EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES
- INAF-Osservatorio Astronomico di Collurania, 64100 Teramo (Italy)
- Dipartimento di Fisica Generale, Universita di Torino, 10125 Torino (Italy)
- Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada (Spain)
- Institut fuer Astronomie, Tuerkenschanzstrasse 17, A-1180 Wien (Austria)
The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, {alpha}, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of low-mass AGB stars (2 M {sub sun}), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C- and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin {sup 13}C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this {sup 13}C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the {sup 13}C({alpha}, n){sup 16}O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T {approx} 1.0 x 10{sup 8} K, with typical densities of (10{sup 6}-10{sup 7}) neutrons cm{sup -3}. Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the {sup 13}C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T {approx} 1.5 x 10{sup 8} K, thus producing larger neutron densities (up to 10{sup 11} neutrons cm{sup -3}). An additional neutron exposure, caused by the {sup 22}Ne({alpha}, n){sup 25}Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the {sup 22}Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, we found an interesting overproduction of some radioactive isotopes, like {sup 60}Fe, as a consequence of the anomalous first {sup 13}C pocket. Finally, light elements (C, F, Ne, and Na) are enhanced at any metallicity.
- OSTI ID:
- 21300717
- Journal Information:
- Astrophysical Journal, Vol. 696, Issue 1; Other Information: DOI: 10.1088/0004-637X/696/1/797; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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