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Context. Observations of the circumstellar envelopes of post-asymptotic giant branch (post-AGB) stars provide information about their mass-loss history during the AGB phase and about the chemistry inside envelopes after that phase. Aims. We analyze visual observations of C2 molecule in absorption and millimeter and sub-millimeter observations of CO molecule in emission with the aim of determining the physical and chemical conditions in the circumstellar shell of the carbon-rich post-AGB star IRAS 22272+5435. Methods. We determined the column densities and excitation structure of C2 from equivalent width analysis of the molecular absorptions. The thermal and density structure of post-AGB shell (AGB remnant) is constrained by multilevel radiative transfer modeling of CO emission lines. The chemical structure of the envelope was computed and then used in the multilevel radiative transfer in C2 for comparison with observed column densities. Results. We estimate the column density of C2 to be 3.2 × 1015 cm−2. From the chemical model we estimate peak abundance of C2 as 6.8 × 10−6 relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10 × 1016 cm. The excitation temperature of the lowest levels of 58 K is consistent with the gas kinetic temperature derived from the CO modeling. The initial abundance of the parent molecule C2H2 inferred from the analysis is found to be 1.27 × 10−5 relative to nucleon density. Conclusions. C2 molecule is a promising tool for probing the temperature structure of the envelopes of post-AGB objects and indirectly for determining the chemical abundance of acetylene.

The excitation of circumstellar C2in carbon-rich post-AGB object IRAS 22272+5435

The excitation of circumstellar C₂in carbon-rich post-AGB object IRAS 22272+5435