SIMBAD references

On the basis that the cometary knots observed in the Helix Nebula form as a result of larger ``parent clouds'' breaking up due to Rayleigh-Taylor instability induced by radiative acceleration of the clouds, we compute characteristics of the cometary knots and of the parent clouds as well. Present observations place constraints on the positions, velocities, and sizes of the parent clouds. Requiring the clouds to produce cometary knots that are stable places further constraints on the properties of parent clouds. We formulate those constraints and show how they further limit the predicted properties of the cometary knots and bring them into agreement with their observed properties.