Diffuse interstellar absorption

It is suspected that 0.2% of interstellar carbon may exist in the form of PAH radical cations and suggested that they may be possible carriers of the visible, diffuse interstellar absorption band (DIB), which extends from 4000 A into the near IR [152], Encouraged by the results on the LCP radical cations, Schutz et al. [153] undertook to calculate the electronic spectra of PAH radical cations starting with naphthalene [(6) in Fig. 3] and anthracene [(7) in Fig. 3]. 

It has been suggested that free polycyllc aromatic hydrocarbon (PAH) molecules can account for unidentified Interstellar emission features In the infrared (8) and for the diffuse Interstellar absorption bands In the visible (9), Also, the possibility of the Interstellar and circumstellar presence of hollow, cage-llke polycarbon molecules with spheroidal shells of hexagonal, graphlte-like sheets of carbon atoms has... 

Abstract We review the potential contribution of single fullerenes and buckyonions to interstellar extinction. Photoabsorption spectra of these molecules are compared with some of the most relevant features of interstellar extinction, the UV bump, the far UV rise and the diffuse interstellar bands. According to semiempirical models, photoabsorption by fullerenes (single and multishell) could explain the shape, width and peak energy of the most prominent feature of the interstellar absorption, the UV bump at 2,175 A. Other weaker transitions are predicted in the optical and near-infrared providing a potential explanation for diffuse interstellar bands. In particular, several fullerenes could contribute to the well known strong DIB at 4,430 A comparing cross sections and available data for this DIB and the UV bump we estimate a density of fullerenes in the diffuse interstellar medium of 0.1-0.2 ppm. These molecules could then be a major reservoir for interstellar carbon. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

Mixtures of fulleranes produced by hydrogenation of solid C60 films under atomic H flux have revealed spectral features that bear striking similarity to those observed in the diffuse interstellar medium, both in the far IR and in the UV spectral windows. Of course, one must be cautious not to overextend the interpretation of laboratory data, for a number of reasons firstly, because electron spectroscopy, the experimental technique used in these studies, differs in several important aspects from the spectroscopic methods employed in observational astronomy, and secondly, because of the specifics of specimen preparation and environmental conditions. In this regard, there is a need to explore the stability of fulleranes to energetic and corpuscular radiation (Cataldo et al. 2009). Nonetheless, our findings lend support to the suggestion of fulleranes as candidates for unidentified emission and absorption features of interstellar and circumstellar media. Whether or not they exist in sufficient abundance is still unclear however, their spectral features make them undoubtedly an ideal model system for laboratory studies of these fascinating astrophysical phenomena. 

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

While dense IS regions are generally well-shielded against high-energy photons, the interaction between molecular ions and photons is relevant to the question of the survival of such ions in the diffuse interstellar medium, where UV irradiation might be expected to be a powerful destructive force. Such effects are, of course, important also for the fate of neutral molecules in the diffuse interstellar radiation field, but UV photo absorption by molecules of moderate size is often more likely to lead to photoionization (itself an important topic, but not covered herein) than to photodissociation. 

Returning to the question which motivated the experiments which in turn led to the discovery of the fullerenes do such carbon clusters exist in nature Astronomical searches for the distinctive fullerene signature of infrared absorption lines have been unsuccessful, and laboratory spectra of fullerenes have not shown any explicit connection to unsolved as-trophysical problems such as the so-called diffuse interstellar bands or other unidentified spectral features.[Ha92]... 

Thus, based on the results presented here, the presence of carbons that are irradiated by energetic ions in space very likely produces carbynoid structures. These could play a role in explaining one of the oldest mysteries in the study of the interstellar medium, namely the identifications of species responsible for the so-called diffuse interstellar bands, a forest of absorption lines seen in many directions of the sky. In fact, among the many suggested materials, linear carbon chains seem to be among the most promising (see e.g. References 2-4.)... 

Simultaneously with these sharp features broad interstellar absorption features were discovered which have now become known as the diffuse interstellar bands (see e.g. Herbig, 1975). The source of the diffuse interstellar bands has remained a mystery for over 50 years. Many detailed suggestions have been made, none of which have been proven. It is, however, undisputed that these absorptions are caused by some constituent of the general interstellar medium, molecular gas or dust or a combination of both. 

Hendel W, Khan ZH, Schmidt W (1986) Hexa-peri-benzocoronene, a candidate for the origin of the diffuse interstellar visible absorption bands Tetrahedron 42 1127-1134... 

M. A. Dopita and R. S. Sutherland, Astrophysics of the Diffuse Universe, Springer-Verlag 2003, give an excellent and lucid account of absorption and emission lines, gas dynamics and other interstellar topics. 

The discovery of C60 by Kroto and coworkers (1985) was motivated in part by the interstellar dust problem. C60 would seem to be an ideal candidate, as it is spherical and graphite-like, it forms spontaneously in harsh environments with carbon dust, and is stable in intense radiation fields, a condition analogous to that found in the diffuse ISM (Kroto and Jura 1992). In fact, the observation of two DIBs at 957.7 and 963.2 nm are tentatively considered the first evidence of C60+ in interstellar dust (Foing and Ehrenfreund 1997). Moreover, a mixture of hydrides of C60 is shown to exhibit spectral features remarkably similar to those seen in the unidentified infrared emission (Stoldt et al. 2001). The UV absorption spectrum of synthetic C60H36 was also observed to possess abroad bump at 217.5 nm (Cataldo 2003). 

Le Bertre (1990) has found that the features at 4430, 5780 and 6284 A are quite strong in the absorption spectrum of the A star companion to the mass-losing carbon star CS 776 (= IRC-20131). The diffuse band at 5797 A is not present in the spectrum of this companion. Again, however, because the star lies in the galactic plane (b = — 0.81°), much of the diffuse bands may be contributed by interstellar instead of circumstellar matter. 

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