Superbubbles and the Galactic evolution of Li, Be and B

TL;DR

This paper presents a unified superbubble model explaining the observed two-slope behavior of Be and B galactic evolution relative to oxygen, supported by energetic particle acceleration and supernova ejecta mixing.

Contribution

It introduces a single model that accounts for both primary and secondary behaviors of Be and B evolution, aligning with observational constraints.

Findings

The Be/O ratio is constant at low metallicity and increases at high metallicity.

The superbubble model explains the transition metallicity and isotopic ratios.

Energetic particles with a flattened low-energy spectrum are key to the model.

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).