Cometary knots in the Helix Nebula

The Cometary Knots in the Helix Nebula have shown themselves in observations with the Hubble Space Telescope's WFPC2 to be well-resolved photoionized surfaces on the central star facing sides of neutral clouds. Material streams backwards from the rims of the ionized surfaces, producing tails. The neutral cores of the Cometary Knots are seen in extinction against the background radiation of the planetary nebula. Calculations of the neutral core masses from the observed extinction indicate masses of about 1.5X10(-5) M. for the best observed objects. We have detected 313 of these objects and project a total number for the entire nebula of 3500. The surface brightness distribution suggests that the surfaces are in at least quasi-equilibrium, arguing that mass loss through the 1.4x10(-9) M. ionized portions is low, thus allowing a lifetime exceeding that of the planetary nebula stage. Various physical models are advanced which may explain the presence of the Cometary Knots. Rayleigh-Taylor instabilities seem the most likely source, arising from either the earliest stages of the planetary nebula development at the ionization boundary between the ionized and neutral components or later when the stellar wind was shocking and shaping the inner part of the nebula. However, we cannot rule out the less likely possibility that these knots are primordial, going back to the original formation of what is now the central star. We even consider that they are the result of mass loss from gigantic comets at distances comparable to our own Oort Cloud of comets, but such a model would require the existence of volatile Earth sized bodies of a nature not usually identified as comets. (C) 1996 American Astronomical Society.