We present the initial results from the Infrared Array Camera (IRAC) imaging survey of planetary nebulae (PNs). The IRAC colors of PNs are red, especially in the 8.0 μm band. Emission in this band is likely due to contributions from two strong H 2 lines and a [Ar III] line in that bandpass. IRAC is sensitive to the emission in the halos as well as in the ionized regions that are optically bright. In NGC 246, we have observed an unexpected ring of emission in the 5.8 and 8.0 μm IRAC bands not seen previously at other wavelengths. In NGC 650 and NGC 3132, the 8.0 μm emission is at larger distances from the central star compared to the optical and other IRAC bands, possibly related to the H 2 emission in that band and the tendency for the molecular material to exist outside of the ionized zones. In the flocculi of the outer halo of NGC 6543, however, this trend is reversed, with the 8.0 μm emission bright on the inner edges of the structures. This may be related to the emission mechanism, where the H 2 is possibly excited in shocks in the NGC 6543 halo, whereas H 2 emission is likely fluorescently excited in the UV fields near the central star.
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Hora et al. (2004) studied this question.
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