[Paper Review] Superbubbles and the Galactic evolution of Li, Be and B

E. Parizot|arXiv (Cornell University)|Jun 7, 2000

Astro and Planetary Science3 citations

TL;DR

This paper proposes the superbubble (SB) model to explain the bimodal Galactic evolution of lithium, beryllium, and boron (Li, Be, B), where spallation of C, N, and O by energetic particles in superbubbles produces these light elements. The model accounts for the observed two-slope behavior in Be/O and B/O vs. O/H—primary at low metallicity and secondary at high metallicity—by assuming a few percent of supernova ejecta mixed in SBs with a flattened particle energy spectrum (E⁻¹), successfully matching observed abundances and the ⁶Li/⁹Be ratio.

ABSTRACT

From a recent re-analysis of the available data, Be and B Galactic evolution appears to show evidence for a two-slope behaviour with respect to O. The inferred Be/O abundance ratio in halo stars is constant at very low metallicity (primary behaviour) and increases proportionally to O/H at high metallicity (secondary behaviour). We show that this can be explained in the framework of one single model, the `superbubble model', in which Li, Be and B are produced by spallation reactions induced by energetic particles accelerated out of a mixture of supernova (SN) ejecta and ambient interstellar medium inside superbubbles (SBs). All the qualitative and quantitative constraints, including the energetics, the value of the transition metallicity and the 6Li/9Be isotopic ratio, are satisfied provided that the energetic particles have a spectrum flattened at low energy (in E^-1) and that the proportion of the SN ejecta inside SBs is of the order of a few percent. This lends support to Bykov's acceleration mechanism inside SBs and to the SB dynamical evolution model of Mac Low & McCray (1988).

Motivation & Objective

• To resolve the long-standing puzzle of the two-slope behavior in Be and B Galactic evolution with respect to oxygen abundance.
• To explain the transition from primary to secondary nucleosynthesis in Li, Be, and B production using a single physical mechanism.
• To test whether the superbubble environment can simultaneously satisfy energetic, isotopic, and abundance constraints from halo and disk stars.
• To assess the viability of the SB model as a unified framework for light element production, replacing ad hoc dual mechanisms.
• To provide a physical basis for the observed ⁶Li/⁹Be ratio and the transition metallicity around 10⁻² to 10⁻¹ solar.

Proposed method

• Modeling Li, Be, and B production via spallation reactions induced by energetic particles (EPs) in superbubbles (SBs), where EPs are accelerated from a mixture of supernova (SN) ejecta and ambient ISM.
• Using a particle energy spectrum of E⁻¹ at low energies to reproduce the observed Be/O and B/O abundance trends across metallicities.
• Introducing a key parameter x, the mass fraction of SN ejecta within SBs, to control the transition from primary to secondary behavior.
• Comparing model predictions with observed stellar abundances of Be and B in halo and disk stars, particularly at low metallicities (e.g., [O/H] < -2).
• Incorporating constraints from the ⁶Li/⁹Be isotopic ratio and the transition metallicity Zₜ ≈ 10⁻²–10⁻¹ Z⊙ to refine model parameters.
• Evaluating the model’s consistency with gamma-ray observations, particularly from future INTEGRAL detections of nuclear de-excitation lines in nearby SBs.

Experimental results

Research questions

• RQ1 Can a single physical model explain the observed two-slope behavior in Be/O and B/O vs. O/H across the Galactic metallicity range?
• RQ2 What particle energy spectrum and ejecta fraction within superbubbles are required to reproduce the observed Li, Be, and B abundances?
• RQ3 How does the ⁶Li/⁹Be isotopic ratio constrain the energetic particle environment in superbubbles?
• RQ4 Is the transition from primary to secondary nucleosynthesis in LiBeB gradual or abrupt, and what does this imply about the underlying acceleration mechanism?
• RQ5 Can the superbubble model reconcile the observed scatter in Be and B abundances with variations in local ejecta fraction x across different stellar populations?

Key findings

• The superbubble model successfully reproduces the two-slope behavior in Be/O and B/O vs. O/H, with primary-like behavior at low metallicity and secondary-like behavior at high metallicity.
• A particle energy spectrum flattened at low energies (E⁻¹) is required to match the observed abundance trends and the ⁶Li/⁹Be ratio.
• The model requires a small but non-negligible fraction of supernova ejecta within superbubbles—approximately 1–3%—to reproduce the observed transition metallicity.
• The transition metallicity Zₜ ≈ 10⁻² to 10⁻¹ Z⊙ is naturally reproduced within the model, consistent with observational constraints.
• The model predicts a continuous transition between primary and secondary regimes, avoiding the sharp discontinuity expected in dual-mechanism scenarios.
• The model is consistent with future gamma-ray observations, as INTEGRAL is expected to detect nuclear de-excitation lines from EP interactions in nearby superbubbles like Orion and Vela.