Mode identification of pulsating white dwarfs using the Hubble Space Telescope

Abstract

We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV stars G226-29 and G185-32 and for the pulsating DBV star PG 1351+489 with the Hubble Space Telescope Faint Object Spectrograph to compare the ultraviolet to the optical pulsation amplitude and determine the pulsation indices. We find that for essentially all observed pulsation modes, the amplitude rises to the ultraviolet as the theoretical models predict for l=1 nonradial g-modes. We do not find any pulsation mode visible only in the ultraviolet, nor any modes whose phase flips by 180° in the ultraviolet, as would be expected if high l pulsations were excited. We find one periodicity in the light curve of G185-32, at 141 s, which does not Ðt theoretical models for the change of amplitude with wavelength of g-mode pulsations.