The Spatial Orientation of Planetary Nebulae Within the Milky Way

Abstract

We analyze the spatial orientation of a homogenous sample of 440 elongated Planetary Nebulae (PNe) in order to determine the orientation of their apparent major axis respect to the Milky Way plane. We present some important geometrical and statistical considerations that have been overlooked by the previous works on the subject. The global distribution of galactic position angles (GPA) of PNe is quantitatively not very different from a random distribution of orientations in the Galaxy. Nevertheless we find that there is at least one region on the sky, toward the galactic center, where a weak correlation may exist between the orientation of the major axis of some PNe and the Galactic equator, with an excess of axes with GPA$\sim 100^{\circ}$. Therefore, we confirm that ``extrinsic'' phenomena (i.e., global galactic magnetic fields, shell compression from motion relative to the Interstellar Medium) do not determine the morphology of PNe on most of the sky, with a possible exception towards the galactic center.