Interstellar medium buckyonions

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. 

A significant fraction of the fullerenes and buckyonions in the interstellar medium could be hydrogenated as discussed by Webster in 1992 (see Fig. 1.4). These molecules, named genetically as fulleranes have deserved attention as potential carriers of diffuse intestellar bands and other interstellar and circumstellar features (Webster 1991, 1992, 1993a). Both, fullerenes and fulleranes have been detected in samples of the Allende meteorite (Becker et al. 1994), see Fig. 1.3b. 

The comparison of the computed cross sections of fullerenes and buckyonions with observations of the UV bump for Ry = 3.1 allow an estimate of the number of these molecules in the diffuse interstellar medium. Let us describe the extinction curve as a + a2x + a37Tx) where 7Tx) is the theoretical cross section computed for each fullerene or buckyonion. Here we assume that indeed the extinction at the energy of the bump is the result of the fullerene plus silicate contributions. We obtain via a least squared fit the relative contribution of the two components (see Fig. 1.6b). The coefficients of this lineal component do not depend significantly on the particular fullerene under consideration taking typical values of a, 1.6 and a2 = 0.07 with a relative error of 20%. 

Indeed, the actual carbon fraction in fullerenes depend of the proper mixture of these molecules in the interstellar medium. It is likely that the number density of fullerenes and buckyonions will decrease with increasing radius (R). A distribution of the type N(full) a R m has been frequently considered in the literature on interstellar grain populations (Mathis et al. 1977). A mixture of fullerenes and buckyonions following such size distribution may reproduce the observed UV bump. The best fits to the shape, peak energy and width of the bump are obtained for m values in the range 2.5 4.5 (Fig. 1.6b). 

The cross section obtained for single fullerenes and buckyonions reproduce the behaviour of the interstellar medium UV extinction curve. A power-law size distribution n(R) R m with in = 3.5 1.0 for these molecules can explain the position and widths observed for the 2,175 A bump and, partly, the rise in the extinction curve at higher energies. We infer ISM densities of 0.2 and 0.1 ppm for small fullerenes and buckyonions (very similar to the densities measured in meteorites). If as expected the cosmic carbon abundance is close to the solar atmosphere value, individual fullerenes may lock up 20-25% of the total carbon in the diffuse interstellar space. 

Hydrogenated fullerenes and buckyonions may produce rotationally based electric dipole microwave radiation under the conditions of the diffuse interstellar medium. These molecules are potential carriers for the anomalous Galactic micro-wave emission recently detected by several cosmic microwave experiments. Their precise contribution to this emission should be fully investigated. 

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