Nucleosynthesis in AGB stars traced by oxygen isotopic ratios I. Determining the stellar initial mass by means of the 17O/18O ratio

Aims. We seek to investigate the O-17/O-18 ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period. Methods. Circumstellar (CO)-C-13-O-16, (CO)-C-12-O-17, and (CO)-C-12-O-18 line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface O-17/O-18 abundance ratio. Results. Stellar evolution models predict the O-17/O-18 ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 M-circle dot. This makes the measured ratio a probe of the initial stellar mass. Conclusions. Observed O-17/O-18 ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the O-17/O-18 ratio to the stellar pulsation period.