Formation of planetary debris discs around white dwarfs – I. Tidal disruption of an extremely eccentric asteroid

Abstract

25%-50% of all white dwarfs (WDs) host observable and dynamically active\nremnant planetary systems based on the presence of close-in circumstellar dust\nand gas and photospheric metal pollution. Currently-accepted theoretical\nexplanations for the origin of this matter include asteroids that survive the\nstar's giant branch evolution at au-scale distances and are subsequently\nperturbed onto WD-grazing orbits following stellar mass loss. In this work we\ninvestigate the tidal disruption of these highly-eccentric (e > 0.98) asteroids\nas they approach and tidally disrupt around the WD. We analytically compute the\ndisruption timescale and compare the result with fully self-consistent\nnumerical simulations of rubble piles by using the N-body code PKDGRAV. We find\nthat this timescale is highly dependent on the orbit's pericentre and largely\nindependent of its semimajor axis. We establish that spherical asteroids\nreadily break up and form highly eccentric collisionless rings, which do not\naccrete onto the WD without additional forces such as radiation or sublimation.\nThis finding highlights the critical importance of such forces in the physics\nof WD planetary systems.\n