ETHOS 1: a high-latitude planetary nebula with jets forged by a post-common-envelope binary central star★

Abstract

We report on the discovery of ETHOS 1 (PN G068.1+11.0), the first spectroscopically confirmed planetary nebula (PN) from a survey of the SuperCOSMOS Science Archive for high-latitude PNe. ETHOS 1 stands out as one of the few PNe to have both polar outflows (jets) travelling at $120\pm10$ km/s and a close binary central star. The lightcurve observed with the Mercator telescope reveals an orbital period of 0.535 days and an extremely large amplitude (0.816 mag) due to irradiation of the companion by a very hot pre-white dwarf. ETHOS 1 further strengthens the long suspected link between binary central stars of planetary nebulae (CSPN) and jets. INT IDS and VLT FORS spectroscopy of the CSPN reveals weak N III, C III and C IV emission lines seen in other close binary CSPN and suggests many CSPN with these weak emission lines are misclassified close binaries. We present VLT FORS imaging and Manchester Echelle Spectrometer long slit observations from which a kinematic model of the nebula is built. An unusual combination of bipolar outflows and a spherical nebula conspire to produce an $X$-shaped appearance. The kinematic age of the jets ($1750\pm250$ yrs/kpc) are found to be older than the inner nebula ($900\pm100$ yrs/kpc) consistent with previous studies of similar PNe. Emission line ratios of the jets are found to be consistent with reverse-shock models for fast low-ionisation emitting regions (FLIERS) in PNe. Further large-scale surveys for close binary CSPN will be required to securely establish whether FLIERS are launched by close binaries.