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 atmosphere of the hot DA white dwarf HZ43, with observations made by the Extreme 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 the signal-to-noise ratio that could be achieved. Recent implementation of a new observing mode designed to circumvent this problem has yielded an approximately fourfold improvement in the effective signal-to-noise ratio. This allows detection of weaker absorption features than was hitherto possible and, in the absence of visible lines, provides dramatically improved upper limits. We find that the He abundance (by number fraction) in HZ43 is less than approximate to 3x10(-7). This is completely 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 the mass and temperature of HZ43. This has important consequences for our understanding of the physical mechanisms that determine the photospheric composition of white dwarfs.