The distribution of relative proper motions of wide binaries in Gaia DR2: MOND or multiplicity?

We examine the distribution of on-sky relative velocities for wide binaries previously assembled from the Gaia DR2 data and focus on the origin of the high-velocity tail of apparently unbound systems, which may be interpreted as evidence for non-Newtonian gravity in the weak field limit. We argue that this tail is instead explicable in terms of a population of hidden triples, i.e. cases where one of the components of the wide binary is itself a close binary unresolved in the Gaia data. In this case, the motion of the photocentre of the inner pair relative to its barycentre affects the apparent relative proper motion of the wide pair and can make pairs that are in fact bound appear to be unbound. We show that the general shape of the observed distributions can be reproduced using simple observationally motivated assumptions about the population of hidden triples.