G 112-43/44: A metal-poor binary star with a unique chemical composition and Helmi stream kinematics

Abstract

Context. Correlations between high-precision elemental abundance ratios and the kinematics of halo stars provide interesting information about the formation and early evolution of the Galaxy. Aims. Element abundances of G 112-43/44, a metal-poor wide-orbit binary star with extreme kinematics, are revisited. Methods. High-precision studies of the chemical compositions of 94 metal-poor dwarf stars in the solar neighbourhood are used to compare abundance ratios for G 112-43/44 with ratios for stars that have similar metallicities, taking into account the effect of deviations from local thermodynamic equilibrium on the derived abundances. Gaia EDR3 data are used to compare the kinematics. Results. The X/Fe abundance ratios of the two components of G 112-43/44 agree within ±0.05 dex for nearly all elements, but there is a hint of a correlation between the difference in [X/H] and the elemental condensation temperature, which may be due to planet-star interactions. The Mg/Fe, Si/Fe, Ca/Fe, and Ti/Fe ratios of G 112-43/44 agree with the corresponding ratios for accreted (Gaia-Enceladus) stars, but Mn/Fe, Ni/Fe, Cu/Fe, and Zn/Fe are significantly enhanced, with Δ [Zn/Fe] reaching 0.25 dex. The kinematics show that G 112-43/44 belongs to the Helmi streams in the solar neighbourhood. In view of this, we discuss if the abundance peculiarities of G 112-43/44 can be explained by chemical enrichment from supernova events in the progenitor dwarf galaxy of the Helmi streams. Interestingly, yields calculated for a helium shell detonation Type Ia supernova model can explain the enhancement of Mn/Fe, Ni/Fe, Cu/Fe, and Zn/Fe in G 112-43/44 and three other α-poor stars in the Galactic halo with abundances from the literature, one of which has Helmi stream kinematics. However, the helium shell detonation model also predicts enhanced abundance ratios of Ca/Fe, Ti/Fe, and Cr/Fe, in disagreement with the observed ratios.