2007A&A...471..205C

This work is conducted as part of the ``SPECPDR'' program dedicated to the study of very small particles and chemistry in photo-dissociation regions with the Spitzer Space Telescope (SST). We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across the illuminated edge of the Horsehead nebula and especially their survival and properties in the HII region. We present spectral mapping observations taken with the Infrared Spectrograph (IRS) at wavelengths 5.2-38µm. The spectra have a resolving power of λ/Δλ=64-128 and show the main aromatic bands, H₂ rotational lines, ionised gas lines and continuum. The maps have an angular resolution of 3.6-10.6"and allow us to study the nebula, from the HII diffuse region in front of the nebula to the inner dense region. A strong AIB at 11.3µm is detected in the HII region, relative to the other AIBs at 6.2, 7.7 and 8.6µm, and up to an angular separation of ∼20"(or 0.04pc) from the ionisation front. The intensity of this band appears to be correlated with the intensity of the [NeII] at 12.8µm and of Hα, which shows that the emitters of the 11.3µm band are located in the ionised gas. The survival of AIB emitters in the HII region could be due to the moderate intensity of the radiation field (G₀∼100) and the lack of photons with energy above ∼25eV. The enhancement of the intensity of the 11.3µm band in the HII region, relative to the other AIBs can be explained by the presence of neutral PAHs. Our observations highlight a transition region between ionised and neutral PAHs observed with ideal conditions in our Galaxy. A scenario where PAHs can survive in HII regions and be significantly neutral could explain the detection of a prominent 11.3µm band in other Spitzer observations.