Morphology and Composition of the Helix Nebula

We present new narrowband filter imagery in Hα and [N II] λ6584, along with UV and optical spectrophotometry measurements from 1200 to 9600 Å of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter, with an inner region containing hot, highly ionized gas that is generally invisible in narrowband images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000 K and 100 L☉, and we estimate a lower limit to the nebular mass to be 0.30 M☉. Abundance measurements indicate the following values: He/H=0.12 (± 0.017), O/H=4.60×10-4 (± 0.18), C/O=0.87 (± 0.12), N/O=0.54 (± 0.14), Ne/O=0.33 (± 0.04), S/O=3.22 × 10-3 (± 0.26), and Ar/O=6.74 × 10-3 (± 0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio that we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the supersolar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5 M☉ progenitor that underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.