Accretion Rate Changes Detected in a Polluted White Dwarf

This letter reports statistically significant changes in the equivalent widths of MgII and CaII lines in the dusty and polluted white dwarf WD 0106-328, based on six epochs of spectroscopy using the VLT and Keck spanning 25 yr. Furthermore, the ratio of these two equivalent widths may also vary, with a 7% probability of being constant. Between 2000 and 2025, both Mg and Ca have experienced decreases in accretion rates, of approximately 20 and 60%, 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 timescales 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 disk of metals, consistent with $α\approx0.1$ 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, disk evolution, and be observed on relatively short timescales.

💡 Research Summary

This paper presents the first robust detection of temporal variations in the metal absorption lines of a polluted white dwarf, WD 0106‑328, using six spectroscopic epochs spanning 25 years (2000–2025) obtained with the VLT (UVES, X‑shooter) and Keck (HIRES). The authors focus on the Mg II 4482 Å and Ca II K lines, measuring equivalent widths (EWs) with high precision via Gaussian profile fitting and Markov‑Chain Monte Carlo (EMCEE) methods, incorporating both statistical and systematic uncertainties (continuum placement, instrument offsets).

Statistical analysis (χ² tests) shows that the EWs of both Ca II and Mg II are highly inconsistent with a constant value (p < 10⁻⁶ and p < 10⁻⁴ respectively), while their ratio has a 7 % probability of being constant, indicating genuine variability. Between the early UVES epochs (2000–2001) and the latest HIRES epoch (2025), Ca II EW declined by ~10 % and Mg II by ~18 %, corresponding to reductions in inferred accretion rates of roughly 20 % (Mg) and 60 % (Ca). The decline is not monotonic; intermediate epochs (2008, 2018) show temporary rebounds, suggesting a fluctuating supply rather than a smooth decay.

Complementary HST/COS ultraviolet spectroscopy (G130M, R ≈ 16 000) reveals the presence of O, Si, and Fe in addition to Mg and Ca. Atmospheric modeling (Koester 2010, 2020) constrained the stellar parameters to Teff ≈ 16 100 K, log g ≈ 8.01, M★ ≈ 0.61 M⊙, R★ ≈ 0.013 R⊙, and a distance of 68.9 pc (consistent with Gaia eDR3). Derived photospheric abundances show Fe dominating the accreted material by mass (