Identical or fraternal twins? : The chemical homogeneity of wide
  binaries from Gaia DR2
  One of the high-level goals of Galactic archaeology is chemical tagging of
stars across the Milky Way to piece together its assembly history. For this to
work, stars born together must be uniquely chemically homogeneous. Wide binary
systems are an important laboratory to test this underlying assumption. Here we
present the detailed chemical abundance patterns of 50 stars across 25 wide
binary systems comprised of main-sequence stars of similar spectral type
identified in Gaia DR2 with the aim of quantifying their level of chemical
homogeneity. Using high-resolution spectra obtained with McDonald Observatory,
we derive stellar atmospheric parameters and precise detailed chemical
abundances for light/odd-Z (Li, C, Na, Al, Sc, V, Cu), $\alpha$ (Mg, Si, Ca),
Fe-peak (Ti, Cr, Mn, Fe, Co, Ni, Zn), and neutron capture (Sr, Y, Zr, Ba, La,
Nd, Eu) elements. Results indicate that 80% (20 pairs) of the systems are
homogeneous in [Fe/H] at levels below 0.02 dex. These systems are also
chemically homogeneous in all elemental abundances studied, with offsets and
dispersions consistent with measurement uncertainties. We also find that wide
binary systems are far more chemically homogeneous than random pairings of
field stars of similar spectral type. These results indicate that wide binary
systems tend to be chemically homogeneous but in some cases they can differ in
their detailed elemental abundances at a level of [X/H] ~ 0.10 dex, overall
implying chemical tagging in broad strokes can work.
