A VERY HOT, HYDROGEN-RICH, WHITE-DWARF PLANETARY NUCLEUS

We report new spectrophotometry and a detailed analysis of the DA whit e dwarf nucleus of the planetary nebula WDHS 1 (PK 197-6-degrees.1). B ased on model atmospheres fits to the Balmer line profiles, the possib le T(eff) for this star ranges from 165,000 K if the helium abundance is zero to approximately 100,000 K if a marginally detectable abundanc e of N(He)/N(H) = 10(-2) is present in the atmosphere; log g approxima tely 7.6 in either case. The nucleus of WDHS 1 is thus the hottest kno wn high gravity star with a hydrogen-rich atmosphere. We present CCD i maging of the nebula showing this to have an angular extent of 22'. At the distance (d) of 1.0 +/- 0.25 kpc estimated from the white dwarf a nalysis, WDHS 1 is one of the largest known planetary nebulae with a d iameter of 6.4 (d/1 kpc) pc. The implied kinematic age of the nebula i s a few times 10(5) yr, which might be consistent with the post-AGB li fetime of the nucleus. We discuss the apparent paucity of objects such as the nucleus of WDHS 1 in comparison with their hydrogen-poor count erparts among planetary nuclei and field white dwarfs.