The everchanging pulsating white dwarf GD358

Abstract

Abstract. We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order k = 8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k = 9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale.

Analysis of Long-Term Single-Site Observations of The Pulsating DB White Dwarf GD 358

“…We see that the main I = 1 modes vary in amplitude from month to month, with some of the modes (such as k = 17) disappearing for several months. We confirm the presence of the k = 19 mode that Kepler et al (2003) report and show that it too has a variable amplitude. We also see a rich spectrum of sum and difference combination peaks that changes from run to run as the parent I = 1 modes change in amplitude.…”

“…For larger amplitude modes, we can also ascertain the reality of a mode by looking for combination peaks (peaks at the sum and/or difference frequency of two modes). et al (1994), Vuille et al (2000), and Kepler et al (2003) have FT plots for GD 358 that look somewhat similar to each other. In Fig.…”

“…While we have seen all of these I = 1 modes before, we have not normally seen the k -13 and 14 modes at such large amplitudes. As a result, these combination peaks hinder our ability to see modes with periods longer than 810 s or shorter than 420 s. However, we see evidence of a mode at k -19 in the 1985 April and 1986 May data sets that was also seen by Kepler et al (2003). The period of this mode is about 854 s, and the period spacing is 44 s, implying that the next trapped mode must be at k = 20 or greater, since we have not found a period spacing minimum yet.…”

Pulsating white dwarfs as astrophysical tools

“…The nearly-uniformlyspaced 1=1 periods in this latter object that were detected in a WET run were the basis for deducing a number of significant astrophysical results. Even though an analysis of subsequent WET runs (Kepler et al 2003) revealed that the pulsation spectrum of GD 358 is rather complicated and changes with time, the basic results remained unchanged.…”

“…This chemically stratified model is the simplest of all the WDs and therefore the easiest to model. Currently, there is a total of 9 pulsating DBs that are known, and the class prototype (GD358) has been extensively studied, including via 3 separate WET runs (e.g., Kepler et al 2003).…”