THE C O ABUNDANCE RATIO IN THE DETACHED CIRCUMSTELLAR ENVELOPES AROUND CARBON STARS/

We present molecular observations of a sample of five 'detached envelo pes' surrounding C-rich evolved stars: S Sct, TT Cyg, U Ant, U Cam and R Scl. The observed lines are (CO)-C-12 J=2-1, HCN J=1-0, HNC J=1-0, SiS J=5-4, HC3N J=10-9, CN N=1-0, CS J=5-4, SiO J=3-2, SO J(N)=65-54, and (CO)-C-13 J=2-1 (observations from the literature, including resul ts for the lines (CO)-C-12 J=1-0, CS J=3-2 and SiO J=2-1, are also tak en into account). We confirm the presence of two emitting shells in S Sct and TT Cyg. We also find a two-shell structure in U Cam and argue that the envelope of R Scl detected in molecular emission is also prob ably detached. The (very probable) detection of HCN emission from the inner envelope of S Sct and its unexpected two-peak structure are rema rkable. In the inner shells of S Sct and U Cam and the outer shells of U Cain and R Scl, the chemistry is found to be dominantly C-rich, fro m the comparison of the line intensity ratios with those usually found in standard evolved stars. We note the lack of detected molecular emi ssion, other than CO lines, from the outer envelopes around S Sct and TT Cyg, for which we cannot conclude about the dominant chemistry. We have then not found any sign of 0-rich chemistry in the observed detac hed shells around C-rich evolved stars.