White dwarf planets in star clusters: gravitational scattering versus mass-loss effects

ABSTRACT White dwarfs are unique laboratories for understanding the formation, evolution, and survivability of planetary systems. Post-main-sequence mass loss will change planetary orbital properties and stir up debris discs, leading to the observed pollution of white dwarf atmospheres. However, to date, very few studies have investigated the impact of the stellar birth environment on white dwarf planetary systems. In this paper we simulate the evolution of giant planets around white dwarf progenitors from their formation in a star-forming region until 1 Gyr, when the most massive stars (>2 M$_\odot$) have left the main sequence. Our simulations self-consistently model N-body interactions between stars and planets, as the stars evolve into white dwarfs within the cluster lifetime. We find that although scattering interactions in dense star-forming regions create free-floating planets, and alter the orbital properties of up to 20 per cent of the surviving planets, the effects of mass loss from the star dominate the dynamics. This behaviour is independent of the stellar density of the birth star-forming region, and largely independent of the initial planet orbital properties. Our simulations produce both captured planets around white dwarfs (potentially similar to WD 0806-661 b) and triple systems with white dwarfs and planets [potentially similar to PSR B1620-26 (AB) b], and our results yield a population synthesis of giant planets from 1 to 100 au that may be relevant to Roman, Gaia, and James Webb Space Telescope observations.