C. F. Claver

Asteroseismology of the DOV star PG 1159 - 035 with the Whole Earth Telescope

Results are reported from 264.1 hr of nearly continuous time-series photometry on the pulsating prewhite dwarf star (DPV) PG 1159 - 035. The power spectrum of the data set is completely resolved into 125 individual frequencies; 101 of them are identified with specific quantized pulsation modes, and the rest are completely consistent with such modal assignment. It is argued that the luminosity variations are certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on the time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. The existing linear theory is invoked to determine, or strongly constrain, many of the fundamental physical parameters describing this star. Its mass is found to be 0.586 solar mass, is rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified.

Whole earth telescope observations of the DBV white dwarf GD 358

We report on the analysis of 154 hours of early continuous high-speed photometry on the pulsating DB white dwarf (DBV) GD 358, obtained during the Whole Earth Telescope (WET) run of 1990 May. The power spectrum of the light curve is dominated by power in the range from 1000 to 2400 microHz with more than 180 significant peaks in the total spectrum. We identify all of the triplet frequencies as degree l = 1, and from the details of their spacings we derive the total stellar mass as 0.61 + or - 0.03 solar mass, the mass of the outer helium envelope as 2.0 + or - 1.0 x 10(exp -6) M(sub *), the absolute luminosity as 0.050 + or - 0.012 solar luminosity and the distance as 42 + or - 3 pc. We find strong evidence for differential rotation in the radial direction -- the outer envelope is rotating at least 1.8 times faster than the core -- and we detect the presence of a weak magnetic field with a strength of 1300 + or - 300 G. We also find a significant power at the sums and differences of the dominant frequencies, indicating nonlinear processes are significant, but they have a richness and complexity that rules out resonant mode coupling as a major cause.

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.

The Masses of White Dwarfs in the Praesepe Open Cluster

We present results from an in-depth photometric and spectroscopic study of white dwarfs in the Praesepe open cluster. From high signal-to-noise ratio spectra, we have estimated log g and Teff for six DA white dwarfs using model atmosphere fits to the Balmer lines. Evolutionary models are then used to determine masses, radii, and cooling times. Good agreement is found with masses determined using gravitational redshifts primarily by Reid and with masses determined from the clusters distance. Included in these comparisons are white dwarfs analyzed in a similar way in the Hyades and Pleiades clusters. The cooling times and cluster ages are then used to determine initial masses (Mi) for each white dwarf. A monotonic initial-final mass relation (Mi-Mf) is determined for these and two other well-studied clusters, although there are indications that more envelope mass is lost in the asymptotic red giant branch thermal pulse phase than current theory predicts. One Praesepe object—LB 5893 (0836+197)—is anomalous in the sense that it has the highest Mf but the highest Teff and the smallest inferred Mi. LB 393 = EG 61 (0837+199) shows Zeeman-split lines indicative of a strong magnetic field, the first such white dwarf found in a star cluster.

A detection of the evolutionary time scale of the DA white dwarf G117- B15A with the whole earth telescope

The time rate of change for the main pulsation period of the 13,000 K DA white dwarf G117 - B15A has been detected using the Whole Earth Telescope (WET). The observed rate of period change, P(dot) = (12.0 + or - 3.5) x 10 to the -15th s/s, is somewhat larger than the published theoretical calculations of the rate of period change due to cooling, based on carbon core white dwarf models. Other effects that could contribute to the observed rate of period change are discussed.

Whole Earth Telescope observations of the white dwarf G29-38 : phase variations of the 615 second period

An extensive set of high-speed photometric observations obtained with the Whole Earth Telescope network is used to show that the complex light curve of the ZZ Zeti (DAV) star G29-38 is dominated by a single, constant amplitude period of 615 s during the time span of these observations. The pulse arrival times for this period exhibit a systematic variation in phase readily explained by light-travel time effects produced by reflex orbital motion about an unseen companion. The best-fit model to the observations indicates a highly eccentric orbit, a period of 109 + or - 13 days and a minimum mass of 0.5 solar mass for the companion. 23 refs.

Whole Earth Telescope observations of the pulsating hot white dwarf PG 1707+427

We report on the analysis of multisite time-series photometry of the pulsating pre-white dwarf (GW Vir star) PG 1707+427, obtained by the Whole Earth Telescope collaboration. This is the last of the known GW Vir stars without surrounding nebulae to be resolved by multisite data. Successful resolution of the pulsation spectrum resulted from the com- bination of high signal-to-noise observations with a large telescope and wide coverage in longitude with smaller telescopes. We find a series of 8 pulsation frequencies (along with two nonlinear combination frequencies), and identify 7 of them as part of a sequence of � = 1 modes, with a common period spacing of 23.0 s. This spacing implies that the mass of PG 1707+427 is 0.57 M� . Preliminary model fits suggest that the mass determined via asteroseismology is consistent with the mass determined from spectroscopy combined with evolutionary tracks.

High-speed photometric observations of the pulsating DA white dwarf GD 165

New high-speed photometric observations of the pulsating DA white dwarf GD 165 are presented. The Fourier spectrum of the light curve of GD 165 exhibits two main regions of power at 120 and 193 s. The presence of a high-amplitude long period mode near ∼1800 s reported by Bergeron and McGraw is not confirmed by these new observations. Light curves obtained with the Canada-France-Hawaii Telescope reveal previously undetected low-amplitude harmonic oscillations. Observations with the Whole Earth Telescope are used to resolve the two principal regions of power. The 120 and 193 s peaks are shown to be multiplets composed of at least three, and possibly five frequency components. The most likely explanation is that these two peaks correspond to nonradial gravity modes with different values of the radial order k and with l=1 or 2 split into 2l+1 components by slow rotation

Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations

Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

Blue edge of the ZZ Ceti instability strip versus mass

In this work we test the theoretical results derived from pulsation models in which the blue edge temperature of the ZZ Ceti instability strip depends on the stellar mass. By using the fundamental parameters (Te{f and M) and the results of the time series photometry of a sample of DA stars, we found all the ZZ Ceti stars in our sample but one (G 226-29) fall inside the instability strip if the theoretical blue edge is shifted by 300 K to higher temperature. Our biggest sources of uncertainty are in the Teff and M determinations, but by assuming that the relative ordering in the temperature among the ZZ Ceti stars is reliable, we conclude that the instability strip boundaries depend on stellar mass.