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 \aas@@fstack { \prime \prime } 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 \mathrm { \hskip { -1.72 pt } { } _ { eff } } = 105 000 \pm 5000 K , \log g = 7.4 \pm 0.4 , and a solar helium abundance ( He = 0.25 \pm 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 , Tol 26 ( PN G298.0 + 34.8 ) , and found that it is a slightly cooler DAOZ WD ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } \approx 85 000 K ) , but this WD shows signatures of gravitational settling of heavy elements .