131 research outputs found
Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
Various studies have emphasized variations of the charge state and
composition of the interstellar polycyclic aromatic hydrocarbon (PAH)
population in photodissociation regions (PDRs). We aim to model the spatial
evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on
the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the
case of the diffuse interstellar medium (ISM). The physical conditions in NGC
7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH
chemical evolution model that includes recent experimental data on PAHs and
describes multiphoton events. We consider a family of compact PAHs bearing up
to 96 carbon atoms. The calculated ionization ratio is in good agreement with
observations in NGC 7023 NW. Within the PDR, PAHs evolve into three major
populations: medium-sized PAHs (50<Nc<90) are normally hydrogenated, larger
PAHs (Nc>90) can be superhydrogenated, and smaller species (Nc<50) are fully
dehydrogenated. In the cavity, where the fullerene C60 was recently detected,
all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical
evolution exhibits a complex non-linear behaviour as a function of the UV
radiation field because of multiphoton events. Steady state for hydrogenation
is reached on timescales ranging from less than a year for small PAHs, up to
10000 years for large PAHs at Av=1. We identified critical reactions that need
more studies. Our new model allows us to rationalize the observational
constraints without any fitting parameter. PAHs smaller than 50 carbon atoms
are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is
obtained for the diffuse ISM. Carbon clusters turn out to be end products of
PAH photodissociation, and the evolution of these clusters needs to be
investigated further to evaluate their impact on the chemical and physical
evolution of PDRs.Comment: 16 pages, 10 figures; Accepted for publication in A&
"Whose data is it anyway?" The implications of putting small area-level health and social data online
International audienceThe planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the radiation pressure on planetary exospheres. In a series of papers, we present with an Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain (1989). In this second part of our work, we present here the density profiles of atomic Hydrogen in planetary exospheres subject to the radiation pressure. We first provide the altitude profiles of ballistic particles (the dominant exospheric population in most cases), which exhibit strong asymmetries that explain the known geotail phenomenon at Earth. The radiation pressure strongly enhances the densities compared with the pure gravity case (i.e. the Chamberlain profiles), in particular at noon and midnight. We finally show the existence of an exopause that appears naturally as the external limit for bounded particles, above which all particles are escaping
Projet de paysage participatif Ă Villandry
Cette recherche sâinscrit dans le cadre du projet intitulĂ© « Participation des populations et renouvellement des pratiques paysagistes. Une approche par lâexpĂ©rimentation et la comparaison des mĂ©thodes » au sein du programme « Paysage et DĂ©veloppement durable II », portĂ© par le MEDDE. LâoriginalitĂ© de ce projet tient en partie Ă la collaboration entre des enseignants-chercheurs de trois Ă©coles de paysage françaises (Angers, Versailles et Bordeaux) et des paysagistes, engagĂ©s ensemble dans deux dĂ©marches expĂ©rimentales. Nous revenons plus particuliĂšrement dans cet article sur lâune des deux expĂ©rimentations, conduite sur la commune de Villandry dans le Val de Loire
Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
Context. In the framework of the interstellar polycyclic aromatic
hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a
fingerprint of single species in this class. Aims. We address the question of
detectability of low energy PAH vibrational bands, with respect to spectral
contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs).
Methods. We extend our extablished Monte-Carlo model of the photophysics of
specific PAHs in astronomical environments, to include rotational and
anharmonic band structure. The required molecular parameters were calculated in
the framework of the Density Functional Theory. Results. We calculate the
detailed spectral profiles of three low-energy vibrational bands of neutral
naphthalene, and four low-energy vibrational bands of neutral anthracene. They
are used to establish detectability constraints based on intensity ratios with
``classical'' AIBs. A general procedure is suggested to select promising
diagnostics, and tested on available Infrared Space Observatory data for the
Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the
far-IR is a challenging, but promising task, especially in view of the
forthcoming launch of the Herschel Space Observatory.Comment: 13 pages, 13 figures, accepted for publication in A&
La convention europĂ©enne au prisme dâune recherche-action de mĂ©diation paysagĂšre en Val de Loire (Villandry)
Cette communication se propose de questionner deux apports majeurs de la Convention européenne du paysage.
Le premier est lâarticulation quâelle construit entre paysage et dĂ©mocratie : initiĂ© par le Conseil de lâEurope dont la mission est de promouvoir les valeurs dĂ©mocratiques, cette convention associe les enjeux de paysage aux enjeux de la dĂ©mocratie locale. Elle fait la promotion des dĂ©marches participatives pour conduire lâaction publique paysagĂšre, sans toutefois donner un contenu prĂ©cis Ă cette participation par et pour le paysage.
Cette absence de dĂ©finition est dâautant plus remarquable que lâautre apport majeur de ce texte rĂ©side dans les dĂ©finitions terminologiques quâil propose sur plusieurs notions-clĂ© de lâaction publique paysagĂšre : paysage, objectifs de qualitĂ© paysagĂšre, politiques publiques du paysage (protection, gestion, amĂ©nagement des paysages).
Notre communication cherchera Ă illustrer et questionner ces deux dimensions de la convention europĂ©enne du paysage Ă partir dâune dĂ©marche de mĂ©diation paysagĂšre actuellement mise en Ćuvre par des chercheurs et/ou paysagistes dans le cadre dâun programme de recherche (volet « paysage et participation » du programme « Paysage et dĂ©veloppement durable », financĂ© par le MinistĂšre de lâĂ©cologie â France) sur une petite commune du Val de Loire (avec le soutien de la mission Loire UNESCO).
Dans un premier temps elle permettra de dĂ©crire un processus en cours de projet de paysage participatif (2012/2014) en sâarrĂȘtant sur les objectifs, les phases, les outils, les limites de cette dĂ©marche. Dans un deuxiĂšme temps, la communication Ă©clairera avec un point de vue critique chacune des dĂ©finitions des termes de la CEP au regard des premiers rĂ©sultats issus de la recherche-action (enquĂȘtes individuelles auprĂšs des habitants, puis ateliers collectifs)
Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium
The competition between the formation and destruction of coronene clusters
under interstellar conditions is investigated theoretically. The unimolecular
nucleation of neutral clusters is simulated with an atomic model combining an
explicit classical force field and a quantum tight-binding approach.
Evaporation rates are calculated in the framework of the phase space theory and
are inserted in an infrared emission model and compared with the growth rate
constants. It is found that, in interstellar conditions, most collisions lead
to cluster growth. The time evolution of small clusters (containing up to 312
carbon atoms) was specifically investigated under the physical conditions of
the northern photodissociation region of NGC 7023. These clusters are found to
be thermally photoevaporated much faster than they are reformed, thus providing
an interpretation for the lowest limit of the interstellar cluster size
distribution inferred from observations. The effects of ionizing the clusters
and density heterogeneities are also considered. Based on our results, the
possibility that PAH clusters could be formed in PDRs is critically discussed.Comment: 14 pages, 14 figures. Astronomy & Astrophysics, accepted for
publicatio
Projet de paysage participatif Ă Villandry : retours dâexpĂ©rience dâune recherche-action
Cette recherche sâinscrit dans le cadre du projet intitulĂ© « Participation des populations et renouvellement des pratiques paysagistes. Une approche par lâexpĂ©rimentation et la comparaison des mĂ©thodes » au sein du programme « Paysage et DĂ©veloppement durable II », portĂ© par le MEDDE. LâoriginalitĂ© de ce projet tient en partie Ă la collaboration entre des enseignants-chercheurs de trois Ă©coles de paysage françaises (Angers, Versailles et Bordeaux) et des paysagistes, engagĂ©s ensemble dans deux dĂ©marches expĂ©rimentales. Nous revenons plus particuliĂšrement dans cet article sur lâune des deux expĂ©rimentations, conduite sur la commune de Villandry dans le Val de Loire
Estimated IR and phosphorescence emission fluxes for specific Polycyclic Aromatic Hydrocarbons in the Red Rectangle
Following the tentative identification of the blue luminescence in the Red
Rectangle by Vijh et al. (2005), we compute absolute fluxes for the vibrational
IR emission and phosphorescence bands of three small polycyclic aromatic
hydrocarbons. The calculated IR spectra are compared with available ISO
observations. A subset of the emission bands are predicted to be observable
using presently available facilities, and can be used for an immediate,
independent, discriminating test on their alleged presence in this well-known
astronomical object.Comment: accepted for publication on A&
Visible photodissociation spectroscopy of PAH cations and derivatives in the PIRENEA experiment
The electronic spectra of gas-phase cationic polycyclic aromatic hydrocarbons
(PAHs), trapped in the Fourier Transform Ion Cyclotron Resonance cell of the
PIRENEA experiment, have been measured by multiphoton dissociation spectroscopy
in the 430-480 nm spectral range using the radiation of a mid-band optical
parametric oscillator laser. We present here the spectra recorded for different
species of increasing size, namely the pyrene cation (C16H10+), the
1-methylpyrene cation (CH3-C16H9+), the coronene cation (C24H12+), and its
dehydrogenated derivative C24H10+. The experimental results are interpreted
with the help of time-dependent density functional theory calculations and
analysed using spectral information on the same species obtained from matrix
isolation spectroscopy data. A kinetic Monte Carlo code has also been used, in
the case of pyrene and coronene cations, to estimate the absorption
cross-sections of the measured electronic transitions. Gas-phase spectra of
highly reactive species such as dehydrogenated PAH cations are reported for the
first time
A general model for the identification of specific PAHs in the far-IR
Context. In the framework of the interstellar PAH hypothesis, far-IR skeletal
bands are expected to be a fingerprint of single species in this class. Aims. A
detailed model of the photophysics of interstellar PAHs is required for such
single-molecule identification of their far-IR features in the presently
available Infrared Space Observatory data and in those of the forthcoming
Herschel Space Observatory mission. Methods. We modelled the detailed
photophysics of a vast sample of species in different radiation fields, using a
compendium of Monte-Carlo techniques and quantum-chemical calculations. This
enabled us to validate the use of purely theoretical data and assess the
expected accuracy and reliability of the resulting synthetic far-IR emission
spectra. Results. We produce positions and intensities of the expected far-IR
features which ought to be emitted by each species in the sample in the
considered radiation fields. A composite emission spectrum for our sample is
computed for one of the most favourable sources for detection, namely the Red
Rectangle nebula. The resulting spectrum is compared with the estimated dust
emission in the same source, to assess the dependence of detectability on key
molecular parameters. Conclusions. Identifying specific PAHs from their far-IR
features is going to be a difficult feat in general, still it may well be
possible under favourable conditions.Comment: 14 pages, 9 figures + 18 pages of online appendix. Accepted for
publication in A&A (09/06/2006
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