HST imaging of hydrogen-poor Ejecta in Abell 30 and Abell 78: Wind-blown cometary structures

Hubble Space Telescope Faint Object Camera images of hydrogen-poor gas in the planetary nebulae A30 and A78 have revealed remarkable "cometary" structures in the [O III] λ5007 line. Most of these cometary structures, consisting of compact (0″.15-0″.5) knots with radial tails several arcseconds in length, are located in an equatorial plane in both nebulae. In addition, two bright, compact (0″.3) polar knots are present in A30, one of them forming a bow shock. Corresponding polar features in A78 are more diffuse. The central stars of both nebulae have energetic winds which are most likely responsible for the "cometary" knot morphology. We interpret this morphology in terms of dense (several thousand electrons cm^-3) H-poor condensations whose outer expanding layers are swept outward by stellar winds. Photoionization modeling indicates that while dense knot cores are mostly heated by atomic photoionization, expanding tenuous gas is heated by photoelectrons ejected from abundant dust grains. Our models predict steep temperature gradients for which there is observational evidence and possible abrupt phase transitions in the expanding gas.