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