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 x 10(15) cm(-2). From the chemical model we estimate peak abundance of C-2 as 6.8 x 10(-6) relative to nucleon density. The absorption of molecular lines originate in the ring between 5 and 10 x 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 C2H2 inferred from the analysis is found to be 1.27 x 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.

We report the discovery of water vapour toward the carbon star V Cygni. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1(11)-0(00) para-water transition at 1113.3430 GHz in the upper sideband of the Band 4b receiver. The observed spectral line profile is nearly parabolic, but with a slight asymmetry associated with blueshifted absorption, and the integrated antenna temperature is 1.69 +/- 0.17 K km s(-1). This detection of thermal water vapour emission, carried out as part of a small survey of water in carbon-rich stars, is only the second such detection toward a carbon-rich AGB star, the first having been obtained by the Submillimeter Wave Astronomy Satellite toward IRC+ 10216. For an assumed ortho-to-para ratio of 3 for water, the observed line intensity implies a water outflow rate similar to 3-6 x 10(-5) Earth masses per year and a water abundance relative to H-2 of similar to 2-5 x 10(-6). This value is a factor of at least 10(4) larger than the expected photospheric abundance in a carbon-rich environment, and - as in IRC+ 10216 - raises the intriguing possibility that the observed water is produced by the vapourisation of orbiting comets or dwarf planets. However, observations of the single line observed to date do not permit us to place strong constraints upon the spatial distribution or origin of the observed water, but future observations of additional transitions will allow us to determine the inner radius of the H2O-emitting zone, and the H2O ortho-to-para ratio, and thereby to place important constraints upon the origin of the observed water emission.

With the TRAM radio telescope we have observed rotational transitions of HCN(1-0), CS(3-2), CS(5-4), and SiO(3-2) in two carbon stars with OH maser emission detected towards them: IRAS 04130+3918 and IRAS 06238 +0904. We present results of modelling the molecular emissions for the latter object. Physical parameters of the envelope were established from a model of the spectral energy distribution. A chemical model was developed for the envelope assuming that it is a genuine carbon star. The observed emissions were fitted with a non-LTE code. We conclude that the observed abundance of the SiO molecule may be explained by the gas-phase chemistry in the envelope.

Aims.The main aim of this paper was to test our (chemical and kinetic) codes, which will be used during self-consistent modelling of dynamics and chemistry in the winds from C-rich AGB stars.

We discuss phenomenon of simultaneous presence of O- and C-based material in surroundings of evolutionary advanced stars. We concentrate on silicate carbon stars and present observations that directly confirm the binary model scenario for them. We discuss also class of C-stars with OH emission detected, to which some [WR] planetary nebulae do belong.

Aims. The main aim of this paper was to test our ( chemical and kinetic) codes, which will be used during self-consistent modelling of dynamics and chemistry in the winds from C-rich AGB stars.Methods. We used the thermodynamical equilibrium code to test the different databases of dissociation constants. We also calculated the equilibrium content of the gas using the kinetic code that includes the chemical network of neutral-neutral reactions. The influence of reaction rates updated using the UMIST database for Astrochemistry 2005 (UDFA05) was tested.Results. The local thermodynamical equilibrium calculations show that the NIST database reproduces equilibrium concentrations fairly well in comparison with previous computations, while consistency for the other, commonly used, dissociation constants is worse.The most important finding is that the steady state solution obtained with the kinetic code for the reactions network is different from the thermodynamical equilibrium solution. In particular, the important opacity sources CN and C-2 are underabundant relative to thermodynamical equilibrium, while O-bearing molecules (like SiO, H2O, and OH) are overabundant. After updating the reaction rates by data from the UDFA05 database, the consistency in O-bearing species becomes much better, however the disagreement in C-bearing species is still present.

We have analysed optical spectra of the carbon-rich post-AGB star IRAS 22272+5435. The circumstellar components of molecular lines of CN (Red System) and C-2 (Phillips and Swan bands) were identified and measured. The mechanism of excitation of the rotational levels of the ground electronic states of CN and C-2 was analysed with a non-LTE code. An empirical model accounting for the observed spectral energy distribution was used as the physical model of the shell. An extensive chemical model was constructed based on the elemental abundances obtained from the analysis of optical atomic lines.

We present a new approach to modelling of chemical evolution of the inner circumstellar envelope of carbon-rich AGB stars. We consider the effect of pulsation-driven shocks on the molecular content of the gas starting from the stellar photosphere, where the chemical composition is assumed to be that at local thermal equilibrium. The chemical kinetic scheme is based on the silicon and sulphur chemistry network of Willacy and Cherchneff (1998).