The hot white dwarf in the peculiar binary nucleus of the planetary nebula EGB6

TL;DR

This study analyzes the hot white dwarf in the planetary nebula EGB6, determining its atmospheric parameters, chemical abundances, and evolutionary status, revealing a solar-like composition and wind-driven element retention.

Contribution

It provides detailed atmospheric modeling of the white dwarf in EGB6, including temperature, gravity, and chemical abundances, and compares it with a similar object, advancing understanding of post-AGB evolution.

Findings

White dwarf temperature: 105,000 K

Solar-like elemental abundances in the WD

Wind-driven mass loss prevents element separation

Abstract

EGB6 is an extended, faint old planetary nebula (PN) with an enigmatic nucleus. The central star (PG0950+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 Teff = 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(dot-M/M_sun/yr) = -11.0 (+1.1)(-0.8) prevents the gravitational separation of elements in the photosphere. The WD has a mass of M/M_sun = 0.58 (+0.12)(-0.04) and its post-AGB age (log(t_evol/yr) = 3.60 (+1.26)(-0.09)) 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, TOL26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (Teff about 85,000 K), but this WD shows signatures of gravitational settling of heavy elements.