The bow-shock and high-speed jet in the faint, 40 arcmin diameter, outer halo of the evolved Helix planetary nebula (NGC 7293)

Abstract

In previous, very deep, optical images of NGC 7293 both a feature that has the morphology of a bow-shock and one with that of a jet were discovered in the faint 40 arcmin diameter halo of the nebula. Spatially resolved longslit profiles of the Halpha and [N II] 6548, 6584 A nebular emission lines from both features have now been obtained. The bow-shaped feature has been found to have Halpha radial velocities close to the systemic heliocentric radial velocity, -27 km/s, of NGC 7293 and is faint in the [N II] 6548, 6584 A emission lines. Furthermore, the full width of these profiles matches the relative motion of NGC 7293 with its ambient interstellar medium consequently it is deduced that the feature is a real bow-shock caused by the motion of NGC 7293 as it ploughs through this medium. The proper motion of the central star also points towards this halo feature which substantiates this interpretation of its origin. Similarly [N II] 6584 A line profiles reveal that the jet-like filament is indeed a collimated outflow, as suggested by its morphology, at around 300 km/s with turbulent widths of around 50 km/s. It's low Halpha/[N II] 6548, 6584 A brightness ratio suggests collisional ionization as expected in a high-speed jet.

7 pages, 6 figures, accepted for publication in MNRAS

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• Herschel imaging of the dust in the Helix Nebula (NGC 7293)
• True versus apparent shapes of bow shocks