Subdwarf B (sdB) stars are extreme horizontal branch stars which are believed to comprise 0.5 solar masses of He (and the usual admixture of metals) with a very thin (less than 2% by mass) layer of H, chemically segregated from the He and heavier elements by settling in the star's large gravitational field. The evolution of the progenitors of these stars is not well understood, relying on unknown mass loss mechanisms on the giant branch to remove almost all the H envelope of the progenitor at the same time as increasing the He core to the mass needed to ignite He burning in the He flash. Fortunately the discovery in the past few years of pulsating sdB stars (the EC14026 stars) (Kilkenny et al. 1997 and its 3 companion papers in the same issue of MNRAS) has opened the prospect of using asteroseismology to examine the interiors of these stars. An example of this approach is the study of the pulsating sdB star PB8783 by O'Donoghue et al. (1998). In the long term, measuring the evolution timescale of the star through pulsation period changes as is being done for several of the variable white dwarfs, will provide the first measurement of the rate of core He burning in any known evolved star.
Feige 48 was discovered to be a pulsator by Koen et al. (1998). Since then, single site data has been obtained every observing season (which is quite long as Feige 48 is quite far north). The best data indicates that Feige 48 pulsates in at least 5 modes and does indeed have (nearly) evenly split modes, at 13 and 26 microhertz. This is unfortunately very close to the daily alias that clutters single site data. Additionally, several of these modes are a daily alias away from those reported in the discovery data (Koen et al, 1998). Feige 48 represents one of the few contrained EC14026 stars, being the object of high resolution Keck spectra (Heber et al. 2000). This places tight constraints on the projected rotation velocity (< 10km/s), limiting any rotational splittings detected.
The goals of the WET Feige 48 observations are to resolve fully the pulsation frequencies present and to identify the quantum numbers of the pulsation modes. We will use this information to model the interior structure of this star and further our understanding of this very interesting class of objects. The WET data should allow us to confirm if any rotationally-split modes exist and find more low-amplitude modes than have been detected in single site data. The more modes there are, the more chance there is of identifying their quantum numbers and the more constraints can be placed on the structure of theoretical models. We have acrued enough observations that we are getting to the limit of where evolutionary changes should be observable in the O-C diagram. Of course to get accurate phases, it is important to have stable, resolved modes. The data acquired during the WET run will greatly aide in this determination.
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