Herschelobservations of extreme OH/IR stars

Abstract

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 H2 O. We search for the presence of isotopologues of H2 O in these objects. Results. We detected the 16 O and 17 O isotopologues of water in these stars, but lines due to H2 18 O are absent. The lack of 18 O is predicted by a scenario where the star has undergone hot-bottom burning which preferentially destroys 18 O relative to 16 O and 17 O. From stellar evolution calculations, this process is thought to occur when the stellar mass is above 5 M ⊙ for solar metallicity. Hence, observations of different isotopologues of H2 O 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 ⊙ . Their high mass-loss rates of ~10-4 M ⊙ yr-1 may affect the enrichment of the interstellar medium and the overall chemical evolution of our Galaxy.