Properties of the Hot DA White Dwarf HZ 43 Based on Far-Ultraviolet [ITAL]ORFEUS[/ITAL]-[ITAL]SPAS II[/ITAL] Observations

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Abstract
We present new far-ultraviolet observations of the 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 between 892 and 1200 Å at a spectral resolution of 3300, is dominated by the Lyman series of hydrogen and shows no evidence of photospheric absorption lines from other ionic species often detected in hot white dwarf stars. The photospheric Lyman line profiles are modeled using synthetic spectra based on improved Stark-broadening calculations; narrow-line cores of interstellar origin help constrain the velocity dispersion in the line of sight. We find an effective temperature of 50,400 ± 100 K and a log g of 7.91±0.06. We also take advantage of ORFEUS's high spectral resolution in order to put stringent upper limits on the abundance of C, N, Si, S, P, and Cl. We find upper limits on the 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 least an order of magnitude below the abundances predicted by radiative levitation calculations. Our analysis refines considerably our knowledge of HZ 43's atmospheric parameters, and we discuss the possible physical mechanisms that might explain the purity of HZ 43's atmosphere. We have also measured the H I velocity dispersion (b-value) in the line of sight of HZ 43 and find a value of 9±2 km s−1.