The Nucleus of the Planetary Nebula EGB 6 as a Post-Mira Binary
  EGB 6 is a faint, large, ancient planetary nebula (PN). Its central star, a
hot DAOZ white dwarf (WD), is a prototype of a rare class of PN nuclei
associated with dense, compact emission-line knots. The central star also shows
excess fluxes in both the near- (NIR) and mid-infrared (MIR). In a 2013 paper,
we used Hubble Space Telescope (HST) images to show that the compact nebula is
a point-like source, located 0".16 (~118 AU) from the WD. We attributed the NIR
excess to an M dwarf companion star, which appeared to coincide with the dense
emission knot. We now present new ground-based NIR spectroscopy, showing that
the companion is actually a much cooler source with a continuous spectrum,
apparently a dust-enshrouded low-luminosity star. New HST images confirm common
proper motion of the emission knot and red source with the WD. The I-band, NIR,
and MIR fluxes are variable, possibly on timescales as short as days. We can
fit the spectral-energy distribution with four blackbodies (the WD, a ~1850 K
NIR component, and MIR dust at 385 and 175 K). Alternatively, we show that the
NIR/MIR SED is very similar to that of Class 0/I young stellar objects. We
suggest a scenario in which the EGB 6 nucleus is descended from a wide binary
similar to the Mira system, in which a portion of the wind from an AGB star was
captured into an accretion disk around a companion star; a remnant of this disk
has survived to the present time, and is surrounded by gas photoionized by UV
radiation from the WD.
