Astronomical evolution mechanism of small size polycyclic aromatic hydrocarbon (PAH) was analyzed using the first principles quantum-chemical calculation. Starting model molecule was benzene (C6H6), which would be transformed to (C5H5) due to carbon void created by interstellar high speed proton attack. In a protoplanetary disk around a young star, molecules would be illuminated by high energy photon and ionized to be cationic-(C5H5). Calculation shows that from neutral to tri-cation, molecule keeps original configuration. At a step of sixth cation, there occurs surprising creation of cyclic-C3H2, which is the smallest PAH. Astronomical cyclic-C3H2 had been identified by radio astronomy. Deep photoionization of cyclic-C3H2 brings successive molecular change. Neutral and mono-cation keep cyclic configuration. At a step of di-cation, molecule was transformed to aliphatic chain-C3H2. Finally, chain-C3H2 was decomposed to pure carbon chain-C3 and two hydrogen atoms. Calculated infrared spectrum of those molecules was applied to observed spectrum of Herbig Ae young stars. Observed infrared spectrum could be partially explained by small molecules. Meanwhile, excellent coincidence was obtained by applying a larger molecules as like (C23H12)2+ or (C12H8)2+. Infrared observation is suitable for larger molecules and radio astronomy for smaller asymmetric molecules. It should be noted that these molecules could be identified in a natural way introducing two astronomical phenomena, that is, void-induced molecular deformation and deep photoionization.
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