  White dwarf (WD) pollution is thought to arise from the tidal disruption of
planetary bodies. The initial fragment stream is extremely eccentric, while
observational evidence suggest that discs are circular or nearly so. Here we
propose a novel mechanism to bridge this gap and show that the fragments can
rapidly circularise through dust or gas drag when they interact with a
pre-existing compact disc. We assume that the tidal stream mainly consists of
small cohesive fragments in the size range 10-1000 m, capable of resisting the
WD tidal forces, whereas the compact discs span a wide mass range. We provide
an analytical model, accompanied by N-body simulations, and find a large
parameter space in fragment sizes and orbital separation that leads to full
circularization. Partial circularization is possible for compact discs that are
several orders of magnitudes less massive. We show that dust-induced
circularization inherently produces gas as tidal fragments collisionally
vaporize the pre-existing dust along their path. We show that ongoing gas
production has a higher probability to occur during the early stages of tidal
disruption events, resulting from the fact that smaller fragments are the first
to circularize. Intermittent gas production however becomes more likely as the
tidal stream matures. This could explain why only a small subset of systems
with dusty compact discs also have an observed gaseous component. Additionally,
the interaction yields fragment erosion by collisional shattering, sputtering,
sublimation and possibly ram-pressure. Material scattered by the collisions
might form a thin dusty halo that evolves through PR drag, in compatibility
with observed infrared variability.
