S-process studies in the light of new experimental cross sections - Distribution of neutron fluences and r-process residuals

Kaeppeler, F.; Beer, H.; Wisshak, K.; Clayton, Donald D.; Macklin, R. L.; Ward, Richard A.

doi:10.1086/160033

Cited by 135 publications

(67 citation statements)

References 22 publications

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“…Case U2 for solar metallicity best reproduces the main component, which accounts for the solar system s-process abundance distribution for A ! 90 (Käppeler et al 1982). Given the relation % 13 C=Fe, the same best fit for the main component is obtained with the ST case and half-solar metallicity (Gallino et al 1998a;Arlandini et al 1999).…”

Section: Neutron Exposures In Agb Starsmentioning

confidence: 75%

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Lugaro

Davis

Gallino

et al. 2003

ApJ

219

235

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The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 M are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized 12 C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C ! O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (, n) reactions on 13 C and 22 Ne during interpulse phases and the thermal pulses, respectively. A 13 C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This 13 C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible 13 C abundances in the pocket. In low-mass AGB stars (1:5 M M 4 M ), a second small burst of neutrons is released by marginal 22 Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4 M < M 8 M ), the 22 Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from 4 He to 210 Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s-process main component in the solar system.

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Section: Neutron Exposures In Agb Starsmentioning

confidence: 75%

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Lugaro

Davis

Gallino

et al. 2003

ApJ

219

235

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“…It occurs when the neutron density is so small that most unstable isotopes have time to decay before capturing a neutron. Since the above cited work by Burbidge et al, an extensive body of work has been dedicated to the phenomenological treatment of the s process and of its signatures in the solar system abundance distribution (Clayton et al 1961, Seeger et al 1965, Clayton & Ward 1974, Käppeler et al 1982. For an account of this research see Käppeler et al (1989) and Meyer (1994).…”

Section: Introductionmentioning

confidence: 99%

“…Hence, the new results from updated stellar models described later (Section 5) find their correspondence in the acknowledgement that the classical analysis of s processes, after its many important contributions in the past, is now superseded. A more complete account of the phenomenological approach in various stages of its development, up to the most recent works with refined cross-section results, can be found in Käppeler et al (1982), Mathews & Ward (1985), Käppeler et al (1989), Wheeler et al (1989), Meyer (1994), and Käppeler (1997Käppeler ( , 1999. This approach will, in any case, continue to be a key guide to nuclear astrophysical processes and problems.…”

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confidence: 99%

Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation

Busso

Gallino

Wasserburg

1999

Annu. Rev. Astron. Astrophys.

1,151

1,182

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We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12 C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13 C(α,n) 16 O, which provides the bulk of the neutron flux at low neutron densities (N n ≤ 10 7 n/cm 3 ), and 22 Ne(α,n) 25 Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal sprocess occurring in all stars. Concerning the 13 C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12 C forming a 13 C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB st...

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“…The production of Rb occurs both via slow neutron capture (the s-process) and rapid neutron capture (the r-process). This is even more complicated by the fact that Rb production by the s-process involves both the weak s-process and the main s-process (Käppeler et al 1982). Moreover, this complexity derives from a variety of stellar environments.…”

Section: Introductionmentioning

confidence: 99%

“…At the same time, the s-process abundance of 85 Rb is sensitive to both the neutron density and temperature of the s-process through the beta decay branching at 85 Kr (Käppeler et al 1982(Käppeler et al , 1989Howard et al 1987;Mathews & Ward 1985). The isotope 85 Kr decays to 85 Rb with a half life of 10.77 yr, depending upon the temperature (Takahashi & Yokoi 1987).…”

Section: Introductionmentioning

confidence: 99%

The Interstellar Rubidium Isotope Ratio Toward Hd169454

Kawanomoto

Aoki

Kajino

et al. 2009

ApJ

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We report here on a new determination of the interstellar 85 Rb/ 87 Rb isotopic ratio for diffuse gas along the line of sight toward HD169454. We have detected no contribution of 87 Rb in the Rb I absorption line at 7800 Å in this cloud. We deduce a lower limit of 85 Rb/ 87 Rb> 2.4 (95% C.L.). This new value differs significantly from the only other reported local interstellar value of 1.21 ± 0.30(1σ ) along a different line of sight, and is marginally consistent with the solar value of 2.43. We discuss the implications of this new isotopic ratio in the context of the various relevant models for the nucleosynthesis of heavy elements. We suggest that material in this cloud may be depleted in the main s-process component.

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S-process studies in the light of new experimental cross sections - Distribution of neutron fluences and r-process residuals

Cited by 135 publications

References 22 publications

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation

The Interstellar Rubidium Isotope Ratio Toward Hd169454

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