Nucleosynthesis in AGB stars traced by oxygen isotopic ratios. I - Determining the stellar initial mass by means of the 17O/18O ratio
Abstract
The aim of this paper is to investigate the 17O/18O 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. Circumstellar 13C16O, 12C17O and 12C18O 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 17O/18O abundance ratio. Stellar evolution models predict the 17O/18O 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. Observed 17O/18O 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 two mass solutions result, though with a larger probability that the low-mass solution is the correct one. Finally, hints at a possible separation between M/S- and C-type stars when comparing the 17O/18O ratio to the stellar pulsation period are presented.
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