Organic Matter in Space Proceedings IAU Symposium No. 251, 2008 S. Kwok & S. Sandford, eds. c  2008 International Astronomical Union doi:10.1017/S1743921308021170 Fullerenes as carriers of extinction, diffuse interstellar bands and anomalous microwave emission Susana Iglesias-Groth Instituto de Astrof´ ısica de Canarias, C/Via L´ actea sn, 38200, La Laguna, Spain email: sigroth@iac.es Abstract. According to semiempirical models, photoabsorption by fullerenes (single and mul- tishell) could explain the shape, width and peak energy of the most prominent feature of the interstellar absorption, the UV bump at 2175 ˚ A. Other weaker transitions are predicted in the optical and near-infrared providing a potential explanation for diffuse interstellar bands. In par- ticular, we find that several fullerenes could contribute to the well known strong DIB at 4430 ˚ 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. We also study the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes. We investigate these molecules as poten- tial carriers of the anomalous (dust-correlated) microwave emission recently detected by several cosmic microwave background experiments. Keywords. Ultraviolet: ISM, (ISM:) dust, extinction, ISM: lines and bands 1. Introduction In 1985 Kroto and Smalley proposed the existence of a new allotropic form of carbon: the fullerenes (Kroto et al. 1985). Their research on samples of vaporized graphite using laser beams, initially aimed to reproduce the chemistry of the atmospheres of carbon enriched giant stars, gave as a result the unexpected discovery of the C 60 (the 60 carbon atoms fullerene). Subsequent experiments by Kroto and Smalley, and others, showed the existence of carbon aggregates with a larger number of atoms (C 70 ,C 84 ,C 240 ) and established that for an even number of atoms larger than 32, these aggregates were stable. While other molecules have serious difficulties to survive in the interstellar medium, the robustness of C 60 and of fullerenes in general, strongly support a long survival in the harsh conditions of interstellar space. Several laboratory experiments (Chhowalla et al. 2003) and theoretical studies (Henrard et al. 1997, Iglesias-Groth 2004) of the photoabsorption by fullerenes and buckyonions (multishell fullerenes) in the UV suggest that these molecules could be responsible of the most intense feature of interstellar absorption, the so-called ultraviolet bump located at 2175 ˚ A. A significant fraction of interstellar carbon (10–30%) could then reside in fullerene related molecules. It has been suggested (Webster 1991, 1992, 1993) that fullerenes could also be a car- rier of diffuse interstellar bands (DIBs). According to theoretical spectra obtained using semiempirical models (Iglesias-Groth et al. 2002, 2003), icosahedric fullerenes and buck- yonions (from C 60 to C 6000 ) present numerous low-intensity bands in the optical and near-infrared, several with wavelengths very similar to well known DIBs. Fullerenes de- serve further study as potential carriers of DIBs. For the moment, only two DIBs may have been identified as caused by the cation of C 60 (Foing & Ehrenfreund 1994). 57 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921308021170 Downloaded from https://www.cambridge.org/core. IP address: 54.161.69.107, on 17 Jun 2020 at 14:54:07, subject to the Cambridge Core terms of use, available at