2023MNRAS.526.5976S

We report the serendipitous discovery of a very metal-poor (VMP) Li-rich giant star (T_eff = 4690 ± 80 K, log g = 1.34 ± 0.13, [Fe/H] = -2.43 ± 0.07). We analyse the Li I 6103 and 6707 Å lines accounting for departures from local thermodynamic equilibrium (NLTE) and correcting for 3D effects using literature data, which yields a lithium abundance log ε_Li = 3.42 ± 0.07. Comparing lithium abundances from the two lines, in 1D NLTE we measure the isotope ratio ⁶Li/⁷Li = 1.64$^{+1.49}_{-1.08}$ per cent. When correcting for 3D effects, we detect the fragile ⁶Li isotope at 2-sigma level and the ratio ⁶Li/⁷Li = 5.65$^{+5.05}_{-2.51}$ per cent. To our knowledge, this is the first ⁶Li/⁷Li measurement in an extremely Li-rich VMP star. The Cameron-Fowler mechanism, which is proposed to produce Li-rich stars, does not imply ⁶Li production and is therefore inconsistent with our measurement when applying 3D corrections. We also derive NLTE abundances for 16 elements, most of which show similar abundances to those found in VMP stars. Sodium is an exception: [Na/Fe]_NLTE1D = 0.07 ± 0.03, which is 0.5 dex higher than what is typical for VMP stars. This star joins the sample of rare Li-rich VMP stars, and we offer a novel way to constrain the source of lithium in such stars through isotope ratio measurements.