1 research outputs found
The puzzle of the CNO isotope ratios in AGB carbon stars
Previous determinations of the oxygen isotopic ratios in AGB carbon stars
were at odds with the existing theoretical predictions. We aim to redetermine
the oxygen ratios in these stars using new spectral analysis tools and further
develop discussions on the carbon and nitrogen isotopic ratios in order to
elucidate this problem. Oxygen isotopic ratios were derived from spectra in the
K-band in a sample of galactic AGB carbon stars of different spectral types and
near solar metallicity. Synthetic spectra calculated in LTE with spherical
carbon-rich atmosphere models and updated molecular line lists were used. The
CNO isotope ratios derived in a homogeneous way, were compared with theoretical
predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code
assuming extra mixing both during the RGB and AGB phases. For most of the stars
the 16O/17O/18O ratios derived are in good agreement with theoretical
predictions confirming that, for AGB stars, are established using the values
reached after the FDU according to the initial stellar mass. This fact, as far
as the oxygen isotopic ratios are concerned, leaves little space for the
operation of any extra mixing mechanism during the AGB phase. Nevertheless, for
a few stars with large 16O/17O/18O, the operation of such a mechanism might be
required, although their observed 12C/13C and 14N/15N ratios would be difficult
to reconcile within this scenario. Furthermore, J-type stars tend to have lower
16O/17O ratios than the normal carbon stars, as already indicated in previous
studies. Excluding these peculiar stars, AGB carbon stars occupy the same
region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram,
showing little spread. This reinforces the idea that these grains were probably
formed in low-mass stars during the previous O-rich phases.Comment: Accepted fo publication in A&