Interstellar medium carbon fraction

We give an estimation of the carbon fraction locked in these molecules. We discuss the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes in various phases of the interstellar medium. These molecules could be the carriers of the anomalous microwave emission detected by Watson et al. (Astrophys. J. 624 L89,2005) in the Perseus molecular complex and Cassasus et al. (2006) in the dark cloud LDN 1622. Hydrogenated forms of fullerenes may account for the dust-correlated microwave emission detected in our Galaxy by Cosmic Microwave Background experiments. 

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). 

Approximately 99 % of the mass of the interstellar medium is in the form of gas (where denser regions are termed interstellar clouds) with the remainder primarily in the form of dust. The total mass of the gas and dust in the interstellar medium is about 15 % of the total mass of visible matter in the Milky Way. The exact nature and origin of interstellar dust grains is unknown, but they are presumably ejected from stars. One likely source is from red giant stars late in their lives. Interstellar dust grains are typically a fraction of a micron across, irregularly shaped, and composed of carbon an or silicates. 

Several important observations of the abundances of the C varieties of molecules In diffuse clouds have recently become available. Hawkins and Jura (1987) determined accurate CH+Z CH" " column density ratios for 4 lines of sight. The inferred ratio (43 4) was found to be remarkably uniform in the different directions, and is a factor of two lower than the terrestrial isotope ratio of 89. If CH" " is indeed formed in a warm region without fractionation, the measured CH / CH ratio should be directly representative of the [ C)/[ C] abundance ratio of carbon in all forms in the interstellar medium. 

Abstract Rate constants for charge transfer processes in the interstellar medium are calculated using ab-initio molecular calculations. Two important reactions are presented the recombination of Si + and Si + ions with atomic hydrogen and helium which is critical in determining the fractional abundances of silicon ions, and the C+ + S -> C + S+ reaction, fundamental in both carbon and sulphur chemistry. 

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