Hydrated Dirty PAHs as Products of Radiative Chemistry in Nebulae

Bernstein et al. (1999) found very similar products in the laboratory when strongly irradiating PAH - water mixture ices: Hydrated PAHs as preferably quinones, ethers (also forming furane like ring structures), and (some) alcohols. In interstellar clouds PAH and water was found. First reactions, however, must have happened at polar grains, the simplest readily to be formed being LiH. The light chemical elements of exploded stars are H, C, O, (He?), Li (in that abundance order), however, C and O always come with some N, although in minor abundance. Thus an additional "dirtification" with some N is probable.

Fig. 12.2. (a) Uracile; primordially probable (sub)-structure with the quinoic property of pyrimidine. (b) Thymine; primordially UNprobable due to its methyl group. Probably developed on the Earth from precursors like uracile. (c) PQQ - pyrrolo-quinolinequinone with three carboxy substitutes. The formic acid residue is found by CIDA in interstellar dust. PQQ-similar molecules are most important catalysts, as they can act as electron donors and acceptors in (at least) three oxidation states.

Fig. 12.2. (a) Uracile; primordially probable (sub)-structure with the quinoic property of pyrimidine. (b) Thymine; primordially UNprobable due to its methyl group. Probably developed on the Earth from precursors like uracile. (c) PQQ - pyrrolo-quinolinequinone with three carboxy substitutes. The formic acid residue is found by CIDA in interstellar dust. PQQ-similar molecules are most important catalysts, as they can act as electron donors and acceptors in (at least) three oxidation states.

Furthermore, we have to point out that over the entire time of production and journey to other stars, the grains take up a lot of radiation dose (some 100 Mrad is an order of magnitude estimate). Thus it is obvious that not only the end-product polymers have to be very stable against radiation; moreover, it is radiative, i.e., epithermal, chemistry that builds up these products at first. Especially these quinoic annealed substances can stabilize free electrons very well (see Sect. 12.3.5). Thus they are the best candidates for organic interstellar dust to enter into our solar system. By the way, there are certainly also a lot of mineral grains in cosmic dust. However, Landgraf et al. (1999) have shown that those microsized particles cannot enter the interior of our solar system for electromagnetic-optical reasons. Thus it is no surprise that we did not detect them.

It seems to be an open question whether in some distances from the active Sun's proton irradiation (not being too energetic) may play a role in reducing the organic matter by proton capture. In this case quinoic systems could be reduced to aromatic (quinol) systems. There is a hint for o-hydroquinone there with the 218-nm UV bump. Later irradiation, in contrary, has an oxidizing effect by dehydrogenation via free radicals. Further polymerization stabilizes its chemical structure against decay and vaporization.

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