2013A&A...556A..46S

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. 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 IRAS22272+5435. 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. We estimate the column density of C₂ to be 3.2x10¹⁵cm^–2. From the chemical model we estimate peak abundance of C₂ as 6.8x10^–6 relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10x10¹⁶cm. The excitation temperature of the lowest levels of 58K 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 1.27x10^–5 relative to nucleon density. 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.