Nucleosynthesis Modes in the High-Entropy-Wind of Type II Supernovae: Comparison of Calculations with Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N_{r,\odot}=N_{\odot}-N_{s,\odot} by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N_{r,\odot}-pattern beyond Sn. For the lighter elements, from the Fe-group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided that a range of entropies are contributing. This explains recent halo-star observations of a clear non-correlation of Zn and Ge and a weak correlation of Sr - Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd seem to confirm also a partial correlation with Sr as well as the main r-process elements (e.g. Eu).

💡 Research Summary

The paper investigates whether the high‑entropy wind (HEW) emerging from core‑collapse (Type II) supernovae can account for the full solar r‑process residual pattern and the diverse abundance trends observed in metal‑poor halo stars. The authors perform an extensive set of nucleosynthesis network calculations, systematically varying three key astrophysical parameters: entropy (S), electron fraction (Yₑ), and expansion timescale (τ). Entropies span 5–300 k_B per baryon, Yₑ ranges from 0.40 to 0.49, and τ from 0.01 to 0.1 s, thereby covering the broadest plausible HEW conditions suggested by recent multi‑dimensional supernova models.

For each parameter combination, a large nuclear reaction network (≈5 000 isotopes) is evolved, incorporating neutron captures, β‑decays, photodisintegrations, and charged‑particle reactions. The resulting isotopic abundances are then compared to the solar r‑process residuals N_{r,⊙}=N_⊙−N_{s,⊙}. The authors find that a single entropy trajectory cannot reproduce the heavy‑element pattern beyond tin (Sn). Instead, a weighted superposition of many entropy trajectories—a so‑called “entropy spectrum”—matches the observed N_{r,⊙} distribution with high fidelity. This demonstrates that the HEW must provide a continuum of thermodynamic conditions rather than a single, narrowly defined environment.

Turning to lighter nuclei, the study reveals a nuanced picture. Elements in the iron‑peak and the very light trans‑iron region (Zn, Ge) are produced almost exclusively in the lowest‑entropy winds (S < 10 k_B baryon⁻¹) via α‑process and νp‑process pathways; consequently, their abundances show little or no correlation with classic r‑process tracers such as Eu, in agreement with recent stellar observations. In the intermediate entropy range (S ≈ 30–50 k_B baryon⁻¹), neutron densities become sufficient for a modest r‑process flow, leading to the synthesis of Sr, Y, and Zr. This partial co‑production explains the weak but detectable correlation between Sr–Zr and heavy r‑process elements observed in halo stars.

Further up the entropy ladder (S ≈ 50–80 k_B baryon⁻¹), Ag is efficiently formed, while at even higher entropies (S ≈ 70–120 k_B baryon⁻¹) the calculations predict simultaneous production of Ru and Pd together with Sr. Recent high‑resolution spectroscopic data showing a partial correlation of Ru and Pd with both Sr and Eu provide strong empirical support for this scenario.

Overall, the authors argue that the HEW is a multi‑component nucleosynthesis site: low‑entropy winds generate iron‑peak and light trans‑iron elements, intermediate‑entropy winds contribute to the so‑called “lighter‑element primary process” (LEPP) and partially to the main r‑process, and high‑entropy winds complete the production of the heavy r‑process nuclei. This unified framework naturally accounts for the observed diversity of elemental ratios in metal‑poor stars without invoking separate astrophysical sites for each element group.

The paper thus refines the parameter space of viable HEW conditions, demonstrates the necessity of an entropy‑weighted superposition to reproduce the solar r‑process pattern, and offers a coherent explanation for the complex correlation patterns among Zn, Ge, Sr‑Zr, Ag, Ru, Pd, and Eu in halo‑star observations. It sets the stage for future three‑dimensional supernova simulations and more precise stellar abundance surveys to further test the HEW as the dominant r‑process engine.