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| - The excitation of circumstellar C 2 in carbon-rich post-AGB object IRAS 22272+5435
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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 C 2 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 C 2 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 C 2 for comparison with observed column densities. Results. We estimate the column density of C 2 to be 3.2 × 10 15 cm -2. From the chemical model we estimate peak abundance of C 2 as 6.8 × 10 -6 relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10 × 10 16 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 C 2H 2 inferred from the analysis is found to be 1.27 × 10 -5 relative to nucleon density. Conclusions. C 2 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.
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