# Constraining nucleosynthesis in neutrino-driven winds using the impact of (α, xn) reaction rates

Athanasios Psaltis, Almudena Arcones, Melina L Avila, Maximilian Jacobi, Camilla Juul Hansen, Linda Lombardo, Zach Meisel, Peter Mohr, Fernando Montes, Wei Jia Ong, et al.

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Athanasios Psaltis, Almudena Arcones, Melina L Avila, Maximilian Jacobi, Camilla Juul Hansen, et al. Constraining nucleosynthesis in neutrino-driven winds using the impact of (α, xn) reaction rates. 10th Conference on Nuclear Physics in Astrophysics, Sep 2022, Geneva, Switzerland. pp.08002, 10.1051/epjconf/202327908002. hal-04457695

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Constraining nucleosynthesis in neutrino-driven winds using the impact of (α, xn) reaction rates

Athanasios Psaltis¹,*, Almudena Arcones¹,², Melina L. Avila³, Maximilian Jacobi¹, Camilla Juul Hansen⁴, Linda Lombardo⁵, Zach Meisel⁶, Peter Mohr⁷, Fernando Montes⁸,⁹, Wei Jia Ong¹⁰, and Hendrik Schatz⁸,⁹,

¹Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstr. 2, Darmstadt 64289, Germany

²GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, Darmstadt 64291, Germany

³Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA

⁴Institute for Applied Physics, Goethe University Frankfurt, Max-von-Laue-Str. 12, Frankfurt am Main 60438, Germany

⁵GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jules Janssen, 92195 Meudon, France

⁶Institute of Nuclear & Particle Physics, Department of Physics & Astronomy, Ohio University, Athens, OH, 45701 USA

⁷Institute for Nuclear Research (ATOMKI), H-4001 Debrecen, Bem tér 18/c, Hungary

⁸National Superconducting Cyclotron Laboratory, East Lansing, MI, 48824, USA

⁹Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements, Michigan State University, East Lansing, Michigan 48824, USA

¹⁰Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore CA 95450

**Abstract.** The lighter heavy elements of the first r-process peak, between strontium and silver, can be synthesized in the moderately neutron-rich neutrino-driven ejecta of either core-collapse supernovae or neutron star mergers via the weak r-process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. We have performed a new impact study to identify the most important (α, xn) reactions that can affect the production of the lighter heavy elements under different astrophysical conditions using new, constrained (α, xn) reaction rates based on the Atomki-V2 αOMP. Our results show how when reducing the nuclear physics uncertainties, we can use abundance ratios to constrain the astrophysical conditions/environment. This can be achieved in the near future, when the key (α, xn) reaction rates will be measured experimentally in radioactive beam facilities.

# 1 Introduction

The astrophysical r-process can produce around half of the elements heavier than iron in explosive environments, such as the merging of a binary neutron star system [1]. Spectroscopic observations of galactic halo metal-poor stars show a large scatter in the light heavy elements of the first r-process peak, between strontium and silver (Z= 38-47), compared to a rather

<footnote>*e-mail: psaltis@theorie.ikp.physik.tu-darmstadt.de</footnote>
robust production of the lanthanides [2]. For this reason, many other processes have been suggested to produce the lighter heavy elements.

A possible scenario is nucleosynthesis in neutrino-driven outflows of core-collapse supernovae or neutron star mergers, via the weak r-process (also known as α-process) [3]. This nucleosynthesis process operates for $T \approx 2-5$ GK and creates heavy elements mainly through α- and proton-induced reactions on neutron-rich nuclei. Refs. [4, 5] have demonstrated that the main nuclear physics uncertainty in weak r-process nucleosynthesis is caused by the (α, n) reaction rates, and more specifically from the α-nucleus potential (αOMP), since these rates are calculated through the statistical Hauser-Feshbach model.

## 2 Impact study of the (α, xn) reaction rates

We recently performed an impact study of the (α, xn) reaction rates to the weak r-process [6], employing the Atomki-V2 αOMP [7]. We explored the phase space of the astrophysical conditions, based on Ref. [8] and used a Monte Carlo technique to systematically vary the (α, xn) reaction rates.

Instead of finding the impact on elemental abundances, as Ref. [9], we explored the impact on abundance ratios, which can be better constrained through observations. Figure 1 shows results of the predicted abundance ratios of Sr/Y and Y/Zr for various astrophysical conditions. We also show the recent observations by Lombardo et al. [10], which includes 52 giant metal-poor stars ([Fe/H] < −1.5), using high resolution spectra from the UVES instrument of the Very Large Telescope (VLT).

<figure><img src="image_2.png" /></figure>

<table><caption>**Figure 1.** Two-dimensional Kernel Density Estimates (KDEs) of the Sr/Y and Y/Zr elemental ratios from Ref. [6] with the addition of stars from the sample of Ref. [10]. In the right, the main properties of each model (electron fraction $Y_e$, entropy per nucleon s and expansion timescale $\tau$) are presented. The contours show the 1 and $2\sigma$ uncertainties for each calculation. See the text for details.</caption><thead><tr><th></th><th>Y<sub>e</sub><br>(k<sub>B</sub>/nuc)</th><th>s</th><th>τ<br>(ms)</th><th></th><th>Y<sub>e</sub><br>(k<sub>B</sub>/nuc)</th><th>s</th><th>τ<br>(ms)</th></tr></thead><tbody><tr><td>MC02</td><td>0.45</td><td>113</td><td>11.9</td><td>MC16</td><td>0.49</td><td>126</td><td>15.4</td></tr><tr><td>MC04</td><td>0.44</td><td>66</td><td>19.2</td><td>MC17</td><td>0.46</td><td>132</td><td>12.4</td></tr><tr><td>MC06</td><td>0.40</td><td>56</td><td>63.8</td><td>MC20</td><td>0.41</td><td>42</td><td>59.3</td></tr><tr><td>MC07</td><td>0.47</td><td>96</td><td>11.6</td><td>MC22</td><td>0.40</td><td>40</td><td>46.7</td></tr><tr><td>MC08</td><td>0.43</td><td>78</td><td>35.0</td><td>MC25</td><td>0.46</td><td>96</td><td>20.9</td></tr><tr><td>MC10</td><td>0.40</td><td>54</td><td>31.0</td><td>MC26</td><td>0.40</td><td>84</td><td>36.2</td></tr><tr><td>MC12</td><td>0.48</td><td>85</td><td>9.7</td><td>MC28</td><td>0.46</td><td>113</td><td>11.9</td></tr><tr><td>MC13</td><td>0.43</td><td>64</td><td>35.9</td><td>MC29</td><td>0.41</td><td>66</td><td>41.4</td></tr><tr><td>MC15</td><td>0.48</td><td>103</td><td>20.4</td><td>MC31</td><td>0.43</td><td>71</td><td>11.4</td></tr></tbody></table>

Based on the analysis above, we have identified 35 (α, xn) reactions which affect the production of the lighter heavy elements in the weak r-process (see Ref. [6] for the complete list). In the following, we discuss experimental plans to measure these important reactions at radioactive ion beam facilities.

## 3 Experimental Plans

Measuring the relevant (α, xn) reaction rates will help us better constrain the relevant astrophysical conditions that produce the lighter heavy elements between strontium and silver via the weak r-process. There is an intense interest in the nuclear astrophysics community
regarding these reactions, and many measurements already have either been proposed or re-
cently been performed in various stable and radioactive ion beam facilities worldwide, such
as in ATOMKI, Argonne, FRIB and TRIUMF. In the following, we shall briefly discuss a
recently approved proposal by the ATLAS PAC at Argonne to perform the first measurement
of the $^{93}$Sr($\alpha$, n)$^{96}$Zr reaction using MUSIC. This reaction has been shown to affect many
elemental ratios under many astrophysical conditions in the impact study of Ref. [6].

The MUSIC (Multi-Sampling Ionization Chamber) detector [11] is a highly-efficient,
self-normalizing active-target system that is capable of measuring cross sections of reactions
relevant for astrophysics. It is sensitive to the energy losses of the particles that travel through
the gas (in our case helium) and has the ability to measure excitation functions using a single
beam energy. For the proposed experiment, the $^{93}$Sr radioactive beam will be provided by the
nuCARIBU neutron-induced fission source.

Recently, MUSIC successfully measured the $^{100}$Mo($\alpha$, xn) reaction, which demonstrated its capability for ($\alpha$, xn) reaction cross sections up to A = 100 [12]. The detector can also be transported to other facilities, such as FRIB, to study important reactions that were identified in our impact study [6].

## 4 Summary & Discussion

Explaining the origin of the lighter heavy elements between strontium and silver is an exciting
open question in nuclear astrophysics. Nucleosynthesis in neutrino-driven ejecta through
the weak r-process provides a possible solution, but there are still uncertainties, both in the
astrophysical conditions and the underlying nuclear physics input.

We recently performed an impact study of the ($\alpha$, xn) reaction rates to the weak r-process using the Atomki-V2 αOMP and identified key reactions that affect the abundance ratios that are observed in metal-poor stars. Future experimental and observational efforts will help to further reduce the relevant reaction rate uncertainties and shed light to the production of the lighter heavy elements in the Cosmos.

## Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project No. 279384907—SFB 1245, the European Research Council Grant No. 677912 EUROPIUM, and the State of Hesse within the Research Cluster ELEMENTS (Project ID 500/10.006)

## References

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[3] A. Arcones, F. Montes, Astrophys. J **731**, 5 (2011)

[4] J. Pereira, F. Montes, Phys. Rev. C **93**, 034611 (2016)

[5] J. Bliss et al., J. Phys. G **44**, 054003 (2017)

[6] A. Psaltis et al., Astrophys. J **935**, 27 (2022)

[7] P. Mohr et al., At. Data Nucl. Data Tables **142**, 101453 (2021)

[8] J. Bliss et al., Astrophys. J **855**, 135 (2018)

[9] J. Bliss et al., Phys. Rev. C **101**, 055807 (2020)

[10] L. Lombardo et al., A&A **665**, A10 (2022)

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[12] W.J. Ong et al., Phys. Rev. C **105**, 055803 (2022)