Abstract
PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 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. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and 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 = 0.59 +/- 0.02 solar masses 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_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 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 l=1 modes and less than 50% for l=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.
Highlights
The star PG 1159-035 was identified by Green in 1977 in a survey for objects with ultraviolet excess, known as the PalomarGreen Survey (Green et al 1986)
Our results show that the low frequency and high amplitude (LFHA) can introduce significant errors in the determinations of the frequencies, amplitudes and phases of the pulsation modes
If the pulsational symmetry axis and the rotational axis are approximately aligned and if the amplitudes of all the pulsation modes of a multiplet are the same, the multiplets appear in the Fourier transform (FT) with a symmetrical design and the relative amplitudes of the components depend on the inclination angle, i, of the rotational axis (Pesnell 1985)
Summary
The star PG 1159-035 was identified by Green in 1977 in a survey for objects with ultraviolet excess, known as the PalomarGreen Survey (Green et al 1986). The Fourier transform of more extensive light curves obtained in the following years, between 1979−1985, allowed the detection of eight pulsation modes (Winget et al 1985); the highest amplitude mode has a period of 516 s. The PG 1159-035 Fourier transform published in 1991 revealed the presence of triplets and multiplets, caused by rotational splitting, allowing the determination of the rotational period of the star (Prot = 1.38 days). The immediate goal of this work was to detect and identify a larger number of pulsating modes in PG 1159-035 from the analysis and comparison of the FTs of photometric data obtained in different years. The analysis of the splitting in frequency in the multiplets of the combined data allows the calculation of the rotation period with higher accuracy and a better estimate of a upper limit for the strength of the star’s magnetic field.
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