IR emission from circumstellar envelopes of C-rich stars

The reliability of a theoretical model that solves the radiative transfer equation in dust clouds surrounding a central star is checked. In particular, it is found that both classical scattering by dust and the back-heating effects are negligible in the radiative transfer when envelopes similar to IRC+10216 are taken into consideration. In addition, new fits of IRC+10216 spectra are presented which were obtained, when the source is in different luminosity phases, under the assumption that amorphous carbon grains are in the circumstellar envelope. The same model is currently used to simulate the emission from carbon-rich sources showing the silicon carbide feature at 11.3 microns

Experimental evidence for amorphous carbon grains in comets

Amorphous carbon grains similar to those produced in the laboratory, but with a higher hydrogen content, appear to be good candidates to simulate both the IR continuum emission and the 3.4 micron band measured for P/Halley. The comparison of the cometary features with those detected in the laboratory for carbon grains characterized by various sp(exp 2)/sp(exp 3) ratios seems to indicate that a prevalent diamond-like (sp(exp 3)) structure should be present in cometary particles. These kinds of solid particles seem also suitable to explain the daily and monthly variations of the 3.4 micron band intensity, relative to the continuum, and, at the same time,- to fulfill the abundance constraints. The same grains appear to be able to reproduce the absorption bands detected in the IR galactic source IRS 7. This result may be considered as a first experimental evidence of a relation existing between interstellar dust and cometary materials

Estimated optical constants of gypsum in the regions of weak absorptions: Application of scattering theories and comparisons to independent measurements

Diffuse reflectance spectra of multiple grain size fractions are used to estimate the optical constants of gypsum over the 0.4–15 μm wavelength region. Two independent scattering theories are used to iteratively determine the imaginary index of refraction from the measured reflectance. We compare the results of these two with each other and with imaginary indices of gypsum reported in the literature. We find that the scattering theory results are more sensitive in the infrared to weak spectral features that are clearly distinguished in the diffuse reflectance spectra. However, we find the scattering results provide a poor determination of the optical constants in the regions of relatively strong absorptions. At visible and near-infrared wavelengths we provide a comparison to the results obtained from analysis of the diffuse reflectance to results obtained from direct transmission measurements of several gypsum crystals having different sample thicknesses. We find the simplest models of the transmission reproduce the observed spectra. The different sample thicknesses for the transmission measurements and different grain sizes in the scattering theories allow evaluation of the imaginary index of refraction over a wide range of values. We find the imaginary index of refraction determined from scattering theories is in remarkable agreement with those obtained from transmission spectra. Different models of the reflectance and transmittance have associated strengths and weaknesses, and we conclude that combining these models provides a more accurate determination of the optical constants of a material when compared to using each separately. We combine the resulting real and imaginary indices of refraction with those reported at infrared wavelengths to provide values covering visual, near-infrared, and infrared wavelengths (0.4–333 μm, 25000–30 cm^(−1))

Raman properties of various carbonaceous materials and their astrophysical implications

It is well known that a large number of celestial objects exhibit, in the range 3 to 12 micron, a family of emission features called unidentified infrared bands (UIR). They usually appear together and are associated with UV sources. Recently various authors have suggested that these features could be attributed to solid carbonaceous materials. Following this interest, a systematic analysis was performed of various types of amorphous carbon grains and polycyclic aromatic hydrocarbons (PAH), produced in lab. Updating results of Raman measurements performed on several carbonaceous materials, chosen according to their astrophysical interest, are presented. The measurements were made by means of a Jobin-Yvon monochromator HG2S and standard DC electronic. The line at 5145 A of an Ar+ laser was used as excitation source

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

Ultraviolet Spectral Changes in Amorphous Carbon Grains Induced by Ion Irradiation

Small carbon grains, processed by UV radiation and cosmic rays, have been proposed as carriers of the 217.5 nm bump present in the interstellar extinction curves (Hecht 1986; Sorrell 1990). In this paper, we present the results of an experiment aimed at simulating, in a -rst approximation, the cosmic-ray irradiation active in space. We have studied the e†ects induced by 3 keV Heions on the UV spectrum of small cosmic analog carbon grains. Two di†erent kinds of grains have been analyzed. They were produced by vapor conden- sation in hydrogen and argon quenching atmospheres. Spectrophotometric measurements have been carried out on grains as they were produced and after ion irradiation in the spectral range 0.19E2 km. Relevant UV spectral changes are observed after ion irradiation: while the UV absorption band shifts from 203 to 215 nm in hydrogenated amorphous carbon grains, an opposite trend is observed for the samples produced in the argon atmosphere. In this case the UV band moves from 240 to 218 nm. These spectral changes are well correlated with the optical gap variations and are therefore interpreted in terms of grain microstructure changes induced by the interactions with ions. At the highest ion Nuence considered, the two carbons tend to have a similar microstructure, as testi-ed by the UV peak position and optical gap values because of a saturation e†ect of the two competitive processes, amorphization and graphitization, which occur in carbon samples during ion irradiation (Compagnini & Calcagno 1996). The results of the present experiment suggest that hydrogenated amorphous carbon grains cannot be transformed into graphite grains by cosmic-ray irradiation. Moreover, the efficiency of ion irradiation in destroying well-ordered aromatic structures poses the problem of the survival itself of polycrystalline or pure graphite particles in the interstellar medium. Subject headings: cosmic rays E dust, extinction E methods: laboratory E ultraviolet: IS

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

Triple F - A Comet Nucleus Sample Return Mission

The Triple F (Fresh From the Fridge) mission, a Comet Nucleus Sample Return, has been proposed to ESA s Cosmic Vision program. A sample return from a comet enables us to reach the ultimate goal of cometary research. Since comets are the least processed bodies in the solar system, the proposal goes far beyond cometary science topics (like the explanation of cometary activity) and delivers invaluable information about the formation of the solar system and the interstellar molecular cloud from which it formed. The proposed mission would extract three samples of the upper 50 cm from three locations on a cometary nucleus and return them cooled to Earth for analysis in the laboratory. The simple mission concept with a touch-and-go sampling by a single spacecraft was proposed as an M-class mission in collaboration with the Russian space agency ROSCOSMOS

VUV-visible measurements on different samples of amorphous carbon

Among various candidate materials for interstellar dust, amorphous carbon (AC) is playing an increasingly important role (Greenstein, 1981; Hecht et al., 1984; Jura, 1983, 1986). Furthermore, recent in situ measurements have clearly shown the presence of carbonaceous grains in the coma of comet Halley (Kissel et al., 1986). Laboratory investigations on AC grains may be very useful to better interpret observations and to support theoretical elaborations. Recently, the authors started an international research program which also includes UV extinction analyses on AC samples, by using synchrotron light. Preliminary results obtained in a first shift of measurements, last June, are given. At the present stage of the data analysis, the authors can only draw some preliminary considerations. A wide band falling at around 240 nm is detected in all the analyzed samples. It intensity seems to decrease with increasing the dust collecting distance. A peak at 150 nm decreases in intensity with increasing the collecting distance. The band seems absent in the samples characterized by a larger amount of dust. A feature at about 200 nm is detected in some samples. At the moment the authors tend to attribute it to the transmission properties of the LiF substrates at the wavelength and/or to some problems in the experimental setup. It is unclear if a hump at 120 nm is real or due to instrumental effects. The profile of the spectra does not show substantial changes when the samples are cooled down to about 100 K. The present results appear to be in general agreement with previous findings, but their analysis is in progress and the interpretation is still on the way