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On the relationship between the H2 emission and the physical structure of planetary nebulae

Mid-IR observations of planetary nebulae (PNe) have revealed diffuse emission associated to their main nebular shells and outer envelopes or haloes. The interpretation of this emission is uncertain because the broad-band mid-IR images may include contributions of different components. In particular, the Spitzer IRAC 8 μm images, that best reveal these nebular features, can include contributions not only of H2 lines, but also those of ionic species, PAH features, and thermal dust continuum emission. To investigate the nature of the emission detected in mid-IR observations of a sample of 10 PNe, we have obtained narrow-band near-IR H2 λ2.122 μm and optical [N II] λ6584 ?A images. The comparison between these images confrm that a significant fraction of the emission detected in the IRAC 8 μm images can be attributed to molecular hydrogen, thus confirming the utility of these mid-IR images to investigate the molecular component of PNe. We have also detected H2 emission from PNe whose physical structure cannot be described as bipolar, but rather as ellipsoidal or barrel-like. These detections suggest that, as more sensitive observations of PNe in the H2 λ2.122 line are acquired, the detection of H2 emission is not exclusive of bipolar PNe, although objects with this morphology are still the brightest H2 emitters. Finally, we remark that the bright H2 emission from the equatorial ring of a bipolar PN does not arise from a photodissociation region shielded from the UV stellar radiation by the ring itself, but from dense knots and clumps embedded within the ionized material of the ring.