Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406). They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System (i.e. the orbit of Pluto), with masses of around 10−5 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
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| - Cometary knot (en)
- Nœud cométaire (fr)
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| - Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406). They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System (i.e. the orbit of Pluto), with masses of around 10−5 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula. (en)
- Les nœuds cométaires sont des objets observés à proximité de plusieurs nébuleuses planétaires, y compris la nébuleuse de l’Hélice (NGC 7293), la nébuleuse de la Lyre (NGC 6720 ou M57), la nébuleuse de l'Haltère (NGC 6853 ou M27), la nébuleuse de l'Esquimau (NGC 2392) et la IC 4406. On suppose que ces objets sont des éléments courants de l’évolution des nébuleuses planétaires, mais cette hypothèse ne peut être confirmée que grâce aux exemples les plus approfondis. Ils sont dans l’ensemble plus grands que le Système solaire en termes de taille (ex. : l’orbite de Pluton), avec des masses comparables à celle de la Terre. Un nœud représente le « revêtement ionisé d’un globule moléculaire poussiéreux et dense » qui forme un objet en forme de croissant, ionisé et illuminé par une étoile centrale, (fr)
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| - Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406). They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System (i.e. the orbit of Pluto), with masses of around 10−5 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula. 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. (en)
- Les nœuds cométaires sont des objets observés à proximité de plusieurs nébuleuses planétaires, y compris la nébuleuse de l’Hélice (NGC 7293), la nébuleuse de la Lyre (NGC 6720 ou M57), la nébuleuse de l'Haltère (NGC 6853 ou M27), la nébuleuse de l'Esquimau (NGC 2392) et la IC 4406. On suppose que ces objets sont des éléments courants de l’évolution des nébuleuses planétaires, mais cette hypothèse ne peut être confirmée que grâce aux exemples les plus approfondis. Ils sont dans l’ensemble plus grands que le Système solaire en termes de taille (ex. : l’orbite de Pluton), avec des masses comparables à celle de la Terre. Un nœud représente le « revêtement ionisé d’un globule moléculaire poussiéreux et dense » qui forme un objet en forme de croissant, ionisé et illuminé par une étoile centrale, avec un rayon ou une queue à l’arrière. Le globule central est au moins 1 000 fois plus dense que le matériau environnant qui l’englobe. Son aspect est similaire à la queue d’une comète qui tourne le dos à son étoile. Cependant, les comètes sont des corps solides bien plus petits en termes de masse et de taille globale.À proximité de la nébuleuse de l’Hélice, le globule poussiéreux central de chaque nœud cométaire apparaît sombre en contraste avec le fond d’une coque sphérique qui émet de la lumière à [O III] 5 007 Angstrom. Ceux qui se trouvent du côté le plus éloigné ne masquent pas cette source de lumière et ne disposent donc pas de cette apparence sombre. (fr)
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