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The planetary nebula NGC 6881 displays in the optical a quadrupolarmorphology consisting of two pairs of highly collimated bipolar lobes alignedalong different directions. An additional bipolar ejection is revealed by thehydrogen molecular emission, but its wide hourglass morphology is verydifferent from that of the ionized material. To investigate in detail thespatial distribution of molecular hydrogen and ionized material within NGC6881, and to determine the prevalent excitation mechanism of the H2 emission,we have obtained new near-IR Br-gamma and H2 and optical H-alpha and [N II]images, as well as intermediate resolution JHK spectra. These observationsconfirm the association of the H2 bipolar lobes to NGC 6881 and find that theprevalent excitation mechanism is collisional. The detailed morphology and verydifferent collimation degree of the H2 and ionized bipolar lobes of NGC 6881not only imply that multiple bipolar ejections have occurred in this nebula,but also that the dominant shaping agent is different for each bipolarejection: a bipolar stellar wind most likely produced the H2 lobes, whilehighly collimated outflows are carving out the ionized lobes into the thickcircumstellar envelope. The asymmetry between the southeast and northwest H2bipolar lobes suggests the interaction of the nebula with an inhomogeneousinterstellar medium. We find evidence that places NGC 6881 in the H II regionSh 2-109 along the Orion local spiral arm.

the astronomical journal

The Unusual Distributions of Ionized Material and Molecular Hydrogen in  NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary  Nebula