The GALAH Survey: A new constraint on cosmological lithium and Galactic lithium evolution from warm dwarf stars

TL;DR

This study presents the largest high-resolution analysis of lithium abundances in over 100,000 stars, revealing patterns of lithium depletion and enrichment across different metallicities and stellar groups, providing new constraints on Galactic and stellar evolution models.

Contribution

It offers the first large-scale comparison of lithium in warm and cool dwarf stars, identifying a consistent offset and new insights into primordial lithium preservation.

Findings

Warm stars have higher Li abundances than cool stars by 0.4 dex.

Li abundance increases with metallicity above [Fe/H] = -0.5.

Cool stars at low metallicity align with the primordial Li from BBN.

Abstract

Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution analysis of Li abundances A(Li) to date, with results for over 100 000 GALAH field stars spanning effective temperatures $5900\,\mathrm{K} \lesssim \rm{T_{eff}} \lesssim7000\,\mathrm{K}$ and metallicities $-3 \lesssim \rm[Fe/H] \lesssim +0.5$. We separated these stars into two groups, on the warm and cool side of the so-called Li-dip, a localised region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)-[Fe/H] plane, but with a roughly constant offset in A(Li) of 0.4 dex, the warm group having higher Li abundances. At $\rm[Fe/H]\gtrsim-0.5$, a significant increasing in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around 0.4 dex relative to the primordial value predicted from Big Bang nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = -1.0 and -0.5, form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.