On the new mechanism of planetary long-period debris formation around white dwarfs

To explain the phenomenon of metal pollution of white dwarfs (WD) photospheres, we compared three main fragmentation mechanisms of small bodies (SB): tidal force, thermal destruction, and sublimation when SB fall on to WD along star-grazing orbits. The temperatures of the WDs lie in the range of 3000-15 000 K. We consider two materials, using their internal strength: crystalline ice and chondrite. We show that inside the Roche limit, ice bodies (ISB) ranging in size from 60 m to 150 km are destroyed by tidal forces. The corresponding sizes of stony bodies (SSB) range from 90 m to 130 km. Bodies of centimetre size are subject to sublimation. The thermal destruction mechanism is effective for bodies whose size lies in the interval where tidal forces and sublimation are not so effective, destroying SSBs smaller than 50 m and ISBs smaller than 1 km near stars with T-eff <= 15 000 K. Such bodies are totally destroyed by thermal tensile stresses long before they reach the Roche limit. There may be observable manifestations of SB falling in the form of short-term flashes of the order of a second from SSB with sizes <= 100 m and WD curtaining with dust tails from ISB, causing WD dimming for a short time of the order of an hour. We conjecture that SB, moving along elongated elliptical orbits at large distances from the star, disintegrates by thermal destruction. The fragments from debris discs have nothing to do with the Roche limit.