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

TL;DR

This study analyzes the faint outer halo of the Helix planetary nebula NGC 7293, confirming a bow-shock caused by its motion through the interstellar medium and identifying a high-speed collimated jet, using spatially resolved spectral profiles.

Contribution

First detailed spectroscopic confirmation of a bow-shock and jet in the outer halo of NGC 7293, elucidating their kinematics and ionization mechanisms.

Findings

Bow-shock velocity matches nebula's motion through interstellar medium

Jet exhibits high speed around 300 km/s with turbulent widths

Ionization suggests collisional processes in the jet

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.