Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud

TL;DR

This study reports the first detection of interstellar lithium in the low-metallicity Small Magellanic Cloud, providing new constraints on primordial lithium abundance and testing Big Bang nucleosynthesis predictions.

Contribution

It presents the first measurement of interstellar lithium in a low-metallicity galaxy, offering an alternative approach to constrain primordial lithium independent of stellar surface effects.

Findings

SMC lithium abundance nearly matches BBN predictions

Results challenge stellar depletion explanations for lithium discrepancy

Data compatible with both standard and non-standard BBN models

Abstract

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial 7Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.