Galactic Cosmic-Ray Induced Production of Lithium in the Small Magellanic Cloud

TL;DR

This paper investigates the contribution of galactic cosmic rays to lithium production in the Small Magellanic Cloud by analyzing gamma-ray data and modeling cosmic-ray interactions, finding cosmic rays account for only a tiny fraction of observed lithium.

Contribution

It presents the first quantitative estimate of lithium produced by cosmic rays in the Small Magellanic Cloud using gamma-ray observations and cosmic-ray interaction models.

Findings

Cosmic rays produce only 0.16% of observed 6Li in the SMC.

Gamma-ray data constrains cosmic-ray induced lithium production.

Other sources likely contribute to lithium abundance in the SMC.

Abstract

Recently, the first lithium detection outside of the Milky Way was made in low-metallicity gas of the Small Magellanic Cloud, which was at the level of the expected primordial value. Part of the observed lithium in any environment has primordial origin, but there is always some post-BBN (Big Bang Nucleosynthesis) contamination, since lithium can also be produced in cosmic-ray interactions with the interstellar medium. Using the fact that processes involving cosmic rays produce lithium, but also gamma rays through neutral pion decay, we use the Small Magellanic Cloud gamma-ray observations by Fermi-LAT to make predictions on the amount of lithium in this galaxy that was produced by galactic cosmic rays accelerated in supernova remnants. By including both fusion processes, as well as spallation of heavier nuclei, we find that galactic cosmic rays could produce a very small amount of lithium. In the case of 6Li isotope (which should only be produced by cosmic rays) we can only explain 0.16% of the measured abundance. If these cosmic rays are indeed responsible for such small lithium production, observed abundances could be the result of some other sources, which are discussed in the paper.