Individual Planetary Nebula (PN) distances are frequently determined by comparing their angular expansion in the sky to the spectroscopically measured radial velocity of the gas. However, even in the case of a perfectly spherical nebula, these velocities are different since the first is a pattern velocity and the second a material velocity. Mellema derived analytical expressions to estimate the magnitude of the difference between the two velocities and found that for the typical PN velocities measured, the pattern velocity is 20-30% higher than the material velocity. PN distances estimated from the expansion parallax may therefore be 20-30% larger than previously thought.
Mellema, Corradi (ING-UK), Schwarz (CTIO-USA), Sanchez-Blazquez (Madrid, Sp) and Gianmanco (IAC, Sp) searched for ring-like structures in the faint halos of Planetary Nebulae. Eight new nebulae with such rings were found, increasing the total sample to ten! This allowed them to apply (small number) statistics to the sample. Given the biases in the original sample, they estimated that rings may be present in a third of all PNs. They suspected that the spacing increases with the age of the system, especially after comparison with the proto-Planetary and Planetary Nebula samples. Such properties would be roughly consistent with the models of Meijerink, Mellema and Simis, but do not rule out other models.
Isosurface representation of the squashing of an intergalactic cloud by the passage of a strong shock. The top left shows the initial cloud with a size of 100 pc, the rest of the sequence shows the evolution in steps of 300,000 years, from the initial flattening to the squashing into a long thin cloud, which would be subject to gravitationial collapse.