DEEP MIXING IN EVOLVED STARS. II. INTERPRETING Li ABUNDANCES IN RED GIANT BRANCH AND ASYMPTOTIC GIANT BRANCH STARS
- Dipartimento di Fisica, Universita di Perugia, and INFN, Sezione di Perugia, Via Pascoli, 06123 Perugia (Italy)
- Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, Campus de Fuente Nueva, 18071 Granada (Spain)
- Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, A-1180 Vienna (Austria)
- Dipartimento di Scienze Ambientali, Seconda Universita di Napoli, Via Vivaldi 43, 81100 Caserta and INFN, Sezione di Napoli (Italy)
We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After outlining the problems affecting our knowledge of the Li content in low-mass stars (M {<=} 3 M{sub sun}), we discuss deep-mixing models for the red giant branch stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M{sub sun}. Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich asymptotic giant branch (AGB) stars. Here, the constraints on extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) {identical_to} log {epsilon}(Li) {approx_equal} 1.5 (and sometimes more). Also, their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, and the C/O and {sup 12}C/{sup 13}C ratios) can be explained. This requires generally moderate efficiencies (M-dot < or approx. 0.3-0.5 x 10{sup -6} M{sub sun} yr{sup -1}) for non-convective mass transport. At such rates, slow extra mixing does not remarkably modify Li abundances in early AGB phases; on the other hand, faster mixing encounters a physical limit in destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be produced; therefore, its destruction on the AGB is modest. Li is then significantly produced by the third dredge up. We also show that effective circulation episodes, while not destroying Li, would easily bring the {sup 12}C/{sup 13}C ratios to equilibrium, contrary to the evidence in most AGB stars, and would burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the common idea that efficient extra mixing drastically reduces the Li content of C stars with respect to K-M giants. This misleading appearance is induced by biases in the data, namely: (1) the difficulty of measuring very low Li abundances in O-rich AGB stars due to the presence of TiO bands and (2) the fact that many, relatively massive (M > 3 M{sub sun}) K- and M-type giants may remain Li-rich, not evolving to the C-rich stages. Efficient extra mixing on the AGB is instead typical of very low masses (M {approx}< 1.5 M{sub sun}). It also characterizes CJ stars, where it produces Li and reduces F and the carbon isotope ratio, as observed in these peculiar objects.
- OSTI ID:
- 21587301
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 741; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABUNDANCE
ALKALI METALS
CARBON 12
CARBON 13
CARBON ISOTOPES
CARBON STARS
DIMENSIONLESS NUMBERS
ELEMENT ABUNDANCE
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GIANT STARS
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
LITHIUM
MAIN SEQUENCE STARS
METALS
MIXING
NUCLEI
NUCLEOSYNTHESIS
RED GIANT STARS
STABLE ISOTOPES
STARS
SYNTHESIS