HIGH-SPEED HUBBLE-SPACE-TELESCOPE ULTRAVIOLET PHOTOMETRY OF 2 DB WHITE-DWARFS - NONRADIAL AND RADIAL PULSATIONS

We observed two DB white dwarf stars with the High Speed Photometer ab oard the Hubble Space Telescope. The two targets, the nonradial pulsat or GD 358, and PG 0112 + 104, a non-pulsating white dwarf with similar temperature, were each observed for 1800 s with a time resolution of 10 ms. We used the F140LP configuration, which gives a broadband respo nse in the UV between 1400 and 3000 angstrom. The data clearly show th e long period (approximately 700 s) pulsations in GD 358. Comparison w ith optical observations obtained two weeks earlier shows that the amp litude of the pulsations in the UV is approximately 1.4 times higher, consistent with nonradial pulsations due solely to temperature changes at constant radius. The high time resolution of these observations al lows us to search for high-frequency pulsations (such as p modes or hi gh overtone radial modes). No firm evidence for high-frequency pulsati ons was seen in either object between 1 and 12 Hz. Correlation analysi s of GD 358 shows no clear signal of multifrequency high overtone radi al pulsations at the 0.00075 mag level, with no individual modes above the 0.0016 mag level. Upper limits for PG 0112 + 104 are approximatel y 2 times higher than for GD 358. Implications of this study for the t heory of white dwarf pulsations are discussed.