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Stars born at the same time in the same place should have formed from gas ofthe same element composition. But most stars subsequently disperse from theirbirth siblings, in orbit and orbital phase, becoming 'field stars'. Here weexplore and provide direct observational evidence for this process in the MilkyWay disc, by quantifying the probability that orbit-similarity among starsimplies indistinguishable metallicity. We define the orbit similarity amongstars through their distance in action-angle space, $\Delta (J,\theta)$, andtheir abundance similarity simply by $\Delta$[Fe/H]. Analyzing a sample of mainsequence stars from Gaia DR2 and LAMOST, we find an excess of pairs with thesame metallicity ($\Delta\mathrm{[Fe/H]}<0.1$) that extends to remarkably largeseparations in $\Delta (J,\theta)$ that correspond to nearly 1 kpc distances.We assess the significance of this effect through a mock sample, drawn from asmooth and phase-mixed orbit distribution. Through grouping such star pairsinto associations with a friend-of-friends algorithm linked by $\Delta(J,\theta)$, we find 100s of mono-abundance groups with $\ge 3$ (to $\gtrsim20$) members; these groups -- some clusters, some spread across the sky -- areover an order-of-magnitude more abundant than expected for a smooth phase-spacedistribution, suggesting that we are witnessing the 'dissolution' of stellarbirth associations into the field.

From birth associations to field stars: mapping the small-scale orbit distribution in the Galactic disc