LIMITS ON THE HYDROGEN LAYER MASS AND CONSEQUENT HELIUM OPACITY IN HOT DA WHITE-DWARF ATMOSPHERES

The results of the ROSAT sky survey of hot H-rich DA white dwarfs demonstrated that, at temperatures above almost-equal-to 40 000 K, most stars contain significant quantities of heavy elements in their atmospheres. Conversely, below 40 000 K most DA white dwarfs appear to have pure H envelopes with, at most, only small traces of any other material. However, ROSAT was unable to exclude a contribution from He to the total photospheric opacity present in these stars. Consequently, the role of He in hot DA white dwarfs remains uncertain. New spectroscopic observations made by the Extreme Ultraviolet Explorer (EUVE) satellite promise to provide direct evidence through a search for photospheric He absorption features. We analyse four well-exposed spectra from the EUVE public archive, finding no evidence for any photospheric He. Using a grid of stratified model atmospheres, we are able to determine lower limits to the mass of the outer H layer, which are significantly larger than those derived from earlier broad-band results.