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

Context. Current three-dimensional (3D) hydrodynamical model atmospheres together with detailed spectrum synthesis, accounting for departures from local thermodynamic equilibrium (LTE), permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile Li-6 isotope in evolved solar-metallicity red giant branch (RGB) stars, not to mention its production in magnetically active targets like HD123351. Aims. A detailed spectroscopic investigation of the lithium resonance doublet in HD123351 in terms of both abundance and isotopic ratio is presented. From fits of the observed spectrum, taken at the Canada-France-Hawaii telescope, with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the Li-6 isotope and to place constraints on its origin. Methods. We derive the lithium abundance A(Li) and the Li-6/Li-7 isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R = 120 000, S/N = 400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical (COBOLD)-B-5 and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). The fitting procedure is repeated with different assumptions and wavelength ranges to obtain a reasonable estimate of the involved uncertainties. Results. We find A(Li) = 1.69 +/- 0.11 dex and Li-6/Li-7 = 8.0 +/- 4.4% in 3D-NLTE, using the line list of Melendez et al. (2012, A&A, 543, A29), 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. Conclusions. Our 2 sigma detection of the Li-6 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 Li-6 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 Li-6 in HD123351.