Neutron producing reactions in stars

Denker, A.; Drotleff, H. W.; Große, M.; Knee, H.; Kunz, R.; Mayer, A.; Seidel, Ralf; Soiné, M.; Wöhr, A.; Wolf, G.; Hammer, J. W.

doi:10.1063/1.47408

Cited by 13 publications

(39 citation statements)

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“…Hence, any 13 C left behind by partial-mixing mechanisms will burn in radiative conditions in the tiny zone (∼1/20 of the intershell) where it was produced, that is, the 13 C pocket; and only the reaction products will be ingested into the convective pulse. This avoids the risk of structural changes from the energy release by the O reaction has recently been revised on experimental grounds by Denker et al (1995), whose results were used by Gallino et al (1998). In the temperature range of interest, this rate is up to a factor of 2 higher than estimated by Caughlan & Fowler (1988).…”

Section: Radiative 13 C Burning and The New Slow-neutron-capture Processmentioning

confidence: 91%

“…However, the enhanced by a factor of 5.5 owing to the higher T b in the pulses. The difference in the 60 Fe yields compared with the earlier model is principally due to the somewhat higher temperatures obtained for the convective pulse region in the new model and to the increased rate for 22 Ne(α,n) 25 Mg reaction (Denker et al 1995).…”

mentioning

confidence: 86%

“…The neutron density reaches a high value of a few times 10 10 n/cm 3 . In Gallino et al (1998), the rates of the 22 Ne(α,n) 25 Mg reaction and of the competing channel 22 Ne(α,γ ) 26 Mg were taken from recent experimental measurements (Käppeler et al 1994, Denker et al 1995, after excluding a possible resonance at 633 keV. In the temperature range of interest, these rates are up to a factor of 3 higher than the Caughlan & Fowler (1988) estimates.…”

Section: Radiative 13 C Burning and The New Slow-neutron-capture Processmentioning

confidence: 99%

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

Busso

Gallino

Wasserburg

1999

Annu. Rev. Astron. Astrophys.

1,157

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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Section: Radiative 13 C Burning and The New Slow-neutron-capture Processmentioning

confidence: 91%

mentioning

confidence: 86%

Section: Radiative 13 C Burning and The New Slow-neutron-capture Processmentioning

confidence: 99%

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

Busso

Gallino

Wasserburg

1999

Annu. Rev. Astron. Astrophys.

1,157

1,182

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“…It possible to have cases where the neutron density is low, but the timescale is long and the neutron exposure is large (case a in the figure) and cases where the neutron density is high but the timescale is short, and the neutron exposure is small (case b in the figure). In fact, these two different cases corresponds for example to the activation of the two neutron sources in AGB the 13 C(α,n) 16 O and the 22 Ne(α,n) 25 Mg reactions discussed below.…”

Section: The Three S-process Golden Rulesmentioning

confidence: 99%

“…The rate of the 13 C(α,n) 16 O reaction in the narrow range of temperatures typical of the radiative 13 C pocket from different literature sources: Angulo et al (NACRE) [21]; La Cognata et al [22]; Heil et al [23]; Guo et al [24]; Denker et al [25]; Pellegriti et al [26]; Johnson et al [27]; Caughlan & Fowler [28]; and Kubono et al [29].…”

Section: Figurementioning

confidence: 99%

Neutron sources and neutron-capture paths in asymptotic giant branch stars

Lugaro

2016

J. Phys.: Conf. Ser.

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Abstract.Roughly half of the abundances of the elements heavier than iron in the cosmos are produced by slow neutron captures (the s process) in hydrostatic conditions when the neutron density is below roughly 10 13 n/cm −3 . While it is observationally well confirmed that asymptotic giant branch (AGB) stars are the main site of the s process, we are still facing many problems in the theoretical models and nuclear inputs. Major current issues are the effect of stellar rotation and magnetic fields and the determination of the rate of the neutron source reactions. I will present these problems and discuss the observational constraints that can help us to solve them, including spectroscopically derived abundances, meteoritic stardust, and stellar seismology. Further, I will present evidence that the s process is not the only neutron-capture process to occur in AGB stars: an intermediate process is also required to explain recent observations of post-AGB stars.

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