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ULTRACAM Observations of Subdwarf B Stars

Simon Jeffery has used a novel camera attached to the William Herschel Telescope (WHT) to obtain the best light curves ever obtained for pulsating subdwarf B stars. These stars pulsate non-radially in several modes with different frequencies and enable astronomers to establish their overall properties and internal structure with unusual precision. The high speed and multicolour characteristics of ULTRACAM together with the 4.2m diameter aperture of the WHT meant that it was possible to measure the amplitudes of several modes at different wavelengths to a precision of better than 0.1%. Comparing the observations with calculations by PhD student Babulakshmanan Ramachandran enabled the oscillation mode of each observed frequency to be identified. Several modes with azimuthal wave numbers l=0,1 and 2 were identified together with, surprisingly, an l=4 mode in both of the stars observed. The latter seems to be visible despite cancellation effects, possibly as a consequence of limb-darkening enhancing the relative flux variation in the disk-centre. With these mode identifications, it becomes possible to compare observations directly with models of the stellar interior and to test those models which demand an enhancement of iron-group elements in the layers of the star responsible for driving pulsations.