Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion

TL;DR

This study provides observational evidence for two distinct Li abundance plateaus in metal-poor stars, revealing a mass- and metallicity-dependent Li depletion, and supports models with atomic diffusion and turbulent mixing aligning with Big Bang nucleosynthesis predictions.

Contribution

It demonstrates the existence of two flat Li plateaus at different metallicities and links Li depletion to stellar mass and metallicity, refining understanding of primordial Li.

Findings

Two well-defined Li plateaus at A_{Li} = 2.18 and 2.27.

Li depletion depends on metallicity and stellar mass.

Models with atomic diffusion and turbulent mixing match observations.

Abstract

We present NLTE Li abundances for 88 stars in the metallicity range -3.5 < [Fe/H] < -1.0. The effective temperatures are based on the infrared flux method with improved E(B-V) values obtained mostly from interstellar NaI D lines. The Li abundances were derived through MARCS models and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, and complemented with reliable equivalent widths from the literature. The less-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall into two well-defined plateaus of A_{Li} = 2.18 (sigma = 0.04) and A_{Li} = 2.27 (sigma = 0.05), respectively. We show that the two plateaus are flat, unlike previous claims for a steep monotonic decrease in Li abundances with decreasing metallicities. At all metallicities we uncover a fine-structure in the Li abundances of Spite plateau stars, which we trace to Li depletion that depends on both metallicity and mass. Models including atomic diffusion and turbulent mixing seem to reproduce the observed Li depletion assuming a primordial Li abundance A_{Li} = 2.64, which agrees well with current predictions (A_{Li} = 2.72) from standard Big Bang nucleosynthesis. Adopting the Kurucz overshooting model atmospheres increases the Li abundance by +0.08 dex to A_{Li} = 2.72, which perfectly agrees with BBN+WMAP.