Year: 2007
Type: article
Abstract: Cosmic structure formation leads to large-scale shocked baryonic flows which are expected to produce a cosmological population of structure-formation cosmic rays (SFCRs). Interactions between SFCRs and ambient baryons will produce lithium isotopes via $\ensuremath{\alpha}+\ensuremath{\alpha}\ensuremath{\rightarrow}^{6,7}\mathrm{Li}$. This pre-galactic (but nonprimordial) lithium should contribute to the primordial $^{7}\mathrm{Li}$ measured in halo stars and must be subtracted in order to arrive to the true observed primordial lithium abundance. In this paper we point out that the recent halo star $^{6}\mathrm{Li}$ measurements can be used to place a strong constraint to the level of such contamination, because the exclusive astrophysical production of $^{6}\mathrm{Li}$ is from cosmic-ray interactions. We find that the putative $^{6}\mathrm{Li}$ plateau, if due to pre-galactic cosmic-ray interactions, implies that SFCR-produced lithium represents ${\mathrm{Li}}_{\mathrm{SFCR}}/{\mathrm{Li}}_{\mathrm{plateau}}\ensuremath{\approx}15%$ of the observed elemental Li plateau. Taking the remaining plateau Li to be cosmological $^{7}\mathrm{Li}$, we find a revised (and slightly worsened) discrepancy between the Li observations and big bang nucleosynthesis predictions by a factor of $^{7}\mathrm{Li}_{\mathrm{BBN}}/^{7}\mathrm{Li}_{\mathrm{plateau}}\ensuremath{\approx}3.7$. Moreover, SFCRs would also contribute to the extragalactic gamma-ray background (EGRB) through neutral pion production. This gamma-ray production is tightly related to the amount of lithium produced by the same cosmic rays; the $^{6}\mathrm{Li}$ plateau limits the pre-galactic (high-redshift) SFCR contribution to be at the level of ${I}_{{\ensuremath{\gamma}}_{\ensuremath{\pi}}\mathrm{SFCR}}/{I}_{\mathrm{EGRB}}\ensuremath{\lesssim}5%$ of the currently observed EGRB.
Authors: Tijana Prodanović, Brian D. Fields
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2016521296