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White dwarf (WD) pollution is thought to arise from the tidal disruption ofplanetary bodies. The initial fragment stream is extremely eccentric, whileobservational evidence suggest that discs are circular or nearly so. Here wepropose a novel mechanism to bridge this gap and show that the fragments canrapidly circularise through dust or gas drag when they interact with apre-existing compact disc. We assume that the tidal stream mainly consists ofsmall cohesive fragments in the size range 10-1000 m, capable of resisting theWD tidal forces, whereas the compact discs span a wide mass range. We providean analytical model, accompanied by N-body simulations, and find a largeparameter space in fragment sizes and orbital separation that leads to fullcircularization. Partial circularization is possible for compact discs that areseveral orders of magnitudes less massive. We show that dust-inducedcircularization inherently produces gas as tidal fragments collisionallyvaporize the pre-existing dust along their path. We show that ongoing gasproduction has a higher probability to occur during the early stages of tidaldisruption events, resulting from the fact that smaller fragments are the firstto circularize. Intermittent gas production however becomes more likely as thetidal stream matures. This could explain why only a small subset of systemswith 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 collisionsmight form a thin dusty halo that evolves through PR drag, in compatibilitywith observed infrared variability.

Circularization of tidal debris around white dwarfs: implications for gas production and dust variability