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 {\mu}m 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 {\lambda}2.122 {\mu}m and optical [N II]
{\lambda}6584 ?A images. The comparison between these images confrm that a
significant fraction of the emission detected in the IRAC 8 {\mu}m 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 {\lambda}2.122 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.
