The Nucleus of the Planetary Nebula EGB 6 as a Post-Mira Binary

TL;DR

EGB 6's nucleus is a post-Mira binary system with a dust-enshrouded companion star, revealing complex interactions and variable infrared emissions, challenging previous assumptions about planetary nebulae central stars.

Contribution

This study identifies a cool, dust-enshrouded companion in EGB 6, providing new insights into binary interactions in planetary nebulae and the survival of accretion disks post-AGB phase.

Findings

The companion is a cool, dust-enshrouded star with a continuous spectrum.

Infrared fluxes are variable on short timescales.

The spectral energy distribution resembles that of young stellar objects.

Abstract

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.