SIMBAD references

We report observations of ¹³CO (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 ¹²CO and ¹³CO 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 ¹²C/¹³C 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 AGB phase. The sixth envelope, which shows a much lower (J=2-1)/(J=1-0) intensity ratio, also has a low, marginally larger, ¹²C/¹³C isotopic ratio. We conclude that it also is a massive object, but in a slightly less evolved stage, just before the superwind phase.