Resumen
NGC 6881 is one of the very few quadrupolar PNe, which is constituted by 2 pairs of bipolar lobes oriented towards different directions. This morphology is indicative of changes in the direction of the ejection of material, suggesting the precession of the equatorial ring. The H_2 and optical morphology of NGC 6881 are very different. To investigate in detail the spatial distribution of molecular and ionized material within NGC 6881 and to determine the prevalent excitation mechanism of the H[2] emission, we have obtained new near-IRBrγ and H_2λ = 2.1218~μm with WHT, optical Hα and [NII] images with NOT, as well as intermediate resolution JHK spectra with TNG. These observations show that NGC 6881 has multiple bipolar lobes formed at different phases of the nebular evolution.
The H_2 images show bipolar lobes resembling an hourglass. Although these lobes are oriented along the mayor axis of the ionized lobes, they are much less collimated. Additionally, the detected northeast lobe is 3-4 times more extended than the ionized lobes. The central region is resolved in our new H[2] images and show the spatial distribution expected for a ring lying outside the ionized material. The different spectral diagnostics indicate that the H[2] emission is produced by shock excitation in the molecule.
