Herschel observations of extreme OH/IR stars The isotopic ratios of oxygen as a sign-post for the stellar mass

Aims. The late stages of stellar evolution are mainly governed by the mass of the stars. Low-and intermediate-mass stars lose copious amounts of mass during the asymptotic giant branch (AGB) which obscure the central star making it difficult to study the stellar spectra and determine the stellar mass. In this study, we present observational data that can be used to determine lower limits to the stellar mass. Methods. Spectra of nine heavily reddened AGB stars taken by the Herschel Space Observatory display numerous molecular emission lines. The strongest emission lines are due to H2O. We search for the presence of isotopologues of H2O in these objects. Results. We detected the O-16 and O-17 isotopologues of water in these stars, but lines due to H-2(18) O are absent. The lack of O-18 is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys O-18 relative to O-16 and O-17. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 M-circle dot for solar metallicity. Hence, observations of different isotopologues of H2O can be used to help determine the lower limit to the initial stellar mass. Conclusions. From our observations, we deduce that these extreme OH/IR stars are intermediate-mass stars with masses of >= 5 M-circle dot. Their high mass-loss rates of similar to 10(-4) M-circle dot yr(-1) may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.