Oxygen Isotopic Abundances in Evolved Stars. IV. Five K Giants
Abstract
Oxygen isotopic ratios have been measured in the atmospheres of Alpha UMa, Beta Gem, Beta UMi, Alpha Ari, and Alpha Ser. It is shown that some of the stars in this and previous samples have probably experienced the helium core flash, and that the O-16/O-17 ratios tend to confirm that 'extramixing' of CNO-cycled material into the stars' envelopes has occurred as a result of the flash. The oxygen isotopic ratios are much smaller than the anomalously high ratios seen in stars in the later thermal-pulsing state of evolution. Hence, the anomalous ratios cannot be explained by the helium core flash. It is proposed that they are due to the addition by the third dredge-up mechanism of helium-burned material which is devoid of both O-17 and O-18 and therefore dilutes the O-17 and O-18 abundances acquired during earlier evolutionary stages. This process may also explain the oxygen isotope ratios seen in barium stars, if these are formed by mass transfer from a thermally pulsing binary companion.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1988
- DOI:
- 10.1086/166047
- Bibcode:
- 1988ApJ...325..768H
- Keywords:
-
- Abundance;
- Asymptotic Giant Branch Stars;
- K Stars;
- Late Stars;
- Oxygen Isotopes;
- Helium;
- Stellar Envelopes;
- Stellar Interiors;
- Astrophysics;
- NUCLEOSYNTHESIS;
- STARS: ABUNDANCES;
- STARS: INTERIORS;
- STARS: LATE-TYPE