Imaging and Spectroscopy of the Helix Nebula: The Ring Is Actually a Disk

Abstract

The structure and conditions within the Helix Nebula have been determinedfrom emission-line images in Hbeta, [O iii], and He ii and usingspectra tracing a radial from the central star almost out to thenorthern boundary of the optical object. The nebula is approximatelylike a thick disk, rather than the torus suggested by low-ionizationions. The central region, previously thought to be a cavity, is filledwith He^+2 with a total gas density comparable to the main ring.The electron temperature in the outer part was determined from [Nii] line ratios to be 9400 +/- 200 K, while [O iii] lines gave 11700+/- 700 K for the bulk of the main ring of intermediate-ionizationmaterial. The Halpha/Hbeta ratio is anomalously low in the centralportion, suggesting that the electron temperature there is very high.The likely source of this elevated central temperature is heatingby photoelectric electrons from grains mixed in with the nebulargas, since this process depends directly on the distance from thecentral star and becomes relatively more important than photoionizationheating at the low nebular densities that apply for the helix. However,it is unlikely that the two-phase condition that can exist when photoelectricheating dominates has produced the cometary knots that freely populatethis nearby planetary nebula. Based on observations obtained at theCerro Tololo Interamerican Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation.