Distant Relatives: The Chemical Homogeneity of Comoving Pairs Identified in Gaia

Abstract

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.