Accretion rate changes detected in a polluted white dwarf

ABSTRACT This letter reports statistically significant changes in the equivalent widths of ${\rm Mg\, {\small II}}$ and ${\rm Ca\, {\small II}}$ lines in the dusty and polluted white dwarf WD 0106–328, based on six epochs of spectroscopy using the Very Large Telescope and Keck spanning 25 yr. Furthermore, the ratio of these two equivalent widths may also vary, with a 7 per cent probability of being constant. Between 2000 and 2025, both Mg and Ca have experienced decreases in accretion rates of approximately 20 per cent and 60 per cent, respectively, but with individual variation during the interim. These metal abundance decreases are the first empirical corroboration of diffusion theory in white dwarfs, which predict sinking time-scales on the order of days for this star. However, the persistent atmospheric metals require a more gradual, circumstellar process, where one possibility is viscous spreading in an ionized disc of metals, consistent with $\alpha \approx 0.1$ within that formalism. The combination of optical and ultraviolet spectroscopy with the Hubble Space Telescope detects all the major rock-forming elements (O, Mg, Si, Fe), and demonstrates that Fe dominates the accreted material by mass, and that it is delivered mostly as pure metal from within a differentiated parent body. This inference is consistent with the possibility that chemically segregated accretion may result from a combination of planetary assembly, fragmentation, and disc evolution, and can be observed on relatively short time-scales.