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

Abstract

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.

White dwarf planetary debris dependence on physical structure distributions within asteroid belts

“…We can provide very rough estimates for the lifetime of a disc of fragments after they have circularized enough Nixon et al 2020;Malamud et al 2021) such that their eccentricity and inclination dispersions are less than about 0.1 rad. In this case, if the fragments are assumed to be equal rocky spheres, then we can read off the disc lifetimes from the appropriate figures in Veras & Heng (2020).…”

“…Further, the process of sublimation is complicated if there is a pre-existing debris disc around the white dwarf. If an incoming body can be captured and embedded into the debris disc (Grishin & Veras 2019;O'Connor & Lai 2020;Malamud et al 2021) it can be shielded from sublimative effects. Since the disc will not be isothermal, the optical depth will vary throughout the disc.…”

“…An inherent assumption in our study is that an incoming asteroid does not first impact an extant disc. This important scenario has been considered in several contexts and different regions of parameter space (Grishin & Veras 2019;O'Connor & Lai 2020;Malamud et al 2021), and may help to explain the origin of the planetesimal orbiting SDSS J1228+1040.…”

“…Further, the interaction of an incoming asteroid with an existing debris disc can further produce gaseous material (Malamud et al 2021). Smaller dust grains entering into a disc can collide with other dust grains resulting in compression shock vaporisation.…”

Rapid destruction of planetary debris around white dwarfs through aeolian erosion

“…Metzger et al 2012), grain-grain collisional vaporization and sputtering. Recently, Malamud, Grishin, & Brouwers (2020) presented another channel of gas production in white dwarf disks, via interactions between an eccentric tidal stream and a pre-existing dusty compact disk.…”