Diffraction-limited 3 μm spectroscopy of iras 04296+3429 and IRAS 05341+0852:: Spatial extent of hydrocarbon dust emission and dust evolutionary sequence

We present 3 mu m spectroscopy of the carbon- rich protoplanetary nebulae IRAS 04296+3429 and IRAS 05341+ 0852, conducted with the adaptive optics system at the Subaru Telescope. We utilize the nearly diffraction- limited spectroscopy to probe the spatial extent of the hydrocarbon dust emitting zone. We find a hydrocarbon emission core extending up to 100-160 mas from the center of IRAS 04296+3429, corresponding to a physical diameter of 400-640 AU, assuming a distance of 4 kpc. However, we find that IRAS 05341+0852 is not spatially resolved with this instrumentation. The physical extent of these protoplanetary nebulae, along with the reanalyzed data of IRAS 22272+5435 published previously, suggests a correlation between the physical extent of the hydrocarbon dust emission and the spectral evolution of the aliphatic to aromatic features in these post- AGB stars. These measurements represent the first direct test of the proposed chemical synthesis route of carbonaceous dust in the circumstellar environment of evolved stars.