2007ApJ...662..389G

We present 3 µm 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.