# Hidden IR structures in NGC 40: signpost of an ancient born-again event

## TL;DR

This paper reveals hidden infrared structures in NGC 40, indicating a toroidal formation likely caused by a late thermal pulse, affecting the nebula's emission characteristics and providing insights into its evolutionary history.

## Contribution

It uncovers a toroidal IR structure in NGC 40 linked to a late thermal pulse, offering new understanding of its morphology and evolution.

## Key findings

- Presence of a toroidal structure mainly seen in PAH emission
- The structure absorbs UV flux, affecting high-excitation lines
- Spectral analysis supports a late thermal pulse scenario

## Abstract

We present the analysis of infrared (IR) observations of the planetary nebula NGC 40 together with spectral analysis of its [WC]-type central star HD 826. Spitzer IRS observations were used to produce spectral maps centred at polycyclic aromatic hydrocarbons (PAH) bands and ionic transitions to compare their spatial distribution. The ionic lines show a clumpy distribution of material around the main cavity of NGC 40, with the emission from [Ar II] being the most extended, whilst the PAHs show a rather smooth spatial distribution. Analysis of ratio maps shows the presence of a toroidal structure mainly seen in PAH emission, but also detected in a Herschel PACS 70 mic image. We argue that the toroidal structure absorbs the UV flux from HD 826, preventing the nebula to exhibit lines of high-excitation levels as suggested by previous authors. We discuss the origin of this structure and the results from the spectral analysis of HD 826 under the scenario of a late thermal pulse.