THE REMARKABLY LOW ABUNDANCE OF HELIUM IN THE ATMOSPHERE OF THE DA WHITE-DWARF HZ43

We report the most sensitive search yet made for helium in the atmosph ere of the hot DA white dwarf HZ43, with observations made by the Extr eme Ultraviolet Explorer (EUVE). Prior upper limits on the presence of He in DA white dwarfs using EUVE spectra have been affected by fixed- pattern efficiency variations in the detector system, which limited th e signal-to-noise ratio that could be achieved. Recent implementation of a new observing mode designed to circumvent this problem has yielde d an approximately fourfold improvement in the effective signal-to-noi se ratio. This allows detection of weaker absorption features than was hitherto possible and, in the absence of visible lines, provides dram atically improved upper limits. We find that the He abundance (by numb er fraction) in HZ43 is less than approximate to 3x10(-7). This is com pletely at odds with the calculations of radiative levitation theory, which predict a He abundance of at least 3 x 10(-6) for a star with th e mass and temperature of HZ43. This has important consequences for ou r understanding of the physical mechanisms that determine the photosph eric composition of white dwarfs.