2006 Whole Earth Telescope Observations of Gd358: A New Look at the Prototype DBV

Abstract

We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 µHz, with the dominant peak at 1234 µHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

PERIODIC VARIATIONS IN THEO–CDIAGRAMS OF FIVE PULSATION FREQUENCIES OF THE DB WHITE DWARF EC 20058–5234

“…The lack of known white dwarf planets is gaining statistical significance and further timing observations of pulsating white dwarfs could place strong constraints on post-main-sequence stellar evolution. Sullivan et al (2008) identified the frequency of A to be an additive combination of the frequencies of E and G, a well-known occurrence in white dwarf pulsators (see, e.g., Provencal et al 2009). Any perturbation to the frequency of E or G should result in an identical perturbation to the frequency of A.…”

Pulsating white dwarfs: new insights

“…A phenomenon reminiscent of these outburst-like events was the sforzando event detected in 1996 for the DBV GD358, in which the star dramatically altered its pulsation characteristics on a timescale of hours(Provencal et al, 2009). …”

Revealing the pulsational properties of the V777 Herculis star KUV 05134+2605 by its long-term monitoring

“…The most famous example is the so-called sforzando effect observed in GD 358. This star showed a high-amplitude sinusoidal light variation for a short period of time instead of the non-linear variability observed just a day before (Kepler et al 2003;Provencal et al 2009). To explain the short-term changes in the pulsational behaviour of a star is challenging.…”