Volume 556, August 2013
|Number of page(s)||9|
|Section||Interstellar and circumstellar matter|
|Published online||23 July 2013|
The excitation of circumstellar C2 in carbon-rich post-AGB object IRAS 22272+5435⋆
1 Department of Astrophysics, N. Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
2 Faculty of Physics and Mathematics, University of Latvia, Raiņa bulvāris 19, 1586 Rīga, Latvia
Received: 28 November 2012
Accepted: 13 May 2013
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.
Key words: stars: AGB and post-AGB / circumstellar matter / stars: mass-loss / line: identification
Table 1 is available in electronic form at http://www.aanda.org
© ESO, 2013
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