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The s process in massive stars, a benchmark for neutron capture reaction rates

Marco Pignatari, Roberto Gallino, Rene Reifarth

TL;DR

The paper addresses the problem of precisely quantifying the weak s-process contribution to solar abundances between Fe and Sr–Zr, where nuclear uncertainties in neutron-capture rates are significant. It adopts a nuclear sensitivity approach by varying 86 $(n,\gamma)$ rates within a 25 $M_\odot$ solar-metallicity massive-star trajectory using the NuGrid $PPN$ post-processing framework, and compares results with updates from KADoNIS and the ASTRAL v0.2 library. The key contributions include identifying the 20 most impactful $(n,\gamma)$ rates, highlighting the dominant role of the seed $^{56}$Fe and the increased importance of light neutron poisons in the C-shell, as well as showcasing localized effects at branching points like $^{85}$Kr and bottlenecks such as $^{68}$Zn; the study also demonstrates how ASTRAL v0.2 can shift abundances by up to a factor of $\sim$1.8 for certain species and generally 10–30% for many others. The findings have significant implications for robust weak s-process predictions in Galactic chemical evolution and guide future experimental efforts to reduce key neutron-capture rate uncertainties, especially near branching points.

Abstract

A clear definition of the contribution from the slow neutron-capture process (s process) to the solar abundances between Fe and the Sr-Zr region is a crucial challenge for nuclear astrophysics. Robust s-process predictions are necessary to disentangle the contribution from other stellar processes producing elements in the same mass region. Nuclear uncertainties are affecting s-process calculations, but most of the needed nuclear input are accessible to present nuclear experiments or they will be in the near future. Neutron-capture rates have a great impact on the s process in massive stars, which is a fundamental source for the solar abundances of the lighter s-process elements heavier than Fe (weak s-process component). In this work we present a new nuclear sensitivity study to explore the impact on the s process in massive stars of 86 neutron-capture rates, including all the reactions between C and Si and between Fe and Zr. We derive the impact of the rates at the end of the He-burning core and at the end of the C-burning shell, where the $^{22}$Ne($α$,n)$^{25}$Mg reaction is is the main neutron source. We confirm the relevance of the light isotopes capturing neutrons in competition with the Fe seeds as a crucial feature of the s process in massive stars. For heavy isotopes we study the propagation of the neutron-capture uncertainties, finding a clear difference of the impact of Fe and Co isotope rates with respect to the rates of heavier stable isotopes. The local uncertainty propagation due to the neutron-capture rates at the s-process branching points is also considered, discussing the example of $^{85}$Kr. The complete results of our study for all the 86 neutron-capture rates are available online. Finally, we present the impact on the weak s process of the neutron-capture rates included in the new ASTRAL library (v0.2).

The s process in massive stars, a benchmark for neutron capture reaction rates

TL;DR

The paper addresses the problem of precisely quantifying the weak s-process contribution to solar abundances between Fe and Sr–Zr, where nuclear uncertainties in neutron-capture rates are significant. It adopts a nuclear sensitivity approach by varying 86 rates within a 25 solar-metallicity massive-star trajectory using the NuGrid post-processing framework, and compares results with updates from KADoNIS and the ASTRAL v0.2 library. The key contributions include identifying the 20 most impactful rates, highlighting the dominant role of the seed Fe and the increased importance of light neutron poisons in the C-shell, as well as showcasing localized effects at branching points like Kr and bottlenecks such as Zn; the study also demonstrates how ASTRAL v0.2 can shift abundances by up to a factor of 1.8 for certain species and generally 10–30% for many others. The findings have significant implications for robust weak s-process predictions in Galactic chemical evolution and guide future experimental efforts to reduce key neutron-capture rate uncertainties, especially near branching points.

Abstract

A clear definition of the contribution from the slow neutron-capture process (s process) to the solar abundances between Fe and the Sr-Zr region is a crucial challenge for nuclear astrophysics. Robust s-process predictions are necessary to disentangle the contribution from other stellar processes producing elements in the same mass region. Nuclear uncertainties are affecting s-process calculations, but most of the needed nuclear input are accessible to present nuclear experiments or they will be in the near future. Neutron-capture rates have a great impact on the s process in massive stars, which is a fundamental source for the solar abundances of the lighter s-process elements heavier than Fe (weak s-process component). In this work we present a new nuclear sensitivity study to explore the impact on the s process in massive stars of 86 neutron-capture rates, including all the reactions between C and Si and between Fe and Zr. We derive the impact of the rates at the end of the He-burning core and at the end of the C-burning shell, where the Ne(,n)Mg reaction is is the main neutron source. We confirm the relevance of the light isotopes capturing neutrons in competition with the Fe seeds as a crucial feature of the s process in massive stars. For heavy isotopes we study the propagation of the neutron-capture uncertainties, finding a clear difference of the impact of Fe and Co isotope rates with respect to the rates of heavier stable isotopes. The local uncertainty propagation due to the neutron-capture rates at the s-process branching points is also considered, discussing the example of Kr. The complete results of our study for all the 86 neutron-capture rates are available online. Finally, we present the impact on the weak s process of the neutron-capture rates included in the new ASTRAL library (v0.2).
Paper Structure (4 sections, 3 figures, 2 tables)

This paper contains 4 sections, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology and nucleosynthesis simulations
  3. Results of the Nuclear sensitivity study
  4. Conclusions

Figures (3)

  • Figure 1: The weak s-process abundances at the end of the C shell for the isotopes (upper panels) and the elements (lower panels) between Fe and Nb. The results obtained using the default $PPN$ model are compared with the results obtained using the recommended rates, upper limits and lower limits provided by the ASTRAL library. The production factors are provided on the left panels, while the corresponding abundance ratios (relative to the default model) are shown in the right panels.
  • Figure 2: The weak s-process abundances at the end of the He core (upper panels) and at the end of the C shell (lower panels). The results obtained using the default model are compared with the same model but multiplying and dividing the $^{25}$Mg(n,$\gamma$)$^{26}$Mg by a factor of 1.3. We report the elemental abundances (left panels) and the isotopic abundance ratios of the test cases normalized to the abundances obtained with the default model.
  • Figure 3: The isotopic ratios of the test cases normalized to the abundances obtained with the default model are reported at the end of the C shell for $^{58}$Fe(n,$\gamma$)$^{59}$Fe (upper panel), $^{68}$Zn(n,$\gamma$)$^{69}$Zn (central panel) and $^{85}$Kr(n,$\gamma$)$^{86}$Kr (lower panel).