Harry S. Shipman

The pulsation modes of the pre-white dwarf PG 1159-035

Context. PG 1159-035, a pre-white dwarf with Teff � 140 000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star. Aims. We have three principal aims: to increase the number of detected and identified pulsation modes in PG 1159-035, study trapping of the star’s pulsation modes, and to improve or constrain the determination of stellar parameters. Methods. We used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods. Results. We identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M/M� = 0.59 ± 0.02 for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at rc/R� = 0.83 ± 0.05. From the multiplet splitting, we calculated the rotational period Prot = 1.3920 ± 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for � = 1 modes and less than 50% for � = 2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.

Watching ZZ Ceti evolve

We report preliminary results from our analysis of the stability of periods observed in the pulsating hydrogen atmosphere white dwarf ZZ Ceti (R548) based on observations that span 37 years from 1970 to 2007. We determine the rate of change of period with time to be dP/dt = (0.8±1.9)×10-15 s/s using the O-C method and dP/dt = (4.3±1.2)×10-15 s/s using the direct non-linear least squares fit NLSPDOT for the dominant period 213.13260643 s after correcting for proper motion. We do not claim either of these values as a measurement at this time, but hope to arrive at a conclusive result in the near future with more observations. The reduced uncertainty for both methods shows the improvement we obtained over the previous evolutionary constraint on ZZ Ceti (Mukadam et al. 2003). These dP/dt values are consistent within uncertainties with the measurement of dP/dt = (3.57 ±0.82)×10-15 s/s for the period 215.2 s observed in another pulsating white dwarf G117-B15A (Kepler et al. 2005). Using the 213 s triplet spacing of 4μHz, we compute the rotation period of ZZ Ceti to be 1.5 days.

Searching for Planets with White Dwarf Pulsations: Spurious Detections

We present 13 years of pulsation timing measurements of the DBV white dwarf EC 20058−5234. Each of the four O—C diagrams mimic the sinusoidal behavior typically attributed to a planet+WD system. However, the amplitude and phase of the O—C variations are inconsistent with each other. We discuss the impact of this result on timing based WD planet searches.

Whole Earth Telescope observations of the DAVs R808 and G38-29

The Whole Earth Telescope, under operation by the Delaware Asteroseismic Research Center, have obtained multi-site observations of two pulsating white dwarf stars. The DAVs R808 and G29-38 both show an abundance of excited modes, mostly clustered around 1050μHz. We present the Fourier analysis of both multi-site campaigns, present the measured periods, discuss their combination modes, and show that amplitude modulation is present in these stars.

PG 1605+072 in WET XCoV22: Support for the multi site spectroscopic telescope

Abstract The Multi-site spectroscopic telescope is a virtual instrument and the name of a collaboration that opens up a new observational window by combining continuous observations of spectroscopic variations and simultaneous photometric monitoring. This constitutes an enormous observational effort, but in return promises to finally provide access to a mode identification for and an asteroseismological analysis of the pulsating sdB star PG 1605+072. Multi-Site Spectroscopic Telescope observations for this object have been secured during a large coordinated campaign in May and June of the year 2002. The frequency resolution and coverage of the photometric time series has been noticeably enhanced by a significant contribution from the Whole Earth Telescope, which was used to observe PG 1605+072 as an alternate target during the WET XCov22 campaign, also conducted in May 2002. This paper briefly outlines the motivation for the MSST project and tries to give a first assessment of the overall quality of the data obtained, with a focus on the Whole Earth Telescope observations.