HST and FUSE Spectroscopy of the DAO - type Central Star LS V + 4621

The DAO-type white dwarf LS V+4621 is the hydrogen-rich central star of the possible planetary nebula Sh 2-216. We have taken high-resolution, high-S/N ultraviolet spectra with STIS aboard the HST and FUSE in order to constrain its photospheric parameters. A detailed spectral analysis by means of state-of-the-art NLTE model-atmosphere techniques is presented which includes the determination the individual abundances of iron-group elements. 1. A brief history of Sh 2-216 and LS V+4621 Sharpless 2-216 (Sh 2-216) has been discovered as a " curious emission-line neb-ula " (Fesen, Blair, & Gull 1981). At a distance of d = 130 pc (Harris et al. 1997), it is the closest possible planetary nebula (PN) known with an apparent diameter of 100 ′. Sh 2-216 has experienced a mild interaction with the interstel-lar medium (ISM; Tweedy, Martos, & Noriega-Crespo 1995). From its distance and proper motion, Kerber et al. (2004) have determined that it has a thin-disk orbit of low inclination and eccentricity. LS V+4621 has been identified as the exciting star of Sh 2-216 by proper motion measurements (Cudworth & Reynolds 1985). Tweedy & Napiwotzki (1992) have demonstrated that LS V+4621, which is the brightest (m V = 12.67) DAO white dwarf (WD 0439+466) known, has the properties (T eff = 90 kK, log g = 7 (cgs), He/H = 0.01 by number) to ionize the surrounding nebula. have reported that a problem (BLP) exists to reproduce lines of the Balmer series (with NLTE model atmospheres) simultaneously at a given T eff in hot DA white dwarfs. Bergeron et al. (1993) found (in LTE calculations for DAO WDs) that the BLP is reduced by the consideration of metal-line blan-keting, and Bergeron et al. (1994) could show clearly that the presence of heavy metals is the source of the BLP – in other words, pure hydrogen models are not well suited for the spectral analysis of hot DA WDs in general.