LAMOST J011939.222−012150.45: The most barium-enhanced CEMP-s turnoff star

Abstract

We have performed chemical abundance analyses for a newly discovered metal-poor turn-off star (T-eff = 6276K, logg = 3.93, [Fe/H] = -2.93), LAMOST J011939.222-012150.45, based on high-resolution and high signal-to-noise ratio spectra in both optical and near-UV obtained by Subaru. Abundances have been derived for 20 elements, including 11 light elements such as C, N, Na, Mg, etc., and 9 neutron-capture elements from Sr to Pb. This object is a carbon-enhanced metal-poor star with a large carbon excess of [C/Fe] = +2.26. LAMOST J011939.222-012150.45 shows extreme enhancement in s-process elements, especially for Ba, La, and Pb ([Ba/Fe] = +3.16 +/- 0.18, [La/Fe] = +2.29 +/- 0.24, [Pb/Fe] = +3.38 +/- 0.12). A very clear radial velocity variation has also been detected, providing evidence of the existence of a companion. Interestingly, even without any scaling, the observed abundance pattern from light to heavy neutron-capture elements agrees well with predictions of accretion from a companion asymptotic giant branch (AGB) star. Considering the evolutionary status of this object, its surface material is very likely to be completely accreted from its AGB companion and has been preserved until today.