PROPERTIES OF THE HOT DA WHITE-DWARF HZ43 BASED ON FAR-ULTRAVIOLET ORFEUS-SPAS-II OBSERVATIONS

We present new far-ultraviolet observations of:he hot, hydrogen-rich, white dwarf (DA) HZ 43 obtained with the Berkeley spectrograph during the ORFEUS-SPAS II mission. The spectrum, which covers the range betwe en 892 and 1200 Angstrom at a spectral resolution of 3300, is dominate d by the Lyman series of hydrogen and shows no evidence of photospheri c absorption lines from other ionic spec,es often detected in hot whit e dwarf stars. The photospheric Lyman line profiles are modeled using synthetic spectra based on improved Stark-broadening calculations; nar row-line cores of interstellar origin help constrain the velocity disp ersion in the line of sight. We find an effective temperature of 50,40 0 +/- 100 K and a log g of 791 +/- 0.06. We also take advantage of ORF EUS's high spectral resolution in order to put stringent tipper limits on the abundance of C, N, Si, S, P and Cl. We find upper limits on th e number abundance relative to hydrogen of 10(-8.0) for C and N, 10(-8 .5) for Si, P, and Cl, and 10(-7.0) for S. These upper limits are at l east an order of magnitude below the abundances predicted by radiative levitation calculations. Our analysis refines considerably our knowle dge of HZ 43's atmospheric parameters, and we discuss the possible phy sical mechanisms that might explain the purity of HZ 43's atmosphere. We have also measured the H I velocity dispersion (b-value) in the lin e of sight of HZ 43 and find a value of 9 +/- 2 km s(-1).