Distant Relatives: The Chemical Homogeneity of Comoving Pairs Identified
  in Gaia
  Comoving pairs, even at the separations of $\mathcal{O}(10^6)\,$AU, are a
predicted reservoir of conatal stars. We present detailed chemical abundances
of 62 stars in 31 comoving pairs with separations of $10^2 - 10^7\,$AU and 3D
velocity differences $< 2 \mathrm{\ km \ s^{-1}}$. This sample includes both
bound comoving pairs/wide binaries and unbound comoving pairs. Observations
were taken using the MIKE spectrograph on the Magellan/Clay Telescope at high
resolution ($\mathrm{R} \sim 45,000$) with a typical signal-to-noise ratio of
150 per pixel. With these spectra, we measure surface abundances for 24
elements, including Li, C, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, Sr, Y, Zr, Ba, La, Nd, Eu. Taking iron as the representative element,
our sample of wide binaries is chemically homogeneous at the level of $0.05$
dex, which agrees with prior studies on wide binaries. Importantly, even
systems at separations $2\times10^5-10^7\,$AU are homogeneous to $0.09$ dex, as
opposed to the random pairs which have a dispersion of $0.23\,$dex. Assuming a
mixture model of the wide binaries and random pairs, we find that $73 \pm 22\%$
of the comoving pairs at separations $2\times10^5-10^7\,$AU are conatal. Our
results imply that a much larger parameter space of phase space may be used to
find conatal stars, to study M-dwarfs, star cluster evolution, exoplanets,
chemical tagging, and beyond.
