Whole Earth Telescope observations of the hot helium atmosphere pulsating white dwarf EC20058-5234

We present the analysis of a total of 177h of high-quality optical time-series photometry of the helium atmosphere pulsating white dwarf (DBV) EC20058-5234. The bulk of the observations (135h) were obtained during a WET campaign (XCOV15) in 1997 July that featured coordinated observing from four southern observatory sites over an 8-d period. The remaining data (42h) were obtained in 2004 June at Mt John Observatory in NZ over a one-week observing period. This work significantly extends the discovery observations of this low-amplitude (few per cent) pulsator by increasing the number of detected frequencies from 8 to 18, and employs a simulation procedure to confirm the reality of these frequencies to a high level of significance (1 in 1000). The nature of the observed pulsation spectrum precludes identification of unique pulsation mode properties using any clearly discernable trends. However, we have used a global modelling procedure employing genetic algorithm techniques to identify the n,l values of eight pulsation modes, and thereby obtain asteroseismic measurements of several model parameters, including the stellar mass (0.55 M_solar) and T_eff (~28200K). These values are consistent with those derived from published spectral fitting: T_eff ~ 28400K and logg ~ 7.86. We also present persuasive evidence from apparent rotational mode splitting for two of the modes that indicates this compact object is a relatively rapid rotator with a period of 2h. In direct analogy with the corresponding properties of the hydrogen (DAV) atmosphere pulsators, the stable low-amplitude pulsation behaviour of EC20058 is entirely consistent with its inferred effective temperature, which indicates it is close to the blue edge of the DBV instability strip. Arguably, our most significant result from this work is the clear demonstration that EC20058 is a very stable pulsator with several dominant pulsation modes that can be monitored for their long-term stability.