From birth associations to field stars: mapping the small-scale orbit
  distribution in the Galactic disc
  Stars born at the same time in the same place should have formed from gas of
the same element composition. But most stars subsequently disperse from their
birth siblings, in orbit and orbital phase, becoming 'field stars'. Here we
explore and provide direct observational evidence for this process in the Milky
Way disc, by quantifying the probability that orbit-similarity among stars
implies indistinguishable metallicity. We define the orbit similarity among
stars through their distance in action-angle space, $\Delta (J,\theta)$, and
their abundance similarity simply by $\Delta$[Fe/H]. Analyzing a sample of main
sequence stars from Gaia DR2 and LAMOST, we find an excess of pairs with the
same metallicity ($\Delta\mathrm{[Fe/H]}<0.1$) that extends to remarkably large
separations in $\Delta (J,\theta)$ that correspond to nearly 1 kpc distances.
We assess the significance of this effect through a mock sample, drawn from a
smooth and phase-mixed orbit distribution. Through grouping such star pairs
into associations with a friend-of-friends algorithm linked by $\Delta
(J,\theta)$, we find 100s of mono-abundance groups with $\ge 3$ (to $\gtrsim
20$) members; these groups -- some clusters, some spread across the sky -- are
over an order-of-magnitude more abundant than expected for a smooth phase-space
distribution, suggesting that we are witnessing the 'dissolution' of stellar
birth associations into the field.
