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Abstract

With typical periods of the order of 10 minutes, the pulsation signatures of ZZ Ceti variables (pulsating hydrogen-atmosphere white dwarf stars) are severely undersampled by long-cadence (29.42 minutes per exposure) K2 observations. Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling precision asteroseismology. We report the discovery of two new ZZ Cetis in long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by three to four nights of follow-up, high-speed (<= 30 s) photometry from the McDonald Observatory, we recover accurate pulsation frequencies for K2 signals that reflected four to five times off the Nyquist with the full precision of over 70 days of monitoring (~0.01 μHz). In turn, the K2 observations enable us to select the correct peaks from the alias structure of the ground-based signals caused by gaps in the observations. We identify at least seven independent pulsation modes in the light curves of each of these stars. For EPIC 220274129, we detect three complete sets of rotationally split l=1 (dipole mode) triplets, which we use to asteroseismically infer the stellar rotation period of 12.7 ± 1.3 hr. We also detect two sub-Nyquist K2 signals that are likely combination (difference) frequencies. We attribute our inability to match some of the K2 signals to the ground-based data to changes in pulsation amplitudes between epochs of observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC 220274129 near the middle of the ZZ Ceti instability strip, with T_eff =11590\pm 200 K and 11810 ± 210 K, and masses 0.57 ± 0.03 M_⊙ and 0.62 ± 0.03 M_⊙, respectively.