Stars that Move Together Were Born Together
  It is challenging to reliably identify stars that were born together outside
of actively star-forming regions and bound stellar systems. However, co-natal
stars should be present throughout the Galaxy, and their demographics can shed
light on the clustered nature of star formation and the dynamical state of the
disk. In previous work we presented a set of simulations of the Galactic disk
that followed the clustered formation and dynamical evolution of 4 billion
individual stars over the last 5 Gyr. The simulations predict that a high
fraction of co-moving stars with physical and 3D velocity separation of $\Delta
r < 20$ pc and $\Delta v < 1.5$ km s$^{-1}$ are co-natal. In this
\textit{Letter}, we use \textit{Gaia} DR2 and LAMOST DR4 data to identify and
study co-moving pairs. We find that the distribution of relative velocities and
separations of pairs in the data is in good agreement with the predictions from
the simulation. We identify 111 co-moving pairs in the Solar neighborhood with
reliable astrometric and spectroscopic measurements. These pairs show a strong
preference for having similar metallicities when compared to random field
pairs. We therefore conclude that these pairs were very likely born together.
The simulations predict that co-natal pairs originate preferentially from
high-mass and relatively young ($< 1$ Gyr) star clusters. \textit{Gaia} will
eventually deliver well-determined metallicities for the brightest stars,
enabling the identification of thousands of co-natal pairs due to disrupting
star clusters in the solar neighborhood.
