Cosmic Ray production of Beryllium and Boron at high redshift

TL;DR

This paper models the non-thermal evolution of light elements like Be and B in the intergalactic medium, incorporating cosmic ray interactions and supernova-driven processes, to explain observed abundances and predict a Be and B plateau at galaxy formation.

Contribution

It introduces a comprehensive model including cosmic ray interactions, supernova processes, and hierarchical structure formation to explain Be and B production at high redshift.

Findings

Be and B are mainly produced by CR interactions in the IGM.

D production is negligible in the model.

A detectable Be and B plateau forms at galaxy formation (z ~ 3).

Abstract

Recently, new observations of Li6 in Pop II stars of the galactic halo have shown a surprisingly high abundance of this isotope, about a thousand times higher than its predicted primordial value. In previous papers, a cosmological model for the cosmic ray-induced production of this isotope in the IGM has been developed to explain the observed abundance at low metallicity. In this paper, given this constraint on the Li6, we calculate the non-thermal evolution with redshift of D, Be, and B in the IGM. In addition to cosmological cosmic ray interactions in the IGM, we include additional processes driven by SN explosions: neutrino spallation and a low energy component in the structures ejected by outflows to the IGM. We take into account CNO CRs impinging on the intergalactic gas. Although subdominant in the galactic disk, this process is shown to produce the bulk of Be and B in the IGM, due to the differential metal enrichment between structures (where CRs originate) and the IGM. We also consider the resulting extragalactic gamma-ray background which we find to be well below existing data. The computation is performed in the framework of hierarchical structure formation considering several star formation histories including Pop III stars. We find that D production is negligible and that a potentially detectable Be and B plateau is produced by these processes at the time of the formation of the Galaxy (z ~ 3).