Lithium evolution in the Galactic thin disc from Main-Sequence and early Red-Giant-Branch stars

TL;DR

This study models lithium evolution in the galactic thin disc, incorporating nova contributions and stellar depletion, and confirms the existence of two lithium abundance plateaus in stars, supporting novae as key lithium sources.

Contribution

It introduces the first analysis including early RGB stars and constrains nova lithium yields within chemical evolution models.

Findings

Confirmation of a second lithium plateau in early RGB stars.

Best-fit nova lithium yield of 2.34×10⁻⁵ M☉ per nova.

Support for novae as primary galactic lithium producers.

Abstract

The role of novae as producers of galactic lithium has been suggested since the 1970s, and it has been reconsidered recently with the detection of $^7$Be in their outbursts. At the same time, stellar models are moving forward to comprehend the discrepancy between the primordial lithium abundance predicted by the standard Big Bang Nucleosynthesis theory and the measured value of old dwarf stars. In this work, we follow the evolution of $^7$Li in the galactic thin disc starting from a primordial value of A(Li)=2.69 dex and applying $^7$Li depletion corrections of the stellar model with overshooting to our chemical evolution models. We use the upper envelope of the observational data to constrain the models. In addition to the dwarf main sequence (MS) stars, our analysis includes, for the first time, the early red-giant-branch (RGB) stars. Besides the renowned Spite plateau of the MS stars at low metallicities, we also confirm the existence of a second A(Li) plateau of the early RGB stars, which can be explained by our model with the corrections from stellar models. Our best-fit model is obtained with an effective averaged $^7$Li yield $^{Li}Y_\mathrm{Nova}=2.34\times 10^{-5} M_\odot$ during the whole lifetime of a nova. This reinforces the possibility that novae are the main galactic $^7$Li source, together with the stellar models' ability to comprehend the "cosmological lithium problem" in this context.