Discovery of a halo around the Helix Nebula NGC 7293 in the WISE all-sky survey

TL;DR

This paper reports the discovery of a large, axisymmetric infrared halo around the Helix Nebula, indicating complex stellar wind and interstellar medium interactions, with modeling suggesting a dynamic mass-loss history.

Contribution

First detection of a 40' infrared halo around NGC 7293 using WISE data, revealing interaction with the ISM and complex stellar wind history.

Findings

Infrared halo has a sharp NE boundary and diffuse SW boundary.

Halo aligns with a NE arc, indicating physical association.

Model fits suggest variable stellar wind and ISM interaction.

Abstract

We report the discovery of an extended halo (40' in diameter) around the planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has an axisymmetric structure with a sharp boundary to the northeast and a more diffuse boundary to the southwest, suggesting an interaction between the stellar wind and the interstellar medium (ISM). The symmetry axis of the halo is well aligned with that of a northeast arc, suggesting that the two structures are physically associated. We have attempted to fit the observed geometry with a model of a moving steady-state stellar wind interacting with the ISM. Possible combinations of the ISM density and the stellar velocity are derived from these fittings. The discrepancies between the model and the observations suggest that the stellar mass loss has a more complicated history, including possible time and angle dependences.