Physics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994hst..prop.5581n&link_type=abstract
HST Proposal ID #5581
Physics
Hst Proposal Id #5581 Hot Stars White Dwarfs
Scientific paper
The pulsation modes of white dwarfs are identified by indices (l,m,k). The mode indices have not been identified with certainty for any pulsation of any DA white dwarf. If the index l of their pulsations could be measured, the masses of their outer hydrogen layers could be determined. This is an important result because the thickness of the hydrogen layer directly tests models for the ejection of planetary nebulae and theories for the spectral evolution of white dwarfs. The amplitudes of the pulsations of DA white dwarfs depend strongly on l at wavelengths less than 3000 A. Time resolved spectroscopy of the pulsations at ultraviolet wavelengths will, therefore, allow a direct determination of l. DN Draconis (= G226-29) is a pulsating DA white dwarf. Three of its pulsation modes are simultaneously excited at periods near 109.3 s; they add together in the light curve to give a beat period of 17.2 hr. We will use the FOS with the blue Digicon and the G160L grating in RAPID readout mode to obtain ultraviolet spectrophotometry of G226-29 with a sampling interval of 10 seconds. We will observe G226-29 for 11 consecutive orbits covering one cycle of its 17.2 hour beat period, accumulating about 7.2 hours of on-target data. Power spectra of the flux variations will yield wavelength-dependent amplitudes for each pulsation and thus the value of l for each pulsation.
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