SIMBAD references

We have acquired imaging of the bipolar outflow M2-9 in the transitions [O III] λ5007, H I λ6563, [N II] λ6584, and [S II] λλ6717, 6731. As a result, we are able to map both excitation and density over the face of the nebula. We find that underlying densities in the lobes increase to n_e∼1.6x10³ cm^–3 at distances r_p∼12"-15" from the central star, after which they again decrease to n_e<200 cm^–3 for r_p>27". Superposed upon this are various density knots, associated with the visual condensations, in which peak values of n_e are of order 6.3x10³ cm^–3. These appear to be responsible for low-excitation shadowing of the outer envelope. All of these features (the density peaks, and broad underlying density variation) are shown to be consistent with a particular model of shock excitation, in which an interior wind is shock-interacting with an external neutral envelope. Such a model appears also to be consistent with trends in [O III]/Hα and [S II]/Hα, which (for the northern lobe at least) appear most consistent with shock excitation.