A thin molecular shell around the carbon star TT Cyg

Interferometric CO(J=1→0 and 2→1) observations reveal a remarkably thin shell of molecular gas around the carbon star TT Cyg, width/radius ≲ 1.″3/34″ ≈ 0.04. It expands at ≈ 13 km s^-1, and contains ∼0.02 M⊙ of gas provided the CO abundance with respect to H₂ is 10^-3 and the distance is 1 kpc. Only about a quarter of the shell has been mapped, but we infer an overall spherical shell with only small, but clear, deviations ai the per cent level. The radial structure of the shell is barely resolved at the arc second level, but there exists weak emission extending a few arc seconds inwards from the peak. A drastic change in mass loss properties, possibly combined with the effects of interacting winds, provides the most likely explanation to the origin of the shell.