  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.
