High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216^{*,**,***,****}

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: rauch@astro.uni-tuebingen.de
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Received: 24 January 2007
Accepted: 11 April 2007

Abstract

Context. We perform a comprehensive spectral analysis of LS V +46°21  in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations.

Aims. LS V +46°21 is the DAO-type central star of the planetary nebula Sh 2-216. High-resolution, high-S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum have been analyzed in order to determine the photospheric parameters and the spectroscopic distance.

Methods. We performed a detailed spectral analysis of the ultraviolet and optical spectrum by means of state-of-the-art NLTE model-atmosphere techniques.

Results. From the N IV – N V, O IV – O VI, Si IV – Si V, and Fe V – Fe VII ionization equilibria, we determined an effective temperature of with high precision. The surface gravity is . An unexplained discrepancy appears between the spectroscopic distance and the parallax distance of LS V +46°21. For the first time, we have identified Mg IV and Ar VI absorption lines in the spectrum of a hydrogen-rich central star and determined the Mg and Ar abundances as well as the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With the realistic treatment of metal opacities up to the iron group in the model-atmosphere calculations, the so-called Balmer-line problem (found in models that neglect metal-line blanketing) vanishes.

Conclusions.Spectral analysis by means of NLTE model atmospheres has presently arrived at a high level of sophistication, which is now hampered largely by the lack of reliable atomic data and accurate line-broadening tables. Strong efforts should be made to improve upon this situation.