The dust never settles: collisional production of gas and dust in evolved planetary systems

TL;DR

This study uses multi-epoch infrared observations to reveal that collisional processes in evolved planetary systems cause significant variability in circumstellar dust and gas, challenging traditional disc models.

Contribution

It demonstrates that optically thin dust driven by collisions explains infrared variability, highlighting the role of planetesimal collisions in debris discs around white dwarfs.

Findings

Infrared flux varies by tens of percent over years.

Collisional dust production correlates with gaseous debris presence.

Variability is linked to planetesimal collision activity.

Abstract

Multi-epoch infrared photometry from Spitzer is used to monitor circumstellar discs at white dwarfs, which are consistent with disrupted minor planets whose debris is accreted and chemically reflected by their host stars. Widespread infrared variability is found across the population of 37 stars with two or more epochs. Larger flux changes occur on longer time-scales, reaching several tens of per cent over baselines of a few years. The canonical model of a geometrically thin, optically thick disc is thus insufficient, as it cannot give rise to the observed behaviour. Optically thin dust best accounts for the variability, where collisions drive dust production and destruction. Notably, the highest infrared variations are seen in systems that show Ca II emission, supporting planetesimal collisions for all known debris discs, with the most energetic occurring in those with detected gaseous debris. The sample includes the only polluted white dwarf with a circumbinary disc, where the signal of the day-night cycle of its irradiated substellar companion appears diluted by dust emission.