SIMBAD references

The structure and conditions within the Helix Nebula have been determined from emission-line images in Hβ, [O III], and He II and using spectra tracing a radial from the central star almost out to the northern boundary of the optical object. The nebula is approximately like a thick disk, rather than the torus suggested by low-ionization ions. The central region, previously thought to be a cavity, is filled with He⁺² with a total gas density comparable to the main ring. The electron temperature in the outer part was determined from [N II] line ratios to be 9400±200 K, while [O III] lines gave 11700 ±700 K for the bulk of the main ring of intermediate-ionization material. The Hα/Hβ ratio is anomalously low in the central portion, suggesting that the electron temperature there is very high. The likely source of this elevated central temperature is heating by photoelectric electrons from grains mixed in with the nebular gas, since this process depends directly on the distance from the central star and becomes relatively more important than photoionization heating at the low nebular densities that apply for the helix. However, it is unlikely that the two-phase condition that can exist when photoelectric heating dominates has produced the cometary knots that freely populate this nearby planetary nebula.