Boron Synthesis in Type Ic Supernovae

We investigate the neutrino-process in an energetic Type Ic supernova (SN Ic) and resultant productions of the light elements including boron and its stable isotopes. SN Ic is a very unique boron source because it can produce boron through not only spallation reactions as discussed in Nakamura & Shigeyama (2004) but also the neutrino-process. The neutrino-process is considered to occur in core-collapse supernovae and previous studies were limited to Type II supernovae (SNe II). Although the progenitor star of an SN Ic does not posses a He envelope so that 7Li production via the neutrino-process is unlikely, 11B can be produced in the C-rich layers. We demonstrate a hydrodynamic simulation of SN Ic explosion and estimate the amounts of the light elements produced via the neutrino-process for the first time, and also the subsequent spallation reactions between the outermost layers of compact SN Ic progenitor and the ambient medium. We find that the neutrino-process in the current SN Ic model produces a significant amount of 11B, which is diluted by 10B from spallation reactions to get closer to B isotopic ratios observed in meteorites. We also confirm that high-temperature mu- and tau-neutrinos and their anti-neutrinos, reasonably suggested from the compact structure of SN Ic progenitors, enhance the light element production through the neutral-current reactions, which may imply an important role of SNe Ic in the Galactic chemical evolution.

💡 Research Summary

This paper presents the first quantitative study of light‑element synthesis in energetic Type Ic supernovae (SNe Ic), focusing on the neutrino‑process (ν‑process) and subsequent spallation reactions. The authors model a 15 M⊙ carbon‑oxygen (C/O) progenitor exploding with an energy of 3 × 10⁵² erg, a configuration reminiscent of SN 1998bw. Using a one‑dimensional relativistic hydrodynamics code, they follow shock propagation, temperature evolution, and matter ejection.

Neutrino emission is assumed to carry a total energy of 3 × 10⁵³ erg with an exponential decay timescale of 3 s. Electron‑type neutrinos (ν_e, (\bar\nu_e)) are given temperatures of 3.2 MeV and 5 MeV, respectively. For the heavy‑flavor neutrinos (ν_μ, τ and their antiparticles) two temperature scenarios are explored: a “standard” case with T = 6 MeV and a “high‑temperature” case with T = 8 MeV, motivated by the compactness of Ic progenitors which should yield hotter neutrino spectra.

A nuclear reaction network comprising 291 isotopes, including all relevant ν‑process channels, is employed. The ν‑process primarily operates in the O/Ne layer (mass coordinate ≈ 4.6–7.4 M⊙), where neutral‑current reactions on ¹²C produce ¹¹B, ¹¹C (which decays to ¹¹B), ¹⁰B, ⁷Li, and ⁷Be. The shock wave destroys much of the freshly synthesized material in inner layers, but the outer C‑rich shells retain a substantial fraction of ¹¹B. The calculated yields are:

• Standard heavy‑flavor temperature: ¹¹B = 2.69 × 10⁻⁷ M⊙, ¹⁰B = 1.29 × 10⁻⁹ M⊙, ⁷Li ≈ 7.4 × 10⁻⁹ M⊙.
• High heavy‑flavor temperature: ¹¹B = 5.46 × 10⁻⁷ M⊙, ¹⁰B ≈ 2.78 × 10⁻⁹ M⊙, ⁷Li ≈ 2.5 × 10⁻⁸ M⊙.

Thus, raising the heavy‑flavor neutrino temperature roughly doubles the production of ¹¹B and other light nuclei via neutral‑current reactions.

The second production channel considered is spallation of the outermost ejecta. Because Ic progenitors have lost their H and He envelopes, the surface consists almost entirely of C and O. A small fraction (≈ 0.3 % of the ejecta mass, ~4 × 10⁻⁴ M⊙) is accelerated to energies > 10 MeV per nucleon. These fast C/O nuclei collide with interstellar medium (ISM) gas (n_H = 1 cm⁻³, n_He = 0.1 cm⁻³). Using the cross sections of Read & Viola (1984), the authors compute spallation yields of ¹¹B = 1.3 × 10⁻⁶ M⊙ and ¹⁰B = 4.38 × 10⁻⁷ M⊙. The spallation‑only ¹¹B/¹⁰B ratio is ≈ 3.

When the ν‑process and spallation contributions are summed, the overall isotopic ratio becomes ¹¹B/¹⁰B ≈ 3.7 for the standard heavy‑flavor temperature and ≈ 4.3 for the high‑temperature case. These values are in good agreement with the solar system meteoritic ratio of 4.05 ± 0.05, indicating that SNe Ic can naturally reproduce the observed boron isotopic composition.

The paper discusses the astrophysical implications. Although SNe Ic are less frequent than SNe II (the observed Ib/c to II ratio in local spirals ranges from 0.058 to 0.41), the per‑event ¹¹B yield from an Ic is comparable to that from a typical SN II (≈ 8 × 10⁻⁷ M⊙). Consequently, SNe Ic are unlikely to dominate the Galactic ¹¹B inventory, but they may contribute significantly in metal‑poor environments where their compact progenitors are surrounded by dense C/O winds. In such settings, the freshly synthesized light elements could be incorporated directly into the next generation of stars, potentially explaining anomalously high Be/B abundances observed in some halo stars (e.g., HD 106038).

Finally, the authors argue that Galactic chemical evolution (GCE) models should be expanded to include a fifth source: SNe Ic, encompassing both ν‑process and CNO spallation contributions. This addition would help resolve longstanding discrepancies, such as the linear Be–Fe trend at low metallicities and the over‑production of ¹¹B in models that only consider SNe II ν‑process and cosmic‑ray spallation.

In summary, the study demonstrates that energetic Type Ic supernovae are viable sites for the synthesis of ¹¹B (and to a lesser extent ⁷Li and ⁹Be) through a combination of hot heavy‑flavor neutrino interactions and high‑energy spallation of C/O ejecta. Their inclusion in GCE calculations is essential for a comprehensive understanding of light‑element abundances and isotopic ratios in the Galaxy.