index - Modélisation, Agrégats, Dynamique Accéder directement au contenu

Derniers dépôts, tout type de documents

Fullerene C 60 is one of the most iconic forms of carbon found in the interstellar medium (ISM). The interstellar chemistry of carbon-rich components, including fullerenes, is driven by a variety of energetic processes including UV and X-ray irradiation, cosmic-ray (CR) bombardment, electron impact, and shock waves. These violent events strongly alter the particle phase and lead to the release of new molecular species in the gas phase. Only a few experimental studies on the shock processing of cosmic analogs have been conducted so far. We explored in the laboratory the destruction of buckminsterfullerene C 60 using a pressure-driven shock tube coupled with optical diagnostics. Our efforts were first devoted to probing in situ the shock-induced processing of C 60 at high temperatures (≤ 4500 K) by optical emission spectroscopy. The analysis of the spectra points to the massive production of C 2 units. A broad underlying continuum was observed as well and was attributed to the collective visible emission of carbon clusters, generated similarly in large amounts. This proposed assignment was performed with the help of calculated emission spectra of various carbon clusters. The competition between dissociation and radiative relaxation, determined by statistical analysis, alludes to a predominance of clusters with less than 40 carbon atoms. Our laboratory experiments, supported by molecular dynamics simulations performed in the canonical ensemble, suggest that C 60 is very stable, and that high-energy input is required to process it under interstellar low-density conditions and to produce C 2 units and an abundance of intermediate-sized carbon clusters. These results provide some insights into the life cycle of carbon in space. Our findings hint that only J-type shocks with velocities above ~100 km s −1 or C-type shocks with velocities above 9 km s −1 can lead to the destruction of fullerenes. Observational tracers of this process remain elusive, however. Our work confirms the potential of shock tubes for laboratory astrophysics.

Continuer la lecture Partager

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

Continuer la lecture Partager

[...]

Continuer la lecture Partager

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. We report James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modeling their kinematics and excitation allowed us to constrain the physical conditions within the gas. We quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

Continuer la lecture Partager

Atrazine is one of the most widely used herbicide molecules in the triazine family. Despite its interdiction in the European Union in 2004, atrazine and its main degradation products remain among the most frequently found molecules in freshwater reservoirs in many European Union countries. Our study aims in obtaining insight into the desorption process of atrazine from the main soil absorbent material: clay. Constrained Molecular Dynamics simulations within the Density Functional Theory framework allow us to obtain a free energy desorption profile of atrazine from a Ca2+-montmorillonite surface. The results are interpreted in terms of atrazine inclination to the clay surface and moreover, in terms of hydration states of the cations present in the clay interlayer as well as the hydration state of the atrazine. The desorption mechanism is driven by atrazine alkyl groups and their sizes because of dispersion stabilizing effects. The highest barrier corresponds to the loss of the isopropyl interaction with the surface.

Continuer la lecture Partager

Sujets

Carbonaceous grains White dwarfs 1 Brown dwarfs Dissipation Polycyclic Aromatic Hydrocarbons Modélisation 22 pole cryogenic ion trap Methods laboratory molecular Astrochimie Modelling Nanoparticles CONFIGURATION-INTERACTION Quantum chemistry Astrochemistry Argile Amorphous CAH Catalyse Benzene dimers Auxiliary density functional theory Excited states Line profiles Agrégats protonés Atomic scattering from surfaces Alanine dipeptide Au147 Car-Parrinello molecular dynamics Atrazine Molecular dynamics Cryogenic ion trap 2 Abundances -ISM Configuration interaction Charge transfer state Threshold algorithm Dynamics Carbon clusters Agrégats protonés uracile-eau Clay mineral PAH DFTB-CI Charge resonance Molecular data Catalysis Benzene Dynamique électronique Chimie quantique BOMD Agrégats moléculaires Champ de forces Approche mixte quantique/classique Atomic data Probability flows ADFT Database Dusty plasma Anharmonic Infrared Spectroscopy Polycyclic aromatic hydrocarbon PAH Disconnectivity Tree Corannulene Dynamique Moléculaire Car-Parrinello Collision Induced Dissociation Molecular clusters Infrared spectroscopy Density Functional Theory Biodegradation Disconnectivity tree Density functional theory Dynamique moléculaire Ammonium/ammonia water clusters HAP Infrared spectra Water clusters Dissociation SCC-DFTB Carbon cluster CID CONSTANTS Density functional based tight binding DFTB Molecular processes Argon Density functional tight binding Agrégats DFTB Agrégats aqueux Agrégats d'eau Dftb QSAR ISM molecules DUST Clusters DFT Barium Chemical shift Optical spectra Clustering Agrégats aqueux d'ammonium/ammoniac Charged system and open shell

Statistiques

Nombre de fichiers déposés

98

Nombre de notices déposées

140