S. J. Kleinman

Understanding the Cool DA White Dwarf Pulsator, G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators.

Whole Earth Telescope Observations of the Interacting White Dwarf Binary System AM CVn: First Results

We report the first results of the Whole Earth Telescope observations of AM CVn in March/April 1990. The Fourier Spectrum of the light curve shows harmonically related peaks. High frequency sidebands with the fine-splitting of 21 µHz are observed for the fundamental period of 1051 s and its 4 lowest harmonics. These have not been observed before. The fundamental period itself is not detected.

A Measurement of the Evolutionary Timescale of the Cool White Dwarf G117-B15A with Wet

We have measured the cooling rate of the 13,000K DA white dwarf G117-B15A by measuring the rate of period change with time for the main pulsation of this ZZ Ceti star, using the Whole Earth Telescope. The observed rate of period change is larger than the predictions of g-mode pulsation theory applied to C/O core white dwarf models.

Whole Earth Telescope Observations of the DBV White Dwarf PG1115+158: Preliminary Results

We present preliminary results from Whole Earth Telescope (WET) data on the DBV star PG1115+158. In the Fourier Transform of this data, we have tentatively identified peaks arranged in multiplets with common frequency splitting. The average period spacing between these multiplets is ~ 40s, very similar to the multiplet spacing found in the DBV GD358. Theoretical models show l = 1 modes with ~ 40s period spacing between modes of consecutive k for white dwarfs with niasses near 0.6M ⊙. If we assume the identified multiplets are l = 1 modes, then the rotation rate of the star is 0.45 days. Furthermore, the deviations of the period spacing between modes from the mean period spacing indicates that, like GD358, the surface Helium layer is thin, less than 10-4 M *.

The intermediate polar PQ Gem-status of WET observations of 1996

A summary of the results on PQ Gem based on WET observations conducted in February 1996 is presented. The 13.9 min spin period of the white dwarf in the system and the orbital side-band at 14.5 min have been detected unambiguously. Searches for other periods in the system gave negative results.

Observation of a Variable, ZZ Ceti White Dwarf: GD154

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K–11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100–1000 seconds.