High-speed Hubble Space Telescope ultraviolet photometry of two DB white dwarfs: Nonradial and radial pulsations

We observed two DB white dwarf stars with the High Speed Photometer aboard the Hubble Space Telescope. The two targets, the nonradial pulsator 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 response in the UV between 1400 and 3000 Å. The data clearly show the long period (∼700 s) pulsations in GD 358. Comparison with optical observations obtained two weeks earlier shows that the amplitude 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 allows us to search for high-frequency pulsations (such as p modes or high overtone radial modes). No firm evidence for high-frequency pulsations was seen in either object between 1 and 12 Hz. Correlation analysis of GD 358 shows no clear signal of multifrequency high overtone radial pulsations at the 0.00075 mag level, with no individual modes above the 0.0016 mag level. Upper limits for PG 0112+104 are approximately 2 times higher than for GD 358. Implications of this study for the theory of white dwarf pulsations are discussed.