The hot white dwarf in the peculiar binary nucleus of the planetary nebula EGB 6

EGB 6 is an extended, faint old planetary nebula (PN) with an enigmatic nucleus. The central star (PG 0950+139) is a hot DAOZ-type white dwarf (WD). An unresolved, compact emission knot was discovered to be located 0.''166 away from the WD and it was shown to be centered around a dust-enshrouded low-luminosity star. It was argued that the dust disk and evaporated gas (photoionized by the hot WD) around the companion are remnants of a disk formed by wind material captured from the WD progenitor when it was an asymptotic giant branch (AGB) star. In this paper, we assess the hot WD to determine its atmospheric and stellar parameters. We performed a model-atmosphere analysis of ultraviolet (UV) and optical spectra. We found T_eff = 105 000 ± 5000 K, log g = 7.4 ± 0.4, and a solar helium abundance (He = 0.25 ± 0.1, mass fraction). We measured the abundances of ten more species (C, N, O, F, Si, P, S, Ar, Fe, Ni) and found essentially solar abundance values, indicating that radiation-driven wind mass-loss, with a theoretical rate of log(Ṁ/M_⊙/yr) = -11.0_-0.8^+1.1, prevents the gravitational separation of elements in the photosphere. The WD has a mass of M/M_⊙ = 0.58_-0.04^+0.12 and its post-AGB age (log(t_evol/yr = 360_-0.09^+1.26)) is compatible with the PN kinematical age of log(t_PN/yr = 4.2). In addition, we examined the UV spectrum of the hot nucleus of a similar object with a compact emission region, Tol 26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (T_eff ≈ 85 000 K), but this WD shows signatures of gravitational settling of heavy elements.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer and the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666.