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factors of a few (compared to the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.Comment: 21 pages 5 figure

Indirect Ultraviolet Photodesorption from CO:N2 Binary Ices — An Efficient Grain-Gas Process

Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.Astronom

Les effets conjugués de la gestion parcellaire et du contexte paysager et de sa dynamique sur les bioagresseurs et les niveaux de régulation biologique

Ce séminaire est une restitution des principales avancées obtenues dans le cadre des projets ANR PEERLESS «viabilité d’une gestion écologique renforcée de la santé des plantes dans les paysages agricoles » (2013-2017) et FRB SEBIOPAG-PHYTO «déterminants agricoles parcellaires et paysagers des variations de niveaux de régulation biologique » (2014-2017). Le séminaire a rassemblé 60 scientifiques, pour moitié extérieure aux unités INRA partenaires de ces projets. Il s'est déroulé à Paris Paris les 27-28 novembre 2017

Initial results from the InSight mission on Mars

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indi- cate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander

Etude numérique de l'instabilité de Vishniac dans les restes de supernovae

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Etude théorique, numérique et expérimentale de la dynamique des chocs d'accrétion dans les variables cataclysmiques magnétiques

Les variables cataclysmiques magnétiques sont des sytèmes binaires serrés contenant une naine blanche fortement magnétisée accrétant de la matière provenant d'un compagnon. La matière est dirigée par les lignes de champ magnétique et tombe avec une vitesse supersonique en formant une colonne d'accrétion au niveau du (des) pôle(s) magnétique(s) de la naine blanche. Lors de l'impact un choc retour est généré et la matière choquée se structure sous l'effet du refroidissement dû au rayonnement. Ce travail présente une étude pluri-disciplinaire de la dynamique de la colonne d'accrétion. Tout d'abord, une approche numérique à l'échelle astrophysique permet d'étudier la dynamique de la colonne d'accrétion. En particulier, le lien avec les observables du système a été abordé. Cette approche est complétée par une approche expérimentale basée sur la génération d'écoulement hydro-radiatifs grâce aux installations de laser de puissance. La pertinence de ces expériences repose sur la construction de lois d'échelle. De nouvelles lois sont exposées dans le cadre de la MHD radiative idéale et résistive. Les résultats du dimensionnement ainsi que l'interprétation de la campagne d'expérience POLAR réalisée en 2012 sur l'installation LULI2000 sont présentés.Magnetic cataclysmic variables are interacting binary systems containing a highly magnetized white dwarf which accretes material from companion. Material is led along magnetic field lines and falls onto magnetic pole(s) of the white dwarf at supersonic speed forming an accretion column. As the material hits the surface, a reverse shock is formed and the shocked region is structured by the cooling effect of radiation processes. This work is a multidisciplinary study of the dynamics of the accretion column. Firstly, a numerical study of the accretion column structure at the astrophysical scale is presented. The observational consequences are discussed. This approach is completed by experiments using radiative flows generated by powerful lasers. The relevance of such experiments is based on the establisment of scaling laws. News scaling laws in the frame of radiative ideal or resistive MHD are exposed. The results of the sizing and the interpretation of the POLAR experimental campaign of 2012 on LULI200 installation are presented.PARIS-Observatoire (751142302) / SudocSudocFranceF