Superwind in evolved OH/IR stars

We report observations of (CO)-C-13 (J=1-0) and (J=2-1) emission towards 5 evolved OH/IR envelopes. Four of them are known to have very weak CO emission compared to their infrared flux, and an anomalously high (J=2-1)/(J=1-0) intensity ratio. Modeling of their (CO)-C-12 and (CO)-C-13 lines is used to test several possible explanations of this behaviour. We conclude that it is most likely due to the recent onset of a superwind phase. Another interesting result of the modeling is the very low C-12/C-13 isotopic ratios we derive in these envelopes, of the order of 3.5 and close to the equilibrium value of the CNO cycle. We interpret this as the signature of massive stars at the end of their AGE phase, The sixth envelope, which shows a much lower (J=2-1)/(J=1-0) intensity ratio, also has a low, marginally larger, C-12/C-13 isotopic ratio. We conclude that it also is a massive object, but in a slightly less evolved stage, just before the superwind phase.