Lithium abundance and⁶Li/⁷Li ratio in the active giant HD 123351

Abstract

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 6 Li isotope in evolved solar-metallicity red giant branch (RGB) stars, not to mention its production in magnetically active targets like HD 123351. Aims. A detailed spectroscopic investigation of the lithium resonance doublet in HD 123351 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 6 Li isotope and to place constraints on its origin. Methods. We derive the lithium abundance A(Li) and the 6 Li/ 7 Li 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 CO 5 BOLD 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 6 Li/ 7 Li = 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 HD 123351. Two other line lists lead to similar results but with inferior fit qualities. Conclusions. Our 2 σ detection of the 6 Li 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 6 Li 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 6 Li in HD 123351.

A detailed study of lithium in 107 CHEPS dwarf stars

“…Generally speaking, NLTE computations within 3D model atmospheres exist, and may be more reliable. However, direct comparison of the 3D and 1D lithium abundance determination results shows that the differences do not exceed 0.02−0.04 dex for the case of red giants (Mott et al 2017), (see discussions on LTE and NLTE in 1D and 3D atmospheres in Asplund et al 2003;Mott et al 2017;Klevas et al 2016). …”

“…First of all, we note that in our case, the effective resolution is restricted by the effects of absorption lines broadening, not only by instrumental broadening and rotation, but also by the macroturbulent motions generated by the convective envelope. The effects of macroturbulence have been well studied for the case of solar atmosphere, and it can be explained using more advanced, but very CPU consumable, 3D modelling (see Mott et al 2017, and references therein). We work in the framework of 1D modelling methods, and to describe the general profile of the Li resonance blend formed by 7 Li+ 6 Li lines, we used a more sophisticated approach in the treatment of the macroturbulent velocity distribution in comparison to the simple Gaussian used in the lithium abundance determination procedure.…”

“…Unfortunately, our spectra are not of sufficiently high S/N to determine 7 Li/ 6 Li ratio with high precision. Usually, the 7 Li/ 6 Li isotopic ratio is determined from analysis of the spectra of much higher S /N ∼ 600 (see Mott et al 2017). Still, for both of the stars we can determine the lower limit of 7 Li/ 6 Li, meaning 7 Li/ 6 Li > 10.…”

Observational constraints on the origin of the elements

“…Consequently, very accurate measurements of the solar abundance are needed that employ sophisticated model atmospheres and spectrum synthesis techniques such as three-dimensional (3D) hydrodynamics and non-local thermodynamic equilibrium (non-LTE) physics. In recent years, with the development of faster and larger computers, it has been possible to develop and implement these methods (Asplund et al 2003;Steffen et al 2015;Klevas et al 2016;Amarsi et al 2016;Mott et al 2017;Amarsi & Asplund 2017;Nordlander et al 2017).…”

The Pristine survey – V. A bright star sample observed with SOPHIE

“…This is not uncommon for metal-rich stars to have a high Li abundace (see e.g. Mott et al 2017). Its A(Li)-LTE abundance is of 2.80 ± 0.10; the 3D-NLTE corretion of 0.05 (Harutyunyan et al 2018) is small when compared to the uncertainty related to the quality of this spectrum.…”