Post main sequence evolution of icy minor planets: Implications for water retention and white dwarf pollution

TL;DR

This study models the evolution of icy minor planets during stellar evolution to understand water retention and its implications for white dwarf pollution, revealing that water often survives but is rarely detected due to dissociation during disruption.

Contribution

It provides a detailed, comprehensive model of icy minor planets' thermal, physical, and chemical evolution through stellar phases, improving understanding of water retention and white dwarf pollution.

Findings

Water can survive in icy minor planets through stellar evolution.

Most water is likely dissociated during tidal disruption, explaining low detection rates.

Future water-rich white dwarf atmospheres are expected to be helium-dominated.

Abstract

Most observations of polluted white dwarf atmospheres are consistent with accretion of water depleted planetary material. Among tens of known cases, merely two cases involve accretion of objects that contain a considerable mass fraction of water. The purpose of this study is to investigate the relative scarcity of these detections. Based on a new and highly detailed model, we evaluate the retention of water inside icy minor planets during the high luminosity stellar evolution that follows the main sequence. Our model fully considers the thermal, physical, and chemical evolution of icy bodies, following their internal differentiation as well as water depletion, from the moment of their birth and through all stellar evolution phases preceding the formation of the white dwarf. We also account for different initial compositions and formation times. Our results show that previous studies have either underestimated or overestimated water retention. We also reaffirm that water can survive in a variety of circumstances and in great quantities, and therefore other possibilities are discussed in order to explain the infrequency of water detections. We find that the most likely explanation is that water does not survive the dissociation of tidally disrupted minor planets, and instead a direct impact onto the white dwarf is required. We show that this possibility is statistically plausible, and predict that if this hypothesis is correct, future observations of extremely water rich atmospheres will involve only helium dominated DB white dwarfs.