9 Summary

We have carried out an atlas of the ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) line emission in a selected sample of AGB and post-AGB stars. The survey observations of 46 circumstellar envelopes reported in this paper significantly improve the data available from previous ¹²CO surveys. The main results we obtained from the work are:

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We have determined interferometric positions for 44 envelopes. Interferometric positions are not available for CL Mon and HD 187785. As a rule, the absolute positional accuracy in the data is estimated to about 0.5''.

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We have set 5$\sigma$ upper limits to the radio continuum at 112GHz of 42 stars and determined the radio flux of CIT6 and 19480+2504 to $\sim 13\,$mJy and $\sim 28\,$mJy, respectively. We tentatively detected $\mu\,$Cep in the radio continuum.

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We have measured terminal and systemic velocities of 44 envelopes. Except perhaps for V CrB, T Cep and R Cas, the difference in the ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) terminal velocities can be considered as negligible.

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The line shapes show for the most part the typical signature of optically thick envelopes. There are, however, line profiles with a strong central spike characteristic of a two-component envelope that in some cases may be related to the presence of detached shells as in U Cam, RS Cnc, R Cas, OH127.8+0.0, 04307+6210, S Cep and possibly T Dra. In addition, we found two line profiles with relatively strong wings, an indirect indication for aspherical mass loss activity, in M1-92 and IRC+10420.

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We have determined ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) flux densities and integrated intensities for almost all the stars in the sample. Consistency in the ($J=1\rightarrow 0$) results was achieved by minimizing possible calibration errors in the single-dish data, that is, by readjusting appropriately the single-dish flux density scale to the interferometric one. In a number of stars we found discrepancies with previous works.

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We analyzed the morphology, size and asymmetry of 46 envelopes. Within the sensitivity of our observations, AGB and post-AGB envelopes can be considered for the most part as of spherical appearance. There is, however, evidence for an inner shell surrounding the central star and/or for a significant envelope asymmetry in about 30% of the envelopes of our sample.

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The position-velocity diagrams of the oxygen rich RS Cnc, M1-92 and R Cas show a remarkable kinematical continuity in both the ¹²CO($J=1\rightarrow 0$) and ¹²CO($J=2\rightarrow 1$) line emission, an indication that the circumstellar dynamics in these stars is confined to an axially symmetric morphology. Further envelopes but with less pronounced kinematical signature are found in Y CVn, RAFGL 2362, T Cep and 23321+6545.

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We have calculated mass loss rates and ¹²CO photodissociation radii for 38 stars. We have also established an empirical relation between the ¹²CO photodissociation radius of an envelope and the measured CO extent at half maximum intensity. This results indicates that very probably the CO extent is given by the envelope radius inside which CO is not photodissociated by interstellar UV radiation.