Flows along cometary tails in the Helix planetary nebula NGC 7293

TL;DR

This paper analyzes ionized gas flows along cometary tails in the Helix nebula, revealing acceleration and coherence of outflows driven by the central star's wind, using multi-wavelength observations including Hubble data.

Contribution

It provides the first detailed kinematic analysis of the complex tail from Knot 32 and compares flows across multiple tails using optical and molecular hydrogen data.

Findings

Ionized gas flows are accelerating away from the central star.

The longest tail remains a coherent outflow despite clumpy medium.

Flows are driven by the star's mildly supersonic AGB wind.

Abstract

Previous velocity images which reveal flows of ionized gas along the most prominent cometary tail (from Knot 38) in the Helix planetary nebula are compared with that taken at optical wavelengths with the Hubble Space Telescope and with an image in the emission from molecular hydrogen. The flows from the second most prominent tail from Knot 14 are also considered. The kinematics of the tail from the more complex Knot 32, shown here for the first time, also reveals an acceleration away from the central star. All of the tails are explained as accelerating ionized flows of ablated material driven by the previous, mildly supersonic, AGB wind from the central star. The longest tail of ionized gas, even though formed by this mechanism in a very clumpy medium, as revealed by the emission from molecular hydrogen, appears to be a coherent outflowing feature.