Spatially Resolved 3 Micron Spectroscopy of IRAS 22272+5435: Formation and Evolution of Aliphatic Hydrocarbon Dust in Proto-Planetary Nebulae^*

We present medium-resolution 3 μm spectroscopy of the carbon-rich proto-planetary nebula IRAS 22272+5435. Spectroscopy with the Subaru Telescope adaptive optics system revealed a spatial variation of hydrocarbon molecules and dust surrounding the star. The rovibrational bands of acetylene (C₂H₂) and hydrogen cyanide (HCN) at 3.0 μm are evident in the central star spectra. The molecules are concentrated in the compact region near the center. The 3.3 and 3.4 μm emission of aromatic and aliphatic hydrocarbons is detected at 600-1300 AU from the central star. The separation of spatial distribution between gas and dust suggests that the small hydrocarbon molecules are indeed the source of solid material and that the gas left over from the grain formation is being observed near the central star. The intensity of aliphatic hydrocarbon emission relative to the aromatic hydrocarbon emission decreases with distance from the central star. The spectral variation is well matched to that of a laboratory analog thermally annealed with different temperatures. We suggest that either the thermal process after the formation of a grain or the variation in the temperature in the dust-forming region over time determines the chemical composition of the hydrocarbon dust around the proto-planetary nebula.