Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances

By M. Busso, Roberto Gallino, D. Lambert, C. Travaglio, V. Smith. 2001

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We present the results of s-process nucleosynthesis calculations for asymptotic giant branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M☉), and we present comparisons of them with observational constraints from high-resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge-up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(α, n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The neutron source 13C is manufactured locally by proton captures on the abundant 12C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge-up episode, when a chemical discontinuity is established between the convective envelope and the He- and C-rich zones. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22Ne(α, n)25Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13C formation, the abundance of 13C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge-up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s [hs]) and that of the Zr-peak ones (light s [ls]), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g., of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s-path at low metallicities drains the Zr and Ba peaks and builds an excess at the doubly magic 208Pb, which is at the termination of the s-path. We then discuss the effects on the models of variations in the crucial parameters of the 13C pocket, finding that they are not critical for interpreting the results. The theoretical predictions are compared with published abundances of s-elements for AGB giants of classes MS, S, SC, post-AGB supergiants, and for various classes of binary stars, which supposedly derive their composition by mass transfer from an AGB companion. This is done for objects belonging both to the Galactic disk and to the halo. The observations in general confirm the complex dependence of neutron captures on metallicity. They suggest that a moderate spread exists in the abundance of 13C that is burnt in different stars. Although additional observations are needed, it seems that a good understanding has been achieved of s-process operation in AGB stars. Finally, the detailed abundance distribution including the light elements (CNO) of a few s-enriched stars at different metallicities are examined and satisfactorily reproduced by model envelope compositions.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/26dda56056023f89a7d33210775e13a55806d4d6

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Citations in the corpus

CONVECTIVE–REACTIVE PROTON–12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS (2010, background): “The first s-process peak elements are enhanced by up to 2dex while Ba and La are not enhanced, causing a ratio of Ba peak to Sr peak elements that is much lower than expected from models and observations of AGB stars (Busso et al. 2001).”
The 85Kr s-Process Branching and the Mass of Carbon Stars (2001, methodology): “…models of AGB evolution and nucleosynthesis, we use the s-process nucleosynthesis results from Gallino et al. (1998), Travaglio et al. (2001), and Busso et al. (2001), which are based on a large set of AGB stellar models, for stars of initial mass between 1.5 and 7 made with the FRANEC…”
The s process: Nuclear physics, stellar models, and observations (2010, background): “For a given metallicity, a spread in the three s-process peaks is observed in stars of spectral type MS, S, C(N) and Ba stars of the Galactic disk (see Abia et al. 2001, 2002; Busso et al. 2001, 1995; Gallino et al. 2005).”
A double stellar generation in the globular cluster NGC 6656 (M 22): Two stellar groups with different iron and s-process element abundances (2009, background): “Barium, yttrium, and zirconium can be considered as a signature of the s processes that occur in intermediate mass AGB stars (Busso et al. 2001), whose wind could have polluted the primordial material from which the second generation of stars in M 22 and NGC 1851 formed.”
s-Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution (2002, methodology): “Busso et al. (2001) demonstrated that a spread in the efficiency of the neutron release in the 13C pocket is necessary to cover the spectroscopic observations at any given metallicity.”
Origin of the Chemical Elements (2010, background): “He-shell flashes (see Section 4.3) and the mixing brought about by them turned out to be the key (Busso et al 2001, Boothroyd 2006).”
Neutron-Capture Elements in the Early Galaxy (2008, methodology): “In essence, the choice of the 13C concentration in the pocket is left as a free parameter, in the range of the upper and lower limits specified above, to be calibrated by observational constraints (Busso et al. 2001, Abia et al. 2002, Gallino et al. 2005, Bisterzo et al. 2006a,b).”
The elemental composition of the Sun - III. The heavy elements Cu to Th (2014, background): “The main neutron source in these stars is thought to be 13C(α,n)16O, although 22Ne(α,n)25Mg can also operate, especially in more massive AGB stars (e.g. Busso et al. 2001; Karakas et al. 2012).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, methodology): “Sr is excluded from the ls elements and Ba from the hs elements because their lines are in general affected by higher uncertainties (Busso et al. 2001).”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, background): “Bonačić Marinović et al. (2007a) find that Galactic disk objects are reproduced by a spread of a factor of two or three in the effectiveness of the 13C pocket, lower than that found by Busso et al. (2001), who needed a spread of a factor of about 20.”
Short-Lived Nuclei in the Early Solar System: A Low Mass Stellar Source? (2003, neutral): “When a 13C pocket was considered, the standard (ST) choices presented by Gallino et al. (1998) and Busso et al. (2001) were adopted, corresponding to the burning of 4 × 10−6 M⊙ of 13C per cycle in low mass stars and of 4 × 10−7 M⊙ in intermediate mass stars.”
THE DISCOVERY AND ANALYSIS OF VERY METAL-POOR STARS IN THE GALAXY (2005, methodology): “A recent prediction of s-process nucleosynthesis models in AGB stars (Gallino et al. 1998; Goriely & Mowlavi 2000; Busso et al. 2001) is that one might expect A nn u.”
Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars (2003, background): “This spread is observed spectroscopically (e.g., Busso et al. 2001; Abia et al. 2001) and may be ascribed to different initial masses, rotationally induced shears, or other types of hydrodynamic instabilities controlling the formation of the 13C pocket.”
s-Process Nucleosynthesis in Carbon Stars (2002, methodology): “This is in agreement with previous findings on the average s-process efficiency by Busso et al. (2001a), derived by other classes of AGB stars and their descendants at various metallicities.”
EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF LOW-MASS ASYMPTOTIC GIANT BRANCH STARS AT DIFFERENT METALLICITIES (2009, methodology): “In the post-process calculations performed by Gallino et al. 1998 (see also Busso et al. 2001; Pignatari et al. 2004; Husti et al. 2008; Bisterzo et al. 2011), the amount of 13C within the pocket (and accordingly the amount of 14N) has been varied parametrically in order to match the observed…”
GALACTIC CHEMICAL EVOLUTION AND SOLAR s-PROCESS ABUNDANCES: DEPENDENCE ON THE 13C-POCKET STRUCTURE (2014, background): “A range of 13C-pocket strengths is needed in order to explain this spread (see, e.g., Busso et al. 2001; Sneden et al. 2008; Käppeler et al. 2011; Bisterzo et al. 2011; Lugaro et al. 2012).”
s‐Process in low‐metallicity stars – I. Theoretical predictions (2010, background): “Spectroscopic observations of disc metallicity stars were done in the past for MS, S, C(N) stars, in order to search for intrinsic AGBs by detecting Tc lines (Smith & Lambert 1988, 1990; Brown et al. 1990; Busso et al. 2001; Abia et al. 2002).”