H2O Isotopologues in Extreme OH/IR Stars

Using Herschel Space Observatory we we observed isotopologues of H2O in extreme OH/IR stars. We detected strong (H2O)-O-16 and (H2O)-O-17 while the (H2O)-O-18 lines are missing, contrary to the overall galactic oxygen abundance in the interstellar medium and the Sun, where O-18 is more abundant than O-17. Theoretical stellar evolution suggests that O-18 is being destroyed during the hot-bottom burning. This implies that these OH/IR stars come from a population of intermediate-mass stars which have an initial mass >= 5 M-circle dot.