2008MNRAS.387..415R

We have undertaken echelle spectroscopy and narrow-band line imaging of the bipolar planetary nebula M 1-8. This has permitted us to map the outflow in [NII] λλ 6548+6583 Å, Hα, and in the v = 1-0 S(1) transition of H₂ at λ 2.122 µm. It has also permitted us to acquire high-resolution spectra for [NII] λ 6583Å, Hα and HeII λ 6560 Å. Our observations support the results of a previous 2MASS analysis by two of the authors (J. P. Phillips and G. Ramos-Larios), and confirm that there is strong H₂ emission outside of the ionized zone, as well as along the major axis of the outflow. Finally, we have investigated the spatial structure of the outflow in low and high excitation lines, and noted evidence for strong ionization stratification within the envelope of the source. We also note that major axis spectra show asymmetries attributable to outflow along the lobes, oriented at an angle i ∼ 35°-40° to the line of sight. Asymmetries along the minor axis, by contrast, appear to be associated with the central collimating disc, and may be interpretable in terms of asymmetries in disc structure, or rotation at an angular velocity of Ω ∼ 1.4 10^–12 rad/s. If the disc arises due to common-envelope evolution, then it seems that angular momentum constraints must be relatively tight, and can only be satisfied given fairly extreme physical assumptions (such as low disc mass, high primary star mass, a low distance to the source and so forth).