PAHs interstellar dust

The yellow substance was studied under deep space conditions. After being subjected to UV-irradiation for about four months, it changed colour to brown, possibly due to enrichment in carbon or carbon-containing compounds. IR analysis showed that the new brown substance showed the same absorption lines as those observed in interstellar dust. A special IR study carried out at Stanford University indicated that the brown substance contained many PAHs. 

The polycyclic aromatic hydrocarbons (PAHs) have been inferred to exist in the interstellar dust by the correlation of their general infrared spectral characteristics with observed celestial infrared emission bands [13-15]. 

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. 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

What is the ultimate fate of the molecular material formed in the envelopes of carbon-rich stars as it heads out into space The dust grains will be processed only slowly by the interstellar radiation held and survive almost intact until they become part of an interstellar cloud. The survival of individual PAHs depends on their size the larger ones withstand radiation much better than do the smaller ones.115 By survival we are referring to the aromatic skeleton the interstellar radiation field will efficiently break H bonds and cause ionization so that unsaturated, ionized PAHs are likely to dominate those found in the diffuse interstellar medium. Such species have been suggested as a source of the DIBs.118,123 Small molecules photodissociate in the interstellar radiation field before the material becomes part of an interstellar cloud. 

PAHs are recognized as cosmochemically important molecules, because they are abundantly detected in interstellar media, carbonaceous chondrites, and interplanetary dust particles. Although FTT reactions [42], the pyrolysis of hydrocarbons such as tlie polymerization of acetylene [138], and ion-molecule reactions [148] liave been accepted as responsible for tlie PAH genesis, the shock process discussed in the present study can be suggested as a strong candidate for PAH formation. Most species of PAHs detected in meteorites and interplanetaiy dust... 

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

A We also developed laser-desorption laser-ionization mass spectrometry for the analysis of adsorbates on surfaces, such as interplanetary dust particles and meteoritic samples. We use one laser to rapidly heat the sample and evaporate molecules from the surface. A second laser intercepts the rising plume of molecules and ionizes those that absorb that color of light. We then weigh the ions using a mass spectrometer. We have analyzed graphite particles extracted from meteorites and found polycyclic aromatic molecules (PAHs). The PAHs have to isotope ratios that match closely the graphite grains, which are believed to be the remnants of the star dust from which our solar system condensed some 4.5 billion years ago. These are the first interstellar molecules observed directly in the laboratory. 

The mid-IR dust emission, particularly the PAH feature at A = 7.7 pm, is a clear tracer of the presence of interstellar matter. The emission shows high contrast against stellar emission at the same wavelength. 

The graphitic particles consist almost entirely of carbon atoms, probably bonded to each other in a variety of complex forms. PAH clusters have been hypothesized for some time and evidence is accumulating that they occur abundantly in the ISM. No specific PAH molecule, however, has as yet been definitively identified in the interstellar medium. Whatever form they may take, dust particles are now thought to be remnants of stellar explosions, in which a star expels part or all of its mass into the ISM. 

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