Amplitude and frequency variability of the pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 observed with the Whole Earth Telescope

Abstract

We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single‐site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes, the amplitudes of which are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (≈40 s) suggests that they are probably caused by non‐radial gravity‐mode pulsations of spherical degree ℓ= 1. If so, PG 1654+160 has a mass around 0.6 M⊙. The time‐scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data.

AT LAST—A V777 HER PULSATOR IN THE KEPLER FIELD

“…Of the 20 known V777 Her pulsators, only a few have been subjected to the persistent follow-up observations required in order to assess their frequency spectra in any detail. Except for the well-studied prototype, only V1063 Tau and V824 Her have been subject to extensive multi-site campaigns (Handler et al 2003), and both stars were found to display substantial variations regarding which modes were excited and their total pulsational amplitude. Of those two, V824 Her was found to exhibit roughly equidistant periods with a mean spacing of ∼40 s. A mean period spacing of 37.1 s was also found from extensive single-site data of SW LMi (Handler et al 2002), and in both cases the obvious interpretation is that of a sequence of g-mode pulsations of degree = 1 in a nominal mass (∼0.6 M ) DB.…”

The pulsating DA white dwarf star EC 14012−1446: results from four epochs of time-resolved photometry

“…Another potential problem is the occurrence of amplitude and frequency variations on short time-scales (e.g. Handler et al 2003), resulting in spurious peaks that complicate the identification of normal modes. On the other hand, the temporal variations of the pulsational spectra of some ZZ Ceti stars may make modes that are not permanently observable, visible at some periods of observation.…”