Exogeological inferences from white dwarf pollutants: the impact of stellar physics

TL;DR

This paper investigates how stellar physics assumptions affect the inferred composition of planetary material accreted onto white dwarfs, highlighting the importance of modeling choices in geological interpretations.

Contribution

It analyzes the impact of convective overshoot and thermohaline mixing assumptions on diffusion timescales, providing insights into more accurate exogeological inferences from white dwarf pollutants.

Findings

Including convective overshoot decreases inferred Fe and O in H-dominated white dwarfs.

Thermohaline mixing also reduces Fe and O abundances in H-dominated systems.

Choice of diffusion model grid significantly affects geological interpretations, especially for He-dominated white dwarfs.

Abstract

Many white dwarfs have accreted material from their own planetary systems. These objects can be used to infer the composition of exoplanetary material and identify evidence for key geological processes. However, the white dwarf atmospheric physics distorts the inferred material composition away from the true composition, mainly through differential atomic diffusion of the accreted metals. Correcting for this effect is essential, but is dependent on various physical assumptions associated with the white dwarf itself. We first focus on the effect of assumptions related to convective overshoot and thermohaline mixing on the atomic diffusion timescales. For white dwarfs with H-dominated atmospheres between 12000 K and 18000 K, we find that including a complete treatment of convective overshoot decreases the inferred Fe and O abundances in accreted material. For these white dwarfs, we also find that including thermohaline mixing decreases Fe and O abundances. For He-dominated systems, the effect of convective overshoot is comparatively minor. We then explore the overall effect of other physical assumptions by comparing publicly available grids of diffusion timescales. We find that the choice of model grid can have a large impact for white dwarfs with He-dominated atmospheres, notably on the inferred core to mantle ratio of accreted material. We identify several systems for which the geological interpretation is robust against these systematics. We also present a `discrepancy metric' which can be used to estimate the potential impact of changing the stellar physics without requiring detailed modelling.