Amplitude Variability as Evidence of Crystallization in GD 518 and Other Massive Pulsating White Dwarfs

In 2013 March we discovered pulsations in the most massive pulsating hydrogen-atmosphere white dwarf to date, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a T-eff = 12,030 +/- 210 K, log g = 9.08 +/- 0.06 white dwarf, which corresponds to a mass of 1.20 +/- 0.03 M-circle dot. Such a massive WD should also be significantly crystallized at this temperature, and may possibly contain an oxygen neon core. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7% in a given night, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, although the pulsation amplitudes change drastically over the 33 days of our discovery observations. We investigate the possibility that these amplitude variations are a consequence of the pulsation modes sampling only the non-crystallized outer mass fraction of the white dwarf (perhaps < 0.05 M-circle dot of material), and thus have very low mode inertia. Amplitude variability could be an observational consequence of a significantly crystallized stellar interior.