Population III Generated Cosmic Rays and the Production of Li6

TL;DR

This paper models the production of Li6 by cosmic rays from Population III stars in the early universe, showing it can explain observed Li6 levels without overproducing Li7, and discusses implications for IGM heating.

Contribution

It presents a new calculation of Li6 production from Pop III cosmic rays within hierarchical structure formation, linking early star formation to observed Li6 abundances.

Findings

Pop III cosmic rays can account for Li6 plateau in metal-poor stars.

The production of Li6 is sensitive to cosmic ray propagation efficiency.

Cosmic ray heating may raise IGM temperatures to around 10^5 K.

Abstract

We calculate the evolution of Li6 generated from cosmic rays produced by an early population of massive stars. The computation is performed in the framework of hierarchical structure formation and is based on cosmic star formation histories constrained to reproduce the observed star formation rate at redshift z \la 6, the observed chemical abundances in damped Lyman alpha absorbers and in the intergalactic medium, and to allow for an early reionization of the Universe at z\sim 11 by Pop III stars as indicated by the third year results released by WMAP. We show that the pregalactic production of the Li6 isotope in the IGM via these Pop III stars can account for the Li6 plateau observed in metal poor halo stars without additional over-production of Li7. Our results depend on the efficiency of cosmic rays to propagate out of minihalos and the fraction of supernovae energy deposited in cosmic rays. We also compute the cosmic ray heating of the IGM gas. In general, we find somewhat high temperatures (of order 10^5 K) implying that the cosmic rays production of Li6 may be required to be confined to the so-called warm-hot IGM.