  Globular clusters exhibit abundance variations, defining `multiple
populations', which have prompted a protracted search for their origin.
Properties requiring explanation include: the high fraction of polluted stars
($\sim 40{-}90$~percent, correlated with cluster mass), the absence of
pollution in young clusters and the lower pollution rate with binarity and
distance from the cluster centre. We present a novel mechanism for late
delivery of pollutants into stars via accretion of sub-stellar companions. In
this scenario, stars move through a medium polluted with AGB and massive star
ejecta, accreting material to produce companions with typical mass ratio $q\sim
0.1$. These companions undergo eccentricity excitation due to dynamical
perturbations by passing stars, culminating in a merger with their host star.
The accretion of the companion alters surface abundances via injected
pollutant. Alongside other self-enrichment models, the companion accretion
model can explain the dilution of pollutant and correlation with intra-cluster
location. The model also explains the ubiquity and discreteness of the
populations and correlations of enrichment rates with cluster mass, cluster age
and stellar binarity. Abundance variations in some clusters can be broadly
reproduced using AGB and massive binary ejecta abundances from the literature.
In other clusters, some high companion mass ratios ($q\gtrsim 1$) are required.
In these cases, the available mass budget necessitates a variable degree of
mixing of the polluted material with the primary star, deviations from model
ejecta abundances or mixing of internal burning products. We highlight the
avenues of further investigation which are required to explore some of the key
processes invoked in this model.
