KIC 11911480: the second ZZ Ceti in the Kepler field

Abstract

We report the discovery of the second pulsating hydrogen-rich (DA) white dwarf in the Kepler field, KIC 11911480. It was selected from the Kepler-INT Survey (KIS) on the basis of its colours and its variable nature was confirmed using ground-based time series photometry. An atmosphere model fit to an intermediate-resolution spectrum of KIC 11911480 places this DA white dwarf close to the blue edge of the empirical boundaries of the ZZ Ceti instability strip: Teff = 12 160 ± 250 K and log g = 7.94 ± 0.10. Assuming a mass–radius relation and cooling models for DA white dwarfs, the atmospheric parameters yield: MWD = 0.57 ± 0.06 M⊙. We also obtained two quarters (Q12 and Q16) of nearly uninterrupted short-cadence Kepler data on this star. We detect a total of six independent pulsation modes with a ≥3σ confidence in its amplitude power spectrum. These pulsations have periods ranging between 172.9 and 324.5 s, typical of the hotter ZZ Ceti stars. Our preliminary asteroseismic study suggests that KIC 11911480 has a rotation rate of 3.5±0.5 days.