Polycyclic aromatic hydrocarbons Interstellar clouds

It is possible that electrons are not the major carrier of negative charge in dense interstellar clouds. It has been suggested that if a large fractional abundance of polycyclic aromatic hydrocarbons (PAHs) exists throughout dense interstellar clouds (see Section I), then electron sticking reactions of the type,... 

Molecular astronomy of carbon molecules is very rich. Of about 120 known interstellar molecules more than three-quarters contain carbon atoms diatomic molecules include CO, CN, C2, CH, CH+, CN+, and CO+ polyatomic include CH2, CH4, C2H2j CH OH, CH3CH2OH, H2CO and HNC large complex unsaturated radicals and polycyclic aromatic hydrocarbons are also detected. These all play a role in the thermochemistry of interstellar clouds. The 2.6-millimeter line of CO diagnoses density and temperature in molecular clouds, as do other molecules. 

Naphthalene, anthracene, and similar hydrocarbons are termed polycyclic aromatic hydrocarbons (PAHs) because they are composed of multiple aromatic rings. PAHs have been found in meteorites and identified in the material surrounding dying stars. Scientists have mixed PAHs with water ice in a vacuum at -260°C to simulate the conditions found in interstellar clouds. To simulate radiation emitted by nearby stars, they shined ultraviolet light on the mixture. About ten percent of the PAHs were converted to alcohols, ketones, and esters— molecules that can be used to form compounds that are important in biological systems. 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

X 10 kg. Analyses of interstellar dust clouds and heavenly bodies fotmd on Earth, such as the Murchison meteorite, showed the presence of a variety of important prebiotic compotmds. such as amino acids, ureas, alcohols. aldehydes, purines, polycyclic aromatic hydrocarbons (PAH), and organic acids,"" highlighting the significance of extraterrestrial carbon delivery to Earth. 

Omont (1986) has outlined the evidence for the presence of polycyclic aromatic hydrocarbons in the interstellar medium and given a full acccount of their properties. He pointed out that PAH ions may be more abundant than free electrons in dense clouds. Lepp and Dalgamo (1987) recognized that in this circumstance the recombinatiwi of positive ions occurs through mutual neutralization with PAH ions and have discussed the consequential changes in the chemistry. Omont (1986) gives the rate coeffrcient fen the process to be approximately... 

It is now generally accepted that many of the neutral molecules observed in interstellar clouds are formed in the gas phase by positive ion-molecule reactions which produce a wide variety of molecular positive ions which then dissociatively recombine with electrons to form the observed neutral molecules. The final neutralization step in the conversion of a positive molecular ion to a neutral molecule may also be the transfer of a proton from the ion to another molecule or the transfer of an electron to the molecular ion from a species of low ionization energy (e.g. a metal atom). Very recently, it has been proposed that negatively-charged macromolecules, PAH" (i.e. negatively- charged polycyclic aromatic hydrocarbons, PAH s), may be present in interstellar... 

The interstellar medium may also appear as dark clouds that absorb starlight. The densities are several 10 particles cm . In such clouds polycyclic aromatic hydrocarbons (PAHs), fullerenes, carbon-chains, diamonds etc. are found. 

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