Classical S-process Research Articles

Solar system Nd isotope heterogeneity: Insights into nucleosynthetic components and protoplanetary disk evolution

High-precision Nd isotope measurements of a diverse set of solar system materials including bulk chondrites and achondrites reveal that their Nd isotope composition is governed by several distinct nucleosynthetic components. The full spectrum of non-radiogenic, mass-independent Nd isotope compositions of solar system materials is best explained by heterogeneous distribution of at least three nucleosynthetic components - the classical s-process component, pure p-process component and an anomalous, previously unidentified s-/r-process component. The 142Nd/144Nd variations in solar system reservoirs specifically fall into three distinct trends - those that result from variations in the s-process component, those resulting from variations in the pure p-process component, and those resulting from coupled s-process and p-process variations. The μ148Nd value, a proxy for s-process variations, as well as μ142Nd that has been corrected for s-process heterogeneity to reflect p-process variations, broadly show an inverse correlation with ε54Cr. The linearity in μ148Nd - ε54Cr space for inner solar system bodies, CI chondrite and Allende-type CAIs possibly suggests the thermally labile nature of some s-process carrier grains unlike the mainstream refractory s-process SiC grains. The p-process carrier for Nd is inferred to be a refractory phase enriched in inner solar system materials through thermal processing. The bulk meteorite regression lines that specifically correspond to s- and p-process heterogeneity, largely define μ142Nd intercepts indistinguishable from terrestrial composition within analytical uncertainty, ruling out resolvable radiogenic μ142Nd excess on Earth that cannot otherwise be accounted for by nucleosynthetic heterogeneity.

Ba isotopic compositions in stardust SiC grains from the Murchison meteorite: Insights into the stellar origins of large SiC grains

We report barium isotopic measurements in 12 large (7–58μm) stardust silicon carbide grains recovered from the Murchison carbonaceous chondrite. The C-, N-, and Si-isotopic compositions indicate that all 12 grains belong to the mainstream population and, as such, are interpreted to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. Barium isotopic analyses were carried out on the Sensitive High Resolution Ion Microprobe – Reverse Geometry (SHRIMP-RG) with combined high mass resolution and energy filtering to eliminate isobaric interferences from molecular ions. Contrary to previous measurements in small (<5μm) mainstream grains, the analyzed large SiC grains do not show the classical s-process enrichment, having near solar Ba isotopic compositions. While contamination with solar material is a common explanation for the lack of large isotopic anomalies in stardust SiC grains, particularly for these large grains which have low trace element abundances, our results are consistent with previous observations that Ba isotopic ratios are dependent on grain size. We have compared the SiC data with theoretical predictions of the evolution of Ba isotopic ratios in the envelopes of low-mass AGB stars with a range of stellar masses and metallicities. The Ba isotopic measurements obtained for large SiC grains from the LS+LU fractions are consistent with grain condensation in the envelope of very low-mass AGB stars (1.25M⊙) with close-to-solar metallicity, which suggests that conditions for growth of large SiC might be more favorable in very low-mass AGB stars during the early C-rich stages of AGB evolution or in stable structures around AGB stars whose evolution was cut short due to binary interaction, before the AGB envelope had already been largely enriched with the products of s-process nucleosynthesis.

Galactic Evolution of Sr, Y, and Zr: A Multiplicity of Nucleosynthetic Processes

We follow the Galactic enrichment of three easily observed light n-capture elements Sr,Y,and Zr.Input stellar yields have been first separated into their respective main and weak s-process,and r-process components.The s-process yields from AGB stars are computed,exploring a wide range of efficiencies of the major neutron source,13C,and covering both disk and halo metallicities.AGBs have been shown to reproduce the main s-component in the solar system.The concurrent weak s-process,which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22Ne n-source,is discussed in detail.Neither the main s-,nor the weak s-components are shown to contribute significantly to the n-capture element abundances observed in unevolved halo stars.We present a detailed analysis of a large database of spectroscopic observations of Sr,Y,Zr, Ba,and Eu for Galactic stars at various metallicities.Spectroscopic observations of Sr,Y,and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of n-capture processes forming Sr,Y and Zr.The observed [Sr,Y,Zr/Ba,Eu] ratio is clearly not flat at low metallicities,as we would expect if Ba,Eu and Sr,Y,Zr all had the same r-process origin.We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr.We find hints for a primary process in low-metallicity massive stars, different from the 'classical s-process' and from the 'classical r-process',that we tentatively define LEPP (Lighter Element Primary Process).This allows us to revise the estimates of the r-process contributions to the solar Sr,Y and Zr abundances,as well as of the contribution to the s-only isotopes 86Sr,87Sr,96Mo.

The 180Ta production puzzle and the classical s-process

We reproduced the observed solar abundance of 180Ta m and 180W in the classical s-process taking into account neutron captures feeding 180Ta and the intermediate state below 1.1 MeV that enables photon coupling between the isomer and the ground state. Intermediate states in 180Ta were analyzed in the framework of the quasiparticle- plus-phonon model and the particle-rotor model.

Thes‐Process Branching at185W

The neutron capture cross section of the unstable nucleus W-185 has been derived from experimental photoactivation data of the inverse reaction W-186(gamma, n) W-185. The new result of sigma = 687 +/- 110 mbarn confirms the theoretically predicted neutron capture cross section of W-185 of sigma approximate to 700 mbarn at kT = 30 keV. A neutron density in the classical s-process of n(n) = (3.8(-0.9)(+1.1)) x 10(8) cm(-3) is derived from the new data for the W-185 branching. In a stellar s-process model, one finds a significant overproduction of the residual s-only nucleus Os-186.

On the Puzzling Origin of the Rare In and Sn Isotopes

The neutron-capture cross sections of the stable isotopes 106Cd,108Cd,114Cd, and 116Cd have been measured relative to gold by means of the activation method, using the quasi-stellar neutron spectrum for kT = 25 keV that can be produced via the 7Li(p, n)7Be reaction near threshold. These data are given with uncertainties between 3% and 6%, five times smaller than quoted for most of the few previous experiments. The present results complement recent measurements on isotopes of tin and a discussion of the stellar β-decay rates in the A = 113/115 region, and allow an update of the nucleosynthetic origin of the Cd/In/Sn isotopes, which is complicated by the s-process branchings at 113Cd,115Cd, and 115In as well as by the many isomers in this mass region. The s-process yields obtained with the classical s-process and with an AGB model exhibit a significant difference with respect to the cadmium and tin abundances, with interesting consequences for the r-process residuals in this mass region. Nevertheless, the origin of the rare, odd isotopes 113In and 115Sn remains unclear. The cross sections for the two light Cd isotopes, 106 and 108, are of interest for testing Hauser-Feshbach extrapolations to the p-process region.

New neutron capture and transmission measurements for 134,136Ba at ORELA and their impact on s-process nucleosynthesis calculations

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 – 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT ≈ 6 – 12 keV) favored by the most recent stellar models of the s-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature, kT = 30 keV, but show significant differences at lower temperatures. We discuss the astrophysical implications of these differences.

Resonance neutron capture and transmission measurements and the stellar neutron capture cross sections of 134Ba and 136Ba.

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of $^{134}\mathrm{Ba}$ and $^{136}\mathrm{Ba}$ at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3--5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT\ensuremath{\approxeq}8-12 keV) favored by the most recent stellar models of the s process. By fitting the data with a multilevel R-matrix code, we determined parameters for 86 resonances in $^{134}\mathrm{Ba}$ below 11 keV and 92 resonances in $^{136}\mathrm{Ba}$ below 35 keV. Astrophysical reaction rates were calculated using these parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature kT=30 keV, but show significant differences at lower temperatures. We determined that these differences were due to the effect of resonances below the energy range of previous experiments and to the use of incorrect neutron widths in a previous resonance analysis. Our data show that the ratio of reaction rates for these two isotopes depends more strongly on temperature than previous measurements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at $^{134}\mathrm{Cs}$ is too low to be consistent with the temperature derived from other branching points. This inconsistency is evidence for the need for more sophisticated models of the s process beyond the classical model. We used a reaction network code to explore the changes in the calculated isotopic abundances resulting from our new reaction rates for an s-process scenario based on a stellar model. These calculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates. \textcopyright{} 1996 The American Physical Society.

S-Process studies with improved cross sections

New experimental techniques for the determination of neutron capture cross sections, e.g. the Karlsruhe 4 pi Barium Fluoride Detector, have reduced the respective uncertainties by a factor five compared to previous methods. With these improved data, detailed analyses of branchings in the s-process path allow the authors to derive constraints on the physical parameters during the s-process. As first examples, the neutron capture cross sections of the s-only isotopes of tellurium and samarium have been determined. The authors discuss the results in the framework of the classical s-process approach and with a stellar model describing the s-process during helium shell burning in low mass stars. The results for tellurium confirm the prediction of the classical approach of a 'local approximation' within the experimental uncertainty of 1%.

A difficulty with Ne-22 as the neutron source for the solar system s-process

view Abstract Citations (21) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A difficulty with Ne-22 as the neutron source for the solar system s-process Despain, K. H. Abstract A consideration of the Ne-22 neutron source as it operates in shell flashes suggests that it cannot produce the bulk of the solar system s-process elements. Shell flashes at low core masses do not liberate enough neutrons per flash to build up solar abundances, while flashes at high core masses liberate the neutrons too rapidly. Classical s-process branches appear to be saturated by neutron capture and many r-process elements are produced. The resulting isotope ratios would be distinctly nonsolar. Publication: The Astrophysical Journal Pub Date: March 1980 DOI: 10.1086/183219 Bibcode: 1980ApJ...236L.165D Keywords: Branching (Physics); Neon Isotopes; Neutron Sources; Nuclear Reactions; Solar System; Stellar Evolution; Abundance; Astrophysics; Krypton 85; Solar Physics; Astrophysics full text sources ADS |

S-process studies - The exact solution

The exact solution to the unbranched s-process network is computed for the range of neutron exposures of astrophysical interest, and the exact solution for the case of an arbitrary degree of degeneracy of the cross-section values is obtained. The validity of the Clayton, Fowler, Hull, and Zimmerman approximate solution is determined by direct comparison with the exact solution. Correction factors for the approximate solution are presented. It is well known that the approximate solution becomes invalid for very small values of the neutron exposure tau. An alternate form of the exact solution developed for samll tau becomes easy to evaluate in that regime. The exact solution of the classical s-process problem can now be evaluated with modern electronic computers to any precision required for arbitrary values of the neutron exposure.

Production of C-14 and neutrons in red giants

We have examined the effects of mixing various amounts of hydrogen-rich material into the intershell convective region of red giants undergoing helium shell flashes. We find that significant amounts of C-14 can be produced via the N-14(n, p)C-14 reaction. If substantial portions of this intershell region are mixed out into the envelopes of red giants, then C-14 may be detectable in evolved stars. We find a neutron flux many orders of magnitude above the flux required for the classical s-process, and thus an intermediate neutron process (i-process) may operate in evolved red giants. In all cases studied we find substantial enhancements of O-17. These mixing models offer a plausible explanation of the observations of enhanced O-17 in the carbon star IRC 10216. For certain physical conditions we find significant enhancements of N-15 in the intershell region.