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 combination 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 $\ell=1$ modes, with a common period spacing of 23.0 s. This spacing implies that the mass of PG 1707+427 is $0.57~M_{\odot}$. Preliminary model fits suggest that the mass determined via asteroseismology is consistent with the mass determined from spectroscopy combined with evolutionary tracks

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 combination 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 l = 1 modes, with a common period spacing of 23.0 s. This spacing implies that the mass of PG 1707+427 is 0.57 Mѳ. Preliminary model fits suggest that the mass determined via asteroseismology is consistent with the mass determined from spectroscopy combined with evolutionary tracks

Observations of the pulsating white dwarf G 185-32

We observed the pulsating hydrogen atmosphere white dwarf G 185-32 with the Whole Earth Telescope in 1992. We report on a weighted Fourier transform of the data detecting 18 periodicities in its light curve. Using the Hubble Space Telescope Faint Object Spectrograph time resolved spectroscopy, and the wavelength dependence of the relative amplitudes, we identify the spherical harmonic degree ( $\ell$) for 14 pulsation signals. We also compare the determinations of effective temperature and surface gravity using the excited modes and atmospheric methods, obtaining $T_{{\rm eff}}=11\,960$ $\pm$ 80 K, $\log g=8.02$ $\pm$ 0.04 and M=0.617 $\pm$ $0.024~M_{\odot}$

A Whole Earth Telescope campaign on the pulsating subdwarf B binary system PG 1336-018 (NY Vir)

We present results from a multisite (‘Whole Earth Telescope’) photometric campaign on PG 1336−018, the close eclipsing binary system containing a pulsating subdwarf B (sdB) star. The main part of the campaign (1999 April) resulted in ∼172 h of observations, representing a coverage of about 47 per cent, and additional data were obtained outside the core campaign. Periodogram analysis shows that the light variations are dominated by three frequencies near 5757, 5585 and 5369 μHz (∼174, 179 and 186 s, respectively), although many frequencies are present, particularly in the range 5000–6000 μHz (∼200–170 s). We identify, with some confidence, 28 frequencies down to a semi-amplitude of 0.0005 in fractional intensity (equivalent to about 0.5 mmag). It is clear that the pulsation frequencies of PG 1336−018 have changed substantially since the 1996 discovery observations were made, and that amplitude changes occur, at least in the dominant three frequencies, on relatively short time-scales (of the order of a day). On the assumption that the pulsating star is phase-locked in the binary system, we have searched for rotational splitting of frequencies near the orbital and half of the orbital period, but the results are confused by aliasing at those frequencies (due to the data gaps caused by the eclipses). A preliminary model qualitatively matches the distribution of frequencies in PG 1336−018, with some good individual correspondences, but cannot be considered adequate because geometric cancellation should hide some of the modes which are apparently detected. Analysis of the pulsations during eclipse recovers three of the strongest modes, but the limited eclipse data — which can, at best, be only about 9 per cent of the total — do not allow mode identification at this stage. Simulations indicate that an overall coverage of about 80 per cent would be required for this to be viable. An attempt was made to determine phase shifts in the pulsation frequencies as a way of directly measuring the size of the binary orbit, but the uncertainties in the method are comparable to the light travel time across the orbit (probably less than a second)

Asteroseismology of RXJ 2117+3412, the hottest pulsating PG 1159 star

The pulsating PG 1159 planetary nebula central star RXJ 2117+3412 has been observed over three successive seasons of a multisite photometric campaign. The asteroseismological analysis of the data, based on the 37 identified $\ell=1$ modes among the 48 independent pulsation frequencies detected in the power spectrum, leads to the derivation of the rotational splitting, the period spacing and the mode trapping cycle and amplitude, from which a number of fundamental parameters can be deduced. The average rotation period is $1.16\pm 0.05$ days. The trend for the rotational splitting to decrease with increasing periods is incompatible with a solid body rotation. The total mass is 0.56 +0.02-0.04 $M_{\odot}$ and the He-rich envelope mass fraction is in the range 0.013-0.078 M*. The luminosity derived from asteroseismology is log( $L/L_{\odot})= 4.05$ +0.23-0.32 and the distance 760 +230-235 pc. At such a distance, the linear size of the planetary nebulae is $2.9\pm 0.9$ pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed

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

Context. PG 1159-035 , a pre-white dwarf with $T_{{\rm eff}}$$\simeq$ 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 $M/M_\odot$ = 0.59 $\pm$ 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 $r_{\rm c}/R_\star$ = 0.83 $\pm$ 0.05. From the multiplet splitting, we calculated the rotational period $P_{\rm rot}$ = 1.3920 $\pm$ 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 $\ell$ = 1 modes and less than 50% for $\ell$ = 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

Whole Earth Telescope observations of BPM 37093: a seismological test of crystallization theory in white dwarfs

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~1.1 MO) to theoretically crystallize while still inside the ZZ Ceti instability strip (Teff ~ 12 000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the “mode trapping” caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized

The pulsations of PG 1351+489

The pulsations of PG 1351+48