High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216

TL;DR

This study presents a detailed ultraviolet and optical spectral analysis of the white dwarf LS V +4621, determining its temperature, gravity, and chemical composition, and highlights the importance of metal opacities in modeling stellar atmospheres.

Contribution

It provides the first identification of Mg IV and Ar VI lines in a hydrogen-rich central star and demonstrates the impact of metal-line blanketing on resolving the Balmer-line problem.

Findings

Effective temperature of 95 +/- 2 kK determined.

Identification of Mg IV and Ar VI absorption lines.

Metal-line blanketing resolves the Balmer-line problem.

Abstract

LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We perform a comprehensive spectral analysis of high-resolution, high-S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum of LS V +4621 by means of state-of-the-art NLTE model-atmosphere techniques in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations. From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization equilibria, we determined an effective temperature of 95 +/- 2 kK with high precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and the parallax distance d = 129 +6/-5 pc of LS V +4621. 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. 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.