G112-43/44: A metal-poor binary star with a unique chemical composition
  and kinematics like the Helmi streams
  G112-43/44, alias BD+00_2058 A and B, is a metal-poor ([Fe/H] = -1.3)
wide-orbit binary star with extreme kinematics. We use high-precision
determinations of the chemical compositions of 94 metal-poor dwarf stars in the
solar neighbourhood to compare abundance ratios for G112-43/44 with ratios for
stars having similar metallicity taking into account the effect of deviations
from local thermodynamic equilibrium on the derived abundances, and Gaia EDR3
data are used to compare the kinematics. The abundances of the two components
of G112-43/44 agree within 0.05 dex for nearly all elements, but there is a
hint of a correlation of the difference in [X/H] with elemental condensation
temperature, which may be due to planet-star interactions. The Mg/Fe, Si/Fe,
Ca/Fe, and Ti/Fe ratios of G112-43/44 agree with the corresponding ratios for
accreted (Gaia-Enceladus) stars, but Mn/Fe, Ni/Fe, Cu/Fe, and Zn/Fe are
significantly enhanced. The kinematics show that G112-43/44 belongs to the
Helmi streams in the solar neighbourhood and in view of this, we discuss if the
abundance peculiarities of G112-43/44 can be explained by chemical enrichment
from supernovae 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 G112-43/44 and three other alpha-poor stars in the Galactic halo, one of
which have Helmi streams kinematics. The helium shell detonation model
predicts, however, also enhanced abundance ratios of Ca/Fe, Ti/Fe, and Cr/Fe in
disagreement with the observed ratios.
