From Historical Perspectives to Some Modern Possibilities

A historical perspective on the study of asymmetries in planetary nebulae (PNs) is presented. We also describe our ongoing work in high resolution spectroscopy of planetaries, and discuss some likely future directions for the study of asymmetrical PNs. The first systematic study of planetary nebulae (PNs) was undertaken at Lick Observatory from 1914 to 1917 and is reported in detail in Volume 13 of their publications. Of particular interest are the extensive papers of Curtis (1918) on the shapes and forms of planetaries and of Campbell & Moore (1918) on their internal motions. Curtis’ discussion was based on plates taken with the Crossley Reflector with a plate scale of 38.7 mm. Many PN are, of course, quite compact so the fine details are lost because of the limited resolution of the emulsion. Curtis obtained a series of graded exposures from which he made drawings of each object, thus overcoming many of the limitations imposed by the non-linearity of photographic emulsions and the limited intensity range that can be accommodated by a single exposure. He recognized that by going to a telescope of much longer focal length “a wealth of minute structural detail would be shown in features that appeared on Crossley plates as diffuse areas and wisps.” From direct photographs alone (even without monochromatic images that showed widely different structures in [O II], [O III], and He II, for example), Curtis noted: “It is evident that we have to do with structures of extraordinary complexities – the aberrant wisps and striae and other minor formal irregularities in such structures as the Ring Nebula in Lyra, NGC 7009, NGC 7026 and others would seem to defy all attempts to analyze the details, whatever hypothesis may be adopted regarding the general form of the structure as a whole.” Curtis (1918) noted that the ring hypothesis fails, as we should expect a large number of elliptical or edge-on forms. The hypothesis of ellipsoidal shells of uniform thickness also fails, as it cannot explain very faint central regions or faintness at the ends of the major axis. He proposed some general classes: ellipsoids or sphere-ring forms, ring shells, ellipsoidal shells, helical objects such as NGC 6543 and NGC 7293, and anomalous forms like NGC 2440. He was unable to recognize such objects as NGC 7027 as shell structures heavily obscured by dust and classified them as “anomalous.” One of the earliest attempts to combine nebular direct photos with kinematical data was that made by Warren K. Green (1917). He compared direct images and radial velocity studies of NGC 6543 and NGC 7009 and tried to interpret them with a theoretical picture of rotating shells of gas. He told me