2002AJ....123..346S

We present new observations of the ionized gas, molecular gas, and cool dust in the Helix Nebula (NGC 7293). The ionized gas is observed in the form of an Hα image, which is constructed using images from the Southern Hα Sky Survey Atlas. The molecular emission was mapped using the H₂ v=1⟶0 S(1) line at 2.122 µm. The far-infrared (FIR) observations were obtained using ISOPHOT on the Infrared Space Observatory. The Hα observations are more sensitive than previous measurements and show the huge extent of the Helix, confirming it as a density-bounded nebula and showing previously unseen point-symmetric structures. The H₂ observations show that the molecular gas follows the distribution of molecular material shown in previous work. The molecular emission is confined to that part of the nebula seen in the classic optical image. Furthermore, comparison of the H₂ emission strength with time-dependent models for photodissociation regions (PDRs) shows that the emission arises from thermal excitation of the hydrogen molecules in PDRs and not from shocks. The FIR observations, at 90 and 160 µm, represent mostly contributions from thermal dust emission from cool dust grains but include a small contribution from ionized atomic lines. Comparison of the FIR emission with the Hα observation shows that the dust and ionized gas are coincident and extend to ∼1100" radius. This equates to a spatial radial extent of more than 1 pc (assuming a distance to the Helix of ∼200 pc). Assuming that the outer layers of the circumstellar shell have spherical symmetry, radiative transfer modeling of the emission in Hα gives a shell mass of ∼1.5 M_☉. However, the modeling does not cover the outermost part of the shell (beyond ∼600" radius), and therefore this is a lower limit for the shell mass. Moreover, the models suggest the need for very large dust grains, with ∼80% of the dust mass in grains larger than 3.5 µm. Comparison of these new observations with previous observations shows the large-scale stratification of the Helix in terms of ionized gas and dust, as well as the coexistence of molecular species inside the ionized zones, where molecules survive in dense condensations and cometary knots.