Nucleosynthesis in AGB stars traced by oxygen isotopic ratios

Abstract

Aims. We seek to investigate the 17 O/ 18 O 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 13 C 16 O, 12 C 17 O, and 12 C 18 O line observations were obtained for a sample of nine stars with various singledish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface 17 O/ 18 O abundance ratio. Results. Stellar evolution models predict the 17 O/ 18 O 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 . This makes the measured ratio a probe of the initial stellar mass. Conclusions. Observed 17 O/ 18 O 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 17 O/ 18 O ratio to the stellar pulsation period.