Spectroscopic Monitoring of Metal Lines in Polluted White Dwarfs

The disruption and accretion of planetary material onto white dwarfs are expected to be inherently dynamic and stochastic, potentially driving variability in the accretion rate and therefore the shape and depth of the photospheric metal absorption lines. This paper presents an 18 yr optical spectroscopic monitoring campaign of five warm (11,000–23,000 K), polluted white dwarfs with sinking timescales of days–months, observed using Magellan/MIKE and SALT/the High Resolution Spectrograph to directly test this prediction. At four of the five systems, no statistically significant variability is detected over baselines of 15–18 yr corresponding to hundreds to thousands of diffusion timescales, with inferred accretion rates stable to within 15%–30% (1 σ ) showing remarkably stable accretion on decadal timescales. This implies that either the processes maintaining the accretion of the disrupted planetary material are stable on the same timescales or currently uncharacterized photospheric processes act to smooth observable abundance variations on these timescales. The one exception, WD 0106−328, shows statistically significant variability in the 4481 Å Mg ii doublet from the ground-based data. Yet no significant equivalent width or abundance changes are seen between two Hubble Space Telescope ultraviolet spectra taken in 2016 and 2025, despite probing a larger set of transitions. This may imply that the ground-based observations witnessed a stochastic excursion from a stable baseline accretion rate, rather than a sustained change in the bulk accretion rate.