AN ENORMOUS PLANETARY-NEBULA SURROUNDING THE X-RAY SOURCE RXJ 2117+34

We report the discovery of a huge planetary nebula surrounding the ROSAT x-ray source RXJ 2117 + 34. The source corresponds to a star which is a member of the PG 1159 spectroscopic class, which are stars that have properties intermediate between planetary nebula nuclei (PNNs) and white dwarf stars. The nebula, which is detected through its [O III] and Halpha emission, has a maximum extent of 13 arcmin. This corresponds to a linear size of 5.3 pc at the published distance of 1.4 kpc. This size is five times greater than the largest previously known planetary nebula S68, which also surround a peculiar hot PG 1159-like PN central star. The expansion age of the RXJ nebula suggests that it is very old (approximately 1.5 X 10(5) yr), which is much older than its central star based on its position in the HR diagram. The morphology of the [O III] emission is interesting, showing a bipolar structure in its central parts and faint linear filaments, suggestive of shocks, in its outer regions. The discovery of an evolved PN around RXJ 2117 confirms that PG 1159 stars are transition objects between PNNs and helium-rich white dwarfs.