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Unraveling the anomaly in the production of $^{60}$Fe nucleus in massive stars

Samapti Lakshan, Le Tan Phuc, Deepak Pandit, Srijit Bhattacharya, Le Thi Quynh Huong, Nguyen Dinh Dang, Balaram Dey, Nguyen Ngoc Anh, Nguyen Quang Hung

TL;DR

The study reevaluates the reported enhancement of $^{60}$Fe production in massive stars by analyzing $^{59}$Fe$(n, extgamma)^{60}$Fe using microscopic nuclear inputs. By employing the EP+IPM framework for nuclear level density and an extended EP+PDM for the extgamma-ray strength function, including a low-energy upbend from temperature-enabled $pp$ and $hh$ excitations, the work demonstrates that the previously claimed MACS enhancement largely stems from the spin-dependent NLD model rather than novel physics in the reaction mechanism. The microscopic approach reproduces the $^{60}$Fe NLD, gSF, and the $(n, extgamma)$ cross section concurrently and yields MACS values significantly lower than Spyrou2024, aligning with Yan2021 within uncertainties. These results emphasize the critical role of fully microscopic nuclear structure inputs in astrophysical rate predictions and challenge the idea of an enhanced $^{60}$Fe production in massive stars.

Abstract

The production of $^{60}$Fe is crucial for nucleosynthesis in massive stars and supernovae. In this work, by using the microscopic EP+IPM (exact pairing plus the independent-particle model) for the nuclear level density (NLD) and extended EP+PDM (exact pairing plus phonon damping model) for the $γ$-ray strength function (gSF), we re-evaluate the substantial enhancement of $^{60}$Fe production recently reported in {\it A. Spyrou et al., Nat. Comm. {\bf 15}, 9608 (2024)}, which was attributed to an unexpectedly large Maxwellian-averaged cross section (MACS). Our analysis demonstrates that this enhancement indeed originates from the choice of NLD, which, despite being constrained to reproduce the total NLD and gSF data, lacks a reliable spin dependence, a critical input for Hauser-Feshbach calculations of nuclear reaction rate. In contrast, our predictions yield a significantly lower MACS, calling the claimed enhancement into question. In particular, our approach highlights the microscopic nature of the low-energy enhancement of the gSF, the so-called upbend resonance, which arises from strong particle-particle ($pp$) and hole-hole ($hh$) excitations that emerge only at finite temperature, thereby further reinsisting on the invalidity of the Brink-Axel hypothesis in this low-energy region. Overall, our study reopens the question on the long-standing problem of $^{60}$Fe production in massive stars.

Unraveling the anomaly in the production of $^{60}$Fe nucleus in massive stars

TL;DR

The study reevaluates the reported enhancement of Fe production in massive stars by analyzing FeFe using microscopic nuclear inputs. By employing the EP+IPM framework for nuclear level density and an extended EP+PDM for the extgamma-ray strength function, including a low-energy upbend from temperature-enabled and excitations, the work demonstrates that the previously claimed MACS enhancement largely stems from the spin-dependent NLD model rather than novel physics in the reaction mechanism. The microscopic approach reproduces the Fe NLD, gSF, and the cross section concurrently and yields MACS values significantly lower than Spyrou2024, aligning with Yan2021 within uncertainties. These results emphasize the critical role of fully microscopic nuclear structure inputs in astrophysical rate predictions and challenge the idea of an enhanced Fe production in massive stars.

Abstract

The production of Fe is crucial for nucleosynthesis in massive stars and supernovae. In this work, by using the microscopic EP+IPM (exact pairing plus the independent-particle model) for the nuclear level density (NLD) and extended EP+PDM (exact pairing plus phonon damping model) for the -ray strength function (gSF), we re-evaluate the substantial enhancement of Fe production recently reported in {\it A. Spyrou et al., Nat. Comm. {\bf 15}, 9608 (2024)}, which was attributed to an unexpectedly large Maxwellian-averaged cross section (MACS). Our analysis demonstrates that this enhancement indeed originates from the choice of NLD, which, despite being constrained to reproduce the total NLD and gSF data, lacks a reliable spin dependence, a critical input for Hauser-Feshbach calculations of nuclear reaction rate. In contrast, our predictions yield a significantly lower MACS, calling the claimed enhancement into question. In particular, our approach highlights the microscopic nature of the low-energy enhancement of the gSF, the so-called upbend resonance, which arises from strong particle-particle () and hole-hole () excitations that emerge only at finite temperature, thereby further reinsisting on the invalidity of the Brink-Axel hypothesis in this low-energy region. Overall, our study reopens the question on the long-standing problem of Fe production in massive stars.

Paper Structure

This paper contains 3 sections, 5 figures, 1 table.

Table of Contents

  1. Results
  2. Discussion
  3. Method

Figures (5)

  • Figure 1: Total (a) and $J=0-3 ~\hbar$ [(b)$-$(e)] NLDs obtained within the EP+IPM in comparison with the experimental data extracted using the $\beta-$Oslo method Spyrou2024 as well as those predicted by two microscopic models, HFBC with readjusted parameters (RIPL-3 corrected) HFBC and HFBCS (Demetriou 2001) HFBCS, for $^{60}$Fe nucleus.
  • Figure 2: a) Total and partial ($E1+M1$ and UBR) gSFs obtained within the extended EP+PDM versus the experimental data extracted using the $\beta-$Oslo method Spyrou2024 as well as the phenomenological gSFs employed Yan et al [Yan (total)] and Spyrou et al [Spyrou (total)] along with the microscopic $M1$ shell-model calculation, for $^{60}$Fe. b) Contributions of $ph$, $pp$, and $hh$ excitations to the EP+PDM UBR gSF.
  • Figure 3: Comparison between cross section of $^{59}$Fe$(n,\gamma)^{60}$Fe (a) and its MACS (b) obtained within the EP+IPM & EP+PDM with those obtained within the two most recent evaluations by Yan et alYan2021 and Spyrou et alSpyrou2024. The uncertainty is shown as a shaded region. The Spyrou's cross section of $^{59}$Fe$(n,\gamma)^{60}$Fe in (a) is calculated using the NLD and gSF employed to determine the MACS in (b).
  • Figure S1: (a) Total and partial ($E1$, UBR and $E1$+UBR) gSFs employed in Spyrous et alSpyrou2024. (b) Reproduction of MACS range (Fig. 1(a) in Spyrou2024) using Spyrou's $E1$ gSF combined with three strong, medium, and weak UBR gSFs in (a) and RIPL-3 corrected NLD. Dotted line is the MACS obtained using HFBCS (Demetriou 2001) NLD and Spyrou's $E1$ + medium UBR gSF.
  • Figure S2: The MACSs obtained using different combinations of NLD and gSF.