Hubble Space Telescope Imaging of the Binary Nucleus of the Planetary Nebula EGB 6

TL;DR

This study uses Hubble imaging to analyze the binary nucleus of planetary nebula EGB 6, revealing a dense, compact emission nebula unexpectedly associated with the M dwarf companion, challenging existing understanding of nebula evolution.

Contribution

First high-resolution imaging resolving the binary components and the dense nebula, providing new insights into nebula morphology and challenging current models of nebula confinement.

Findings

The compact nebula is superposed on the M dwarf companion, far from the white dwarf's radiation.

The nebula's electron density remains extremely high over three decades.

A significant mid-infrared excess indicates large dust shells around the system.

Abstract

EGB 6 is an ancient, low-surface-brightness planetary nebula. The central star, also cataloged as PG 0950+139, is a very hot DAOZ white dwarf (WD) with an apparent M dwarf companion, unresolved from the ground but detected initially through excesses in the JHK bands. Its kinematics indicates membership in the Galactic disk population. Inside of EGB 6 is an extremely dense emission knot -- completely unexpected since significant mass loss from the WD should have ceased ~10^5 y ago. The electron density of the compact nebula is very high (2.2x10^6 cm^-3), as indicated by collisional de-excitation of forbidden emission lines. Hubble Space Telescope imaging and grism spectroscopy are reported here. These resolve the WD and apparent dM companion -- at a separation of 0."166, or a projected 96(+204,-45) AU at the estimated distance of 576(+1224, -271) pc (using the V magnitude). Much to our surprise, we found that the compact emission nebula is superposed on the dM companion, far from the photoionizing radiation of the WD. Moreover, a a striking mid-infrared excess has recently been reported in Spitzer/IRAC and MIPS bands, best fit with two dust shells. The derived ratio L_IR/L_WD = 2.7 x 10^-4 is the largest yet found for any WD or planetary nucleus. The compact nebula has maintained its high density for over three decades. We discuss two possible explanations for the origin and confinement of the compact nebula, neither of which is completely satisfactory. This leaves the genesis and confinement of the compact nebula an astrophysical puzzle, yet similar examples appear in the literature.