The s-process in massive stars: the Shell C-burning contribution

Pignatari, M.; Gallino, R.; Baldovin, C.; Michael, Wiescher; Herwig, Falk; Heger, Alexander; Heil, M.; Käppeler, F.

doi:10.22323/1.028.0061

Cited by 3 publications

(8 citation statements)

References 15 publications

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“…In this section, we study the impact of the present Mg MACS on full stellar model calculations for a 25 M star, with solar metallicity, which were performed with an updated postprocessing code described in Ref. [51].…”

Section: A Impact On S-process Abundancesmentioning

confidence: 99%

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

Massimi¹,

Koehler²,

Bisterzo³

et al. 2012

Phys. Rev. C

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We have measured the neutron capture cross sections of the stable magnesium isotopes 24,25,26 Mg in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in 25 Mg and 26 Mg were recorded using the total energy method based on C 6 2 H 6 detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for 24 Mg, 25 Mg, and 26 Mg are 3.8±0.2 mb, 4.1±0.6 mb, and 0.14±0.01 mb, respectively.

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Section: A Impact On S-process Abundancesmentioning

confidence: 99%

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

Massimi¹,

Koehler²,

Bisterzo³

et al. 2012

Phys. Rev. C

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“…[5]), takes place during the presupernova evolution of massive stars with M 8M . Neutrons are mainly provided by the 22 Ne(α, n) 25 Mg reaction during convective core He burning and during convective shell C burning. One has to consider that also 13 C(α, n) 16 O and 17 O(α, n) 20 Ne reactions contribute to a minor extent, recycling neutrons previously captured by the abundant 12 C and 16 O.…”

Section: A Stellar Modelsmentioning

confidence: 99%

“…Later on, when 4 He has been depleted to about 10% in mass fraction and the central temperature has increased up to T 8 ≈ 2.5, 22 Ne is produced by further α captures on 18 O. The 22 Ne(α, n) 25 Mg reaction becomes an efficient neutron source only in the last phases of core He burning, close to He exhaustion, when central temperatures of T 8 = 3-3.5 are reached [21].…”

Section: A Stellar Modelsmentioning

confidence: 99%

“…First, the 13 C(α, n) 16 O reaction occurs under radiative conditions during the intervals between convective He-shell burning episodes. While the 13 C reaction provides most of the neutron exposure at low temperatures of 1 × 10 8 K (kT ∼ 8 keV) and relatively low neutron densities (n n 10 7 cm −3 ), the resulting abundances are modified by a second burst of neutrons from the 22 Ne(α, n) 25 Mg reaction, which is marginally activated during the next convective instability, when a short neutron burst with peak densities of n n 10 10 cm −3 is released at temperatures of 2.7 × 10 8 K (kT ∼ 23 keV). Although this second neutron burst accounts only for a few percent of the total exposure, it is essential for adjusting the final abundance patterns of the s-process branchings.…”

Section: Introductionmentioning

confidence: 99%

“…Another non-negligible contribution to the final s yields may come from a partial tiny convective He burning shell at the outer border of the C shell, whose existence depends on the adopted stellar model [2,3]. Neutrons are mainly produced by the 22 Ne(α, n) 25 Mg reaction in both cases, but at rather different temperatures and neutron densities. During core He burning, neutrons are produced near core He exhaustion at temperatures of T = (2.5-3.5) × 10 8 K for about 10 4 years with neutron densities < ∼ 10 6 cm −3 , whereas the higher temperatures of T = (1.0-1.4) × 10 9 K during the subsequent carbon shell burning phase give rise to peak values of about 10 12 cm −3 [4][5][6].…”

Section: Introductionmentioning

confidence: 99%

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Neutron capture cross sections for the weaksprocess in massive stars

Heil¹,

Käppeler²,

Uberseder³

et al. 2008

Phys. Rev. C

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Neutron capture nucleosynthesis in massive stars plays an important role in galactic chemical evolution as well as for the analysis of abundance patterns in very old metal-poor halo stars. The so-called weak s-process component, which is responsible for most of the s abundances between Fe and Sr, turned out to be very sensitive to the stellar neutron capture cross sections in this mass region and, in particular, of isotopes near the seed distribution around Fe. Activation measurements in a quasistellar neutron spectrum corresponding to a thermal energy of kT = 25 keV have been carried out on 58 Fe, 59 Co, 64 Ni, 63 Cu, and 65 Cu. By a series of repeated irradiations with different experimental conditions, uncertainties between 3.0% and 4.6% could be achieved, factors of 2 to 3 more accurate than previous data. Compared to previous measurements, severe discrepancies were found for 63,65 Cu. The consequences of these results have been studied by detailed model calculations for convective core He burning and convective shell C burning in massive stars.

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The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars

Pignatari,

Gallino,

Meynet

et al. 2008

Preprint

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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

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The s-process in massive stars: the Shell C-burning contribution

Cited by 3 publications

References 15 publications

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

Neutron capture cross sections for the weaksprocess in massive stars

The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars

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