[Paper Review] Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits

A. Mott, M. Steffen+3 more|arXiv (Cornell University)|Apr 21, 2017

Stellar, planetary, and galactic studies51 references17 citations

TL;DR

This study presents a 3D non-LTE analysis of the lithium resonance doublet in the magnetically active red giant HD 123351 using high-resolution CFHT spectroscopy. It reports a 2σ detection of $^{6}\mathrm{Li}/^{7}\mathrm{Li} = 8.0 \pm 4.4\%$ and $A(\mathrm{Li}) = 1.69 \pm 0.11$ dex, challenging standard stellar evolution models due to the unexpected presence of the fragile $^{6}\mathrm{Li}$ isotope in an evolved star.

ABSTRACT

Current three-dimensional (3D) hydrodynamical model atmospheres together with NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not to mention its production in magnetically active targets like HD123351. From fits of the observed CFHT spectrum with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical CO5BOLD and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and 6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Meléndez et al. (2012), updated with new atomic data for V I, which results in the best fit of the lithium line profile of HD123351. Two other line lists lead to similar results but with inferior fit qualities. Our 2-sigma detection of the 6Li isotope is the result of a careful statistical analysis and the visual inspection of each achieved fit. Since the presence of a significant amount of 6Li in the atmosphere of a cool evolved star is not expected in the framework of standard stellar evolution theory, non-standard, external lithium production mechanisms, possibly related to stellar activity or a recent accretion of rocky material, need to be invoked to explain the detection of 6Li in HD123351.

Motivation & Objective

• To determine the lithium abundance and $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ isotopic ratio in the magnetically active red giant HD 123351 using high-resolution spectroscopy.
• To assess the reliability of $^{6}\mathrm{Li}$ detection by comparing synthetic line profiles from 1D and 3D model atmospheres under non-LTE conditions.
• To evaluate the impact of different line lists and atomic data on the derived isotopic ratio and abundance.
• To constrain the origin of the observed $^{6}\mathrm{Li}$, given its incompatibility with standard stellar evolution theory.
• To identify systematic uncertainties arising from blending lines and model assumptions in isotopic ratio determination.

Proposed method

• Synthetic spectra were computed using 3D CO 5 BOLD and 1D LHD model atmospheres with non-LTE treatment for lithium line formation.
• High-resolution CFHT spectra ($R = 120,000$, S/N ≈ 400) were fitted to synthetic profiles using multiple line lists, including updated atomic data from Lawler et al. (2014).
• The fitting procedure was repeated across different wavelength ranges and assumptions to quantify systematic uncertainties in $A(\mathrm{Li})$ and $^{6}\mathrm{Li}/^{7}\mathrm{Li}$.
• The Meléndez et al. (2012) line list, updated with new $\log gf$ values for V i, provided the best fit to the observed Li-line profile.
• Statistical analysis and visual inspection of fits were used to validate the $^{6}\mathrm{Li}$ detection at the 2σ level.
• Results were cross-checked with alternative line lists to assess robustness and identify potential errors, such as spurious negative $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ ratios from flawed blends.

Experimental results

Research questions

• RQ1 What is the true $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ isotopic ratio in the evolved, magnetically active giant HD 123351?
• RQ2 How do 3D non-LTE model atmospheres improve the accuracy of lithium isotopic ratio determination compared to 1D LTE models?
• RQ3 To what extent do uncertainties in line list composition and atomic data affect the derived $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ ratio?
• RQ4 Can the observed $^{6}\mathrm{Li}$ abundance be explained by standard stellar evolution, or does it require non-standard processes?
• RQ5 What are the implications of the $^{6}\mathrm{Li}$ detection for lithium production mechanisms in active, evolved stars?

Key findings

• The $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ isotopic ratio in HD 123351 is measured as $8.0 \pm 4.4\%$ using 3D-NLTE modeling with the Meléndez et al. (2012) line list, corrected for updated V i $\log gf$ values.
• The lithium abundance is determined as $A(\mathrm{Li}) = 1.69 \pm 0.11$ dex, consistent with the 3D model atmosphere parameters $T_{\rm eff} = 4780$ K, $\log g = 3.20$, and $[\mathrm{Fe}/\mathrm{H}] = 0.00$.
• The Meléndez et al. (2012) line list with updated V i data produced the best fit to the observed Li-line profile, while the Reddy et al. (2002) list led to unphysical negative $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ ratios due to erroneous blends.
• The detection of $^{6}\mathrm{Li}$ at the 2σ level is robust, supported by visual inspection and statistical analysis across multiple fitting configurations.
• The observed $^{6}\mathrm{Li}/^{7}\mathrm{Li}$ ratio is inconsistent with standard stellar evolution, which predicts near-total destruction of $^{6}\mathrm{Li}$ in red giants after the first dredge-up.
• Possible explanations include recent accretion of rocky planetary material (requiring ~50 Earth masses) or flaring-induced spallation, though the latter is considered unlikely due to insufficient energy output.