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

C. T. Nguyen, G. Cescutti, F. Matteucci, F. Rizzuti, A. Mucciarelli, D. Romano, L. Magrini, A. J. Korn, A. Bressan, L. Girardi

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.

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

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 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 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 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 Li yield 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 comprehend the "cosmological lithium problem" in this context.

Paper Structure

This paper contains 10 sections, 4 equations, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. Sample selection
  3. Predictions from stellar models
  4. Chemical evolution model
  5. Model for thin disc
  6. Correction from stellar models
  7. Results
  8. Dwarf main sequence stars
  9. Early RGB stars
  10. Conclusions

Figures (10)

  • Figure 1: Left panel: the sample of 7 347 field stars from 2021AA...651A..84M, with our selected subsamples highlighted in red and blue. Right panel: the mass distribution with metallicity ([Fe/H]) of stars in our selected samples. The blue error bar indicates the mean uncertainty value of our selected dwarf-MS sample. The black vertical line marks $\mathrm{[Fe/H]}=-0.8$ dex.
  • Figure 2: Left panel: the complete sample of Li detection adopted from the GALAH DR4 catalogue (grey dots). Our two selected samples are highlighted in blue and red. Top-right panel: zoom in on our selected early RGB stars sample, with the colour bar indicating A(Li). Bottom-right panel: our selected dwarf-MS stars sample. See the text for details of the selections.
  • Figure 3: The variation of surface Li abundance with $\log g$ of four selected stellar models. The square symbol marks the location where $\log g\sim 4.1$ on the MS, while the circle symbol indicates $\log g \sim 2.8$ on the early RGB. The black line indicates $\log g\sim 2$ around the RGB bump.
  • Figure 4: Depletion of Li abundance up to the MS (blue) and after the 1DU at the early RGB phase (red). The models are taken from 2025AA...696A.136N, including the depletion during the PMS phase due to the envelope overshoot.
  • Figure 5: Chemical evolution models of Li with different adopted values of original Li abundance and novae yields, as listed in the figure legend. An illustration model with $-0.4$ dex depletion from the primordial $\mathrm{A(Li)}=2.69$ model (magenta line) is plotted as the dotted line.
  • ...and 5 more figures