s-process nucleosynthesis of 142^Nd: crisis of the classical model (original) (raw)
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THE WEAK s -PROCESS IN MASSIVE STARS AND ITS DEPENDENCE ON THE NEUTRON CAPTURE CROSS SECTIONS
The Astrophysical Journal, 2010
The slow neutron capture process in massive stars (weak s process) produces most of the s-process isotopes between iron and strontium. Neutrons are provided by the 22 Ne(α,n) 25 Mg reaction, which is activated at the end of the convective He-burning core and in the subsequent convective C-burning shell. The s-process-rich material in the supernova ejecta carries the signature of these two phases. In the past years, new measurements of neutron capture cross sections of isotopes beyond iron significantly changed the predicted weak s-process distribution. The reason is that the variation of the Maxwellian-averaged cross sections (MACS) is propagated to heavier isotopes along the s path. In the light of these results, we present updated nucleosynthesis calculations for a 25 M star of Population I (solar metallicity) in convective He-burning core and convective C-burning shell conditions. In comparison with previous simulations based on the Bao et al. compilation, the new measurement of neutron capture cross sections leads to an increase of s-process yields from nickel up to selenium. The variation of the cross section of one isotope along the s-process path is propagated to heavier isotopes, where the propagation efficiency is higher for low cross sections. New 74 Ge, 75 As, and 78 Se MACS result in a higher production of germanium, arsenic, and selenium, thereby reducing the s-process yields of heavier elements by propagation. Results are reported for the He core and for the C shell. In shell C-burning, the s-process nucleosynthesis is more uncertain than in the He core, due to higher MACS uncertainties at higher temperatures. We also analyze the impact of using the new lower solar abundances for CNO isotopes on the s-process predictions, where CNO is the source of 22 Ne, and we show that beyond Zn this is affecting the s-process yields more than nuclear or stellar model uncertainties considered in this paper. In particular, using the new updated initial composition, we obtain a high s-process production (overproduction higher than 16 O, ∼100) for Cu, Ga, Ge, and As. Using the older abundances by Anders & Grevesse, also Se, Br, Kr, and Rb are efficiently produced. Our results have important implications in explaining the origin of copper in the solar abundance distribution, pointing to a prevailing contribution from the weak s-process in agreement with spectroscopic observations and Galactic chemical evolution calculations. Because of the improvement due to the new MACS for nickel and copper isotopes, the nucleosynthesis of copper is less affected by nuclear uncertainties compared to heavier s-process elements. An experimental determination of the 63 Ni MACS is required for a further improvement of the abundance prediction of copper. The available spectroscopic observations of germanium and gallium in stars are also discussed, where most of the cosmic abundances of these elements derives from the s-process in massive stars.
Neutron capture nucleosynthesis in AGB stars
Astrophysical implications of the laboratory study of presolar materials, 1997
Recent AGB models including diffusive overshoot or rotational effects suggest the partial mixing (PM) of protons from the H-rich envelope into the C-rich layers during the third dredge-up. In order to study the impact of such a mixing on the surface abundances, nucleosynthesis calculations based on stellar AGB models are performed for different assumptions of protons (ranging from X mix p = 10 −6 to 0.7) in the PM zone. For high proton-to-12 C abundance ratios, light nuclei such as fluorine and sodium are efficiently produced, while heavier sprocess nuclei are synthesized for lower proton-to-12 C ratios. In the framework of the PM model, assuming a smooth exponentially decreasing proton profile, the surface 19 F abundance evolution is correlated with that of s-process nuclei in agreement with observations. However, as a function of the surface C/O abundance ratio, the surface 19 F enrichment remains difficult to reconcile with observations in AGB stars. Sodium is predicted to be efficiently produced in a small region of the PM zone with proton-to-12 C abundance ratio of about 10, but with large overproduction factors (up to fifty times higher than the sodium left over by the hydrogen burning shell). The primary 13 C pocket formed in the PM zone at low proton-to-12 C ratios is responsible for an efficient production of s-process nuclei. A table of elemental overabundances predicted at the surface of AGB stars at four different metallicities is presented. All the nucleosynthesis calculations are shown to suffer from major nuclear reaction rate uncertainties, in particular, 13 C (p , γ) 14 N , 14 N (n , p) 14 C and 22 Ne (α , n) 25 Mg. The major uncertainties associated with the amount of protons mixed into the C-rich zone are found in the extent of the PM zone rather than in the adopted H profile. Finally, the PM scenario predicts that low-metallicity AGB stars enriched in s-process elements should exhibit a large overproduction of Pb and Bi compared to other s-isotopes. The search of such Pb-stars is highly encouraged.
s-Process nucleosynthesis in AGB stars: A Test for stellar evolution
We study the slow neutron capture process (s process) in Asymptotic Giant Branch (AGB) stars using three different stellar evolutionary models computed for a 3 M ⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13 C(α,n) 16 O reaction that occurs in radiative conditions. A tiny region rich in 13 C (the 13 C pocket) is created by proton captures on the abundant 12 C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local protons over 12 C number ratio, p/ 12 C ∼ > 0.3, most of the 13 C nuclei produced are further converted by proton capture to 14 N. Besides, 14 N nuclei represent a major neutron poison. We find
Neutron Capture in Low-Mass Asymptotic Giant Branch Stars: Cross Sections and Abundance Signatures
Astrophysical Journal, 1999
The recently improved information on the stellar (n,gamma) cross sections of neutron-magic nuclei at N = 82, and in particular of 142Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. Particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that - in certain cases - the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modelling. Thus, the s process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions for describing the low neutron density provided by the 13C source.
A New Study ofs‐Process Nucleosynthesis in Massive Stars
The Astrophysical Journal, 2000
We present a comprehensive study of s-process nucleosynthesis in 15, 20, 25, and 30 M ⊙ stellar models having solar-like initial composition. The stars are evolved up to ignition of central neon with a 659 species network coupled to the stellar models. In this way, the initial composition from one burning phase to another is consistently determined, especially with respect to neutron capture reactions. The aim of our calculations is to gain a full account of the s-process yield from massive stars. In the present work, we focus primarily on the s-process during central helium burning and illuminate some major uncertainties affecting the calculations. We briefly show how advanced burning can significantly affect the products of the core helium burning s-process and, in particular, can greatly deplete 80 Kr that was strongly overproduced in the earlier core helium burning phase; however, we leave a complete analysis of the s-process during the advanced evolutionary phases (especially in shell carbon burning) to a subsequent paper. Our results can help to constrain the yield of the s-process material from massive stars during their pre-supernova evolution.
2001
We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,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 s-enriched material is subsequently mixed with the envelope by the III 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, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency. The theoretical predictions are compared with published abundances of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB super...
The Astrophysical …, 2008
The goal of this paper is to analyze the impact of a primary neutron source on the s−process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary 14 N. Part of the nitrogen produced in the H−burning shell diffuses by rotational mixing into the He core where it is converted to 22 Ne providing additional neutrons for the s process. We present nucleosynthesis calculations for a 25 M ⊙ star at [Fe/H] = −3, −4, where in the convective core He−burning about 0.8 % in mass is made of primary 22 Ne. The usual weak s−process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s−process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y and Zr is reached. On the other hand, the s−process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of 22 Ne and by nuclear uncertainties, first of all by the 22 Ne(α,n) 25 Mg reaction. Finally, assuming that 22 Ne is primary in the considered metallicity range, the s−process efficiency
Neutron Capture Elements ins‐Process–Rich, Very Metal‐Poor Stars
The Astrophysical Journal, 2001
We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7mb −1 , significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed