Lithium evolution in the Galactic thin disc from main-sequence and early red-giant-branch stars

The role of novae as producers of galactic lithium-7 (Li) has been suggested since the 1970s, and it has been reconsidered recently with the detection of beryllium-7 in their outbursts. At the same time, stellar models are moving forward to help us understand the discrepancy between the primordial Li abundance predicted by the standard Big Bang nucleosynthesis theory and the measured value of old dwarf stars. For this work, we followed the evolution of Li in the Galactic thin disc starting from a primordial value of A(Li)= 2.69 dex and applying Li depletion corrections of the stellar model with overshoot to our chemical evolution models. We used the upper envelope of the observational data to constrain the models. In addition to the dwarf main-sequence (MS) stars, our analysis included, for the first time, the early red-giant-branch (RGB) stars. In addition to 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 was obtained with an effective averaged Li yield L Y Nova = 2.34 × 10 -5 M ⊙ during the whole lifetime of a nova. This reinforces the possibility that novae are the main galactic Li source, together with the stellar models’ ability to resolve the ‘cosmological Li problem’ in this context.