2008AJ....135.1441R

The planetary nebula NGC 6881 displays in the optical a quadrupolar morphology consisting of two pairs of highly collimated bipolar lobes aligned along different directions. An additional bipolar ejection is revealed by the hydrogen molecular emission, but its wide hourglass morphology is very different from that of the ionized material. To investigate in detail the spatial distribution of molecular hydrogen and ionized material within NGC 6881, and to determine the prevalent excitation mechanism of the H₂ emission, we have obtained new near-IR Brγ and H₂ and optical Hα and [N II] images, as well as intermediate resolution JHK spectra. These observations confirm the association of the H₂ bipolar lobes to NGC 6881 and find that the prevalent excitation mechanism is collisional. The detailed morphology and very different collimation degree of the H₂ and ionized bipolar lobes of NGC 6881 not only imply that multiple bipolar ejections have occurred in this nebula, but also that the dominant shaping agent is different for each bipolar ejection: a bipolar stellar wind most likely produced the H₂ lobes, while highly collimated outflows are carving out the ionized lobes into the thick circumstellar envelope. The asymmetry between the southeast and northwest H₂ bipolar lobes suggests the interaction of the nebula with an inhomogeneous interstellar medium. We find evidence that places NGC 6881 in the H II region Sh 2-109 along the Orion local spiral arm.