On the electronic structure of small carbon grains of astrophysical interest

In a previous paper Mennella et al. (1995a) studied the evolution of the UV spectrum of small carbon grains due to thermal annealing in the range 250-800°C. The spectral variations were interpreted in terms of internal structural rearrangement of the grains caused by hydrogen loss. The electronic transitions (Ï-Ï* and Ï-Ï*) of the sp2 clusters forming the grains were indicated as the major factors responsible for determining their extinction properties. In this paper we present the results of new measurements aimed at probing the heat-induced structural changes. The thermal evolution of the optical gap and of the Raman spectrum, both sensitive to the sp2 clustering degree, confirms that the observed spectral changes do depend on structural variations. In fact, the Ï electron delocalization of the sp2 clusters determines a link between structural and electronic properties in carbons. We find a basic correlation between the UV peak position and the optical gap. It is interpreted in terms of a dependence of the dipole matrix momentum of Ï transitions on the sp2 cluster size. The attribution of the spectral changes to the grain internal structure is corroborated by morphological analyses. Scanning and transmission electron microscope images show that the fluffy structure of the samples as well as the dimension and the shape of the single grains do not change after the annealing process. In the astrophysical context, the present results can be relevant for the attribution of the 217.5 nm feature, as they show that the internal structure of carbon grains, having sizes similar to those expected for the "bump" carriers, controls the interaction with UV photons

Laboratory experiments on cosmic dust analogues: the structure of small carbon grains

In this paper we present new results of our experiments aimed to study the internal structure of cosmic analogue carbon grains. The samples, produced by arc discharge between two carbon electrodes in an argon atmosphere, were annealed in the temperature range 250-780°C in order to produce modification of the internal grain structure. These changes were monitored by analysing the variations of the extinction profile between 190 and 2600 nm and of the optical gap as a function of the annealing temperature. The shift of the UV peak position towards longer wavelengths. the overall increase of the extinction coefficient and the closing of the gap as the temperature increases are all consistent with the evolution of carbon grains outlined by Mennella et al. (Astrophys. J., 444, 288, 1995 ; Astrophys. J. Suppl. Ser., 100, 149, 1995). It provides a growth in number and size of the sp2 clusters forming the grains during the annealing. The relevance of the electronic structure of the aromatic clusters in the extinction processes and the dependence of the energy Ï transitions on their size are confirmed by the present results. These results may be relevant in the context of interstellar bump attribution, as they show that the internal structure of small carbon grains is dominant in extinction processes. © 1995

Analysis of cosmic materials: Results on carbon and silicate laboratory analogues

Carbon and silicates are two of the main components of cosmic dust. They change nature through different evolutionary phases, according to the cosmic environment and the experienced processing. To understand the evolution of cosmic materials the study of "laboratory analogues" represents a powerful tool. In this context, systematic analyses are performed at the cosmic physics laboratory of Naples on solid particles, synthesised and processed under carefully controlled conditions. Different kinds of carbon and silicate samples are produced under various environmental conditions and exposed to processes (e.g. thermal annealing, UV irradiation and ion bombardment). The comparative analysis of the results allows us to link intrinsic properties (such as chemical composition and structure) to the optical behaviour of grains. This study offers the opportunity to interpret observations concerning the composition of small bodies in the Solar System, such as spectroscopic results obtained for comets by the Infrared Space Observatory (ISO). Several open questions remain, however, unsolved and await results from new laboratory experiments. ©1999 COSPAR. Published by Elsevier Science Ltd

Mesure du flux de poussières émises par la comète 46P/Wirtanen dans le cadre de la mission spatiale ROSETTA

GIADA est un des instruments de la mission ROSETTA, il est dédié à l'étude de l'évolution du flux des grains cométaires et de leurs propriétés dynamiques en fonction de la position du noyau. Parmi les modules qui composent cet instrument, le système MBS a pour mission de mesurer le flux en masse de poussières au moyen de microbalances à cristaux de quartz (QCM). L'objectif principal de cette thèse est le développement, la production et la caractérisation du MBS. Dans le but de déterminer la capacité de QCM à mesurer la déposition de masses sous formes de particules solides, quelques caractéristiques fondamentales (sensibilité en fonction de la température et masse des grains, efficacité de l'adhésion en fonction de la vitesse du grain) devront être étudiés. Un travail préliminaire a été d'optimiser les expériences pour le modèle de vol du QCM. Dans cette phase, on a utilisé des modèles de capteurs de laboratoire et on a étudié quelques matériaux pour produire un film adhésif dans la prospective d'améliorer l'efficacité de récolte du capteur. Pour la première fois, la sensibilité du QCM aux matériaux solides a été déterminée. La valeur trouvée est proche de la valeur nominale. En étudiant l'efficacité d'adhésion des grains dans différentes configurations du capteur (avec et sans film adhésif) on a pu montrer que le film augmente l'efficacité de l'adhésion, principalement pour les grains rapides. Malgré tout, le signal de sortie en fonction de la température est pratiquement 10 fois moins stable en présence d'un film. Ces résultats ont suggéré d'adopter une configuration finale sans film pour le MBS. Pendant la mission, on prévoit pour les grains cométaires direct une révélation quotidienne et au moins mensuelle pour les grains indirects. L'activité effectuée durant cette thèse a été cruciale pour le développement et la calibration du MBS, de plus, les résultats obtenus de ce travail ont validé la microbalance comme capteur de poussière en environnement spatial.AIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF

Dehydrogenation study of cosmic-dust analogue grains

New infrared and optical-absorption measurements performed on hydrogenated amorphous-carbon grains are presented. In particular, the spectral modifications produced by dehydrogenation of the samples are analysed. We interpret the observed variations in terms of an increasing dimension of the graphitic clusters disordely arranged to form carbon grains. The results support the hypothesis that the behaviour of the interstellar extinction in the ultraviolet can be attributed to dehydrogenated amorphous-carbon grains. © 1993 Società Italiana di Fisica

Infrared reflectance spectra of Martian analogues

Reflectance spectroscopy in the infrared is a powerful tool to characterise the optical properties of analogue materials for Mars. In this work we present laboratory studies of four Martian analogues: calcite, montmorillonite, palagonite and andesite. Diffuse and specular reflectance and transmittance spectra in the IR are shown. The spectral effect produced by small grains clung to the larger ones is analysed for calcite. Significant variations in the diffuse reflectance spectrum, especially in the reststrahlen region, are observed. The diffuse reflectance spectra of the other materials show the shift of the reststrahlen features with Si content in the materials. Optical constants of andesite are retrieved, by using the Lorentz model, from the specular reflectance spectrum. These values are fundamental in performing modelling of the radiance coming from the planet Mars

Interstellar extinction: a parametrical study by using laboratory data

In this paper we present a comparative study between laboratory data, astronomical observations and theoretical models in order to provide a contribution to the characterization of cosmic-dust properties. In particular, we propose a parametrization of the amorphous-carbon extinction curve. The laboratory extinction measurements, performed in the extreme ultraviolet spectral region, are compared to the UV interstellar extinction curves provided by the IUE satellite. The aim of our work was to define a model that could describe both observational and laboratory data. To this purpose we referred to the model developed by Fitzpatrick and Massa. The second step of the study was the determination of extinction coefficients, for two different size distributions of amorphous-carbon grains, by means of the optical constants, n and k, derived by Rouleau and Martin. © 1993 Società Italiana di Fisica

Extinction signatures of amorphous carbon grains from the vacuum UV to the far-IR

Among the various components of cosmic dust, amorphous carbon grains play a relevant role. Together with other carbon-based materials. they have been invoked to interpret a number of spectral features observed in the interstellar and circumstellar environments. In order to validate, on a more quantitative basis, the reliability of such hypotheses. it is necessary to analyse in detail the optical properties of "analogue" grains produced in the laboratory. In the present paper, we report, for the first time, a complete spectral analysis of the extinction properties measured on various kinds of amorphous carbon grains, from the vacuum UV to the far-IR. These data can be complemented with other experimental results obtained on the same samples by means of different analytical techniques. This approach allows us to propose a self-consistent interpretation about the actual physico-chemical properties of our cosmic dust analogues. The resulting scenario provides a powerful tool to interpret astronomical observations concerning cosmic dust. © 1995