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.

中文翻译：

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白矮星周围潮汐碎片的环化：对气体产生和尘埃变异的影响

白矮星 (WD) 污染被认为是由行星体的潮汐破坏引起的。最初的碎片流非常古怪，而观察证据表明圆盘是圆形的或几乎是圆形的。在这里，我们提出了一种新的机制来弥合这一差距，并表明当碎片与预先存在的光盘相互作用时，它们可以通过灰尘或气体阻力快速循环。我们假设潮汐流主要由大小范围为 10-1000 m 的小粘性碎片组成，能够抵抗 WD 潮汐力，而光盘的质量范围很广。我们提供了一个分析模型，伴随着 N 体模拟，并在碎片大小和轨道分离方面找到了一个大的参数空间，导致完全环化。对于质量小几个数量级的光盘，部分圆化是可能的。我们表明，随着潮汐碎片沿着它们的路径碰撞汽化预先存在的尘埃，尘埃引起的环化本质上会产生气体。我们表明，在潮汐破坏事件的早期阶段，持续的天然气生产发生的可能性更高，这是因为较小的碎片首先被环化。然而，随着潮汐流的成熟，间歇性的天然气生产变得更有可能。这可以解释为什么只有一小部分带有灰尘光盘的系统也有观察到的气体成分。此外，相互作用通过碰撞破碎、溅射、升华和可能的撞击压力产生碎片侵蚀。