ON THE NEED FOR DEEP-MIXING IN ASYMPTOTIC GIANT BRANCH STARS OF LOW MASS
- Dipartimento di Fisica, Universita di Perugia (Italy)
- Departamento de Fisica Teorica y del Cosmos, Universidad de Granada (Spain)
- INAF, Osservatorio di Teramo (Italy)
The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the {sup 26}Al/{sup 27}Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both red giant branch (RGB) and asymptotic giant branch (AGB) stages, is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low-mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the {sup 26}Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of Population I stars above about 1.2 M{sub sun}, this consumes {sup 3}He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.
- OSTI ID:
- 21451055
- Journal Information:
- Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 717; ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABUNDANCE
ALUMINIUM 26
ALUMINIUM 27
ALUMINIUM ISOTOPES
ATMOSPHERES
BETA DECAY RADIOISOTOPES
BETA-PLUS DECAY RADIOISOTOPES
CARBIDES
CARBON COMPOUNDS
CARBON STARS
CHALCOGENIDES
DIMENSIONLESS NUMBERS
ELEMENTS
EVEN-ODD NUCLEI
EVOLUTION
GALAXIES
HELIUM 3
HELIUM ISOTOPES
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MAIN SEQUENCE STARS
MASS
NONMETALS
NUCLEAR REACTIONS
NUCLEI
NUCLEOSYNTHESIS
ODD-EVEN NUCLEI
ODD-ODD NUCLEI
OXIDES
OXYGEN
OXYGEN COMPOUNDS
PHOTOSPHERE
RADIOISOTOPES
SECONDS LIVING RADIOISOTOPES
SILICON CARBIDES
SILICON COMPOUNDS
SOLAR ATMOSPHERE
STABLE ISOTOPES
STAR EVOLUTION
STARS
STELLAR ATMOSPHERES
SYNTHESIS
YEARS LIVING RADIOISOTOPES