Cometary knot

At optical wavelengths, the knots are seen as "the ionized skin of a dense, dusty molecular globule" forming a crescent-shaped head that is ionized and illuminated by the central star, with a trailing spoke or tail. In molecular hydrogen and carbon monoxide data, the tails of cometary knots are observed to be highly molecular. The central globule is at least 1000 times denser than the surrounding material that streams past it. The appearance is analogous to the tail of a comet that faces away from its star, but comets are solid bodies and much smaller in overall size and mass.

Globules located far and close to the central star present different characteristics. On the near side of the Helix Nebula, the central dusty globule of each cometary knot appears dark against the background as it absorbs the [O III] 5007 Angstrom light emitted in the nebular envelope. Those on the far side do not obstruct this light source and so do not have this dark appearance. In addition, globules near the central star appear to have a distinct trailing tail, whereas those located farther do no exhibit such defined tails.

The origin of cometary knots in planetary nebulae is still unknown and subject to active research. It is unclear whether they were created during the Asymptotic Giant Branch (AGB) phase and somehow managed to survive the AGB-PN transition, or if they were created when the star has already become a planetary nebula. The latter case would imply that the conditions in the planetary nebula host would have, at a certain point, triggered the formation of molecular clumps in its nebular envelope. Therefore, understanding the formation and evolution of cometary knots would not only give an insight into the physical properties of the planetary nebula host, but would also help draw a more detailed picture of the stellar evolution of low to intermediate mass stars.

Relation to other photoevaporation flows

Cometary knots are one type of ionized photoevaporation flow, which is characteristically associated with planetary nebulae, but several other types of photoevaporation flows (proplyds, cometary globules, elephant trunks, and champagne flows) are known from H II regions such as the Orion Nebula. Cometary knots are described as more advection-dominated than the other varieties, which are recombination-dominated or dust-dominated. The distinction can be made in terms of the formula for the "dynamic ionization balance within a photoevaporation flow", F_* ≈ μn₀ + αn₀²h. Here F_* is the "ionizing photon flux incident on the outside of the flow", μ is the "initial velocity of the flow", α is the "recombination coefficient", n₀ is the "peak ionized density in the flow", and h, which is approximately 0.1 r₀, is the "effective thickness of the flow". In advection-dominated flows, μn₀ is greater than αn₀²h, and most of the incoming photons reach the ionization front and ionize fresh gas. In other flows, most photons fail to reach the ionization front, and instead balance recombinations in the flow.