Cometary knot

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Cometary knots in the Helix Nebula Helix-nebula-detail-hubble.jpg
Cometary knots in the Helix Nebula

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). [1] [2] They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. [2] They are generally larger than the size of the Solar System (i.e. the orbit of Pluto), with masses of around 0.00001 times the mass of the Sun, which is comparable to the mass of the Earth. [1] [3] [4] There are about 40,000 cometary knots in the Helix Nebula. [5]

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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. [6] In molecular hydrogen and carbon monoxide data, the tails of cometary knots are observed to be highly molecular. [4] The central globule is at least 1000 times denser than the surrounding material that streams past it. [6] 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. [6] In addition, globules near the central star appear to have a distinct trailing tail, whereas those located farther do no exhibit such defined tails. [5]

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. [5] 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. [7]

Reports in more distant objects

Several structures have been described as cometary knots or cometary globules that surround R Coronae Borealis, which is a peculiar star described as potentially the result of a white dwarf merger or final helium shell flash that periodically dims due to a build-up of carbon dust surrounding it, acting as a 'natural coronograph'. [8]

Three-dimensional modelling of NGC 6337, a planetary nebula with a close binary nucleus, suggests the presence of a "thick ring with radial filaments and knots." The cometary knots represent large density fluctuations in a slowly expanding toroid. [9]

Related Research Articles

<span class="mw-page-title-main">Planetary nebula</span> Type of emission nebula created by dying red giants

A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.

<span class="mw-page-title-main">Orion Nebula</span> Diffuse nebula in the constellation Orion

The Orion Nebula is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion, and is known as the middle "star" in the "sword" of Orion. It is one of the brightest nebulae and is visible to the naked eye in the night sky with apparent magnitude 4.0. It is 1,344 ± 20 light-years (412.1 ± 6.1 pc) away and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light-years across. It has a mass of about 2,000 times that of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.

<span class="mw-page-title-main">Helix Nebula</span> Planetary nebula in the constellation Aquarius

The Helix Nebula is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth. The distance, measured by the Gaia mission, is 655±13 light-years. It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are similar to the Dumbbell Nebula, varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture, as well as the "Eye of Sauron".

<span class="mw-page-title-main">H II region</span> Large, low-density interstellar cloud of partially ionized gas

An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

<span class="mw-page-title-main">Proplyd</span> Dust ring surrounding large stars thousands of solar radii wide

A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth.

<span class="mw-page-title-main">NGC 2346</span> Planetary nebula in the constellation Monoceros

NGC 2346 is a planetary nebula near the celestial equator in the constellation of Monoceros, less than a degree to the ESE of Delta Monocerotis. It is informally known as the Butterfly Nebula. The nebula is bright and conspicuous with a visual magnitude of 9.6, and has been extensively studied. Among its most remarkable characteristics is its unusually cool central star, which is a spectroscopic binary, and its unusual shape.

<span class="mw-page-title-main">Stingray Nebula</span> Planetary nebula in the constellation Ara

The Stingray Nebula is the youngest-known planetary nebula, having appeared in the 1980s. The nebula is located in the direction of the southern constellation Ara, and is located 18,000 light-years away. Although it is some 130 times the size of the Solar System, the Stingray Nebula is only about one tenth the size of most other known planetary nebulae. The central star of the nebula is the fast-evolving star SAO 244567. Until the early 1970s, it was observed on Earth as a preplanetary nebula in which the gas had not yet become hot and ionized.

Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.

<span class="mw-page-title-main">NGC 2438</span> Planetary nebula in the constellation Puppis

NGC 2438 is a planetary nebula in the southern constellation of Puppis. Parallax measurements by Gaia put the central star at a distance of roughly 1,370 light years. It was discovered by William Herschel on March 19, 1786. NGC 2438 appears to lie within the cluster M46, but it is most likely unrelated since it does not share the cluster's radial velocity. The case is yet another example of a superposed pair, joining the famed case of NGC 2818.

<span class="mw-page-title-main">NGC 7027</span> Planetary nebula in the constellation Cygnus

NGC 7027, also known as the Jewel Bug Nebula, is a very young and dense planetary nebula located around 3,000 light-years from Earth in the constellation Cygnus. Discovered in 1878 by Édouard Stephan using the 800 mm (31 in) reflector at Marseille Observatory, it is one of the smallest planetary nebulae and by far the most extensively studied.

<span class="mw-page-title-main">Protoplanetary nebula</span> Nebula surrounding a dying star

A protoplanetary nebula or preplanetary nebula is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars.

<span class="mw-page-title-main">Sh2-279</span> Emission nebula in the constellation Orion

Sh2-279 is an HII region and bright nebulae that includes a reflection nebula located in the constellation Orion. It is the northernmost part of the asterism known as Orion's Sword, lying 0.6° north of the Orion Nebula. The reflection nebula embedded in Sh2-279 is popularly known as the Running Man Nebula.

<span class="mw-page-title-main">NGC 6302</span> Bipolar planetary nebula in the constellation Scorpius

NGC 6302 is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae. The spectrum of NGC 6302 shows that its central star is one of the hottest stars known, with a surface temperature in excess of 250,000 degrees Celsius, implying that the star from which it formed must have been very large.

<span class="mw-page-title-main">NGC 6751</span> Planetary nebula in the constellation Aquila

NGC 6751, also known as the Glowing Eye Nebula, is a planetary nebula in the constellation Aquila. It is estimated to be about 6,500 light-years away.

<span class="mw-page-title-main">NGC 2022</span> Planetary nebula in the constellation Orion

NGC 2022 is a planetary nebula in the equatorial constellation of Orion, located at a distance of 8.21 kilolight-years from the Sun. It was first observed by William Herschel on December 28, 1785, who described it as: considerably bright, nearly round, like a star with a large diameter, like an ill-defined planetary nebula. In medium-sized amateur telescopes it looks like a small grayish patch of light. It is not very bright but it is still easy to spot it in the eyepiece. Even in a telescope as small as 80mm it can just be seen using a narrowband filter such as an OIII filter as a 'fuzzy' star. The object has the shape of a prolate spheroid with a major to minor axis ratio of 1.2, an apparent size of 28″, and a halo extending out to 40″, which is about the angular diameter of Jupiter as seen from Earth.

<span class="mw-page-title-main">NGC 1501</span> Planetary nebula in the constellation Camelopardalis

NGC 1501 is a complex planetary nebula located in the constellation of Camelopardalis, it was discovered on the 27th August 1787 by William Herschel. It is also known as the Oyster Nebula.

<span class="mw-page-title-main">NGC 6905</span> Planetary nebula in the constellation Delphinus

NGC 6905, also known as the Blue Flash Nebula, is a planetary nebula in the constellation Delphinus. It was discovered by William Herschel in 1784. The central star is 14.0 mag. The distance of the nebula, as with most planetary nebulae, is not well determined and estimates range between 1.7 and 2.6 kpc.

<span class="mw-page-title-main">NGC 6778</span> Planetary nebula in the constellation Aquila

NGC 6778 is a planetary nebula (PN) located about 10,300 light years away from the Sun in the equatorial constellation of Aquila. It is positioned 5° to the SSW of the prominent star Delta Aquilae. This nebula was discovered by German astronomer Albert Marth during the period 1863–1865. English astronomer John Herschel may have mistakenly catalogued it as NGC 6785, as nothing can be found now at the coordinates he gave for it. In the New General Catalogue it was described as a "small, elongated, ill-defined disc".

<span class="mw-page-title-main">NGC 6337</span> Planetary nebula in the constellation Scorpius

NGC 6337, the Ghostly Cheerio or Cheerio Nebula, is a toroidal planetary nebula in the constellation Scorpius. It appears as a ring-shaped (annular) transparent nebula resembling a piece of the breakfast cereal Cheerios, hence the name. Filament and knots, and a faint shell surround the ring. Its magnitude is 11.90; its position in Scorpius is right ascension 17h 22m15.67s, declination -38° 29' 01.73". The Ghostly Cheerio has a redshift value of -0.000236.

References

  1. 1 2 Nemiroff, R.; Bonnell, J., eds. (13 April 2008). "Curious Cometary Knots in the Helix Nebula". Astronomy Picture of the Day . NASA.
  2. 1 2 C.R. O'Dell; et al. (2003). "Knots in planetary nebulae" (PDF). Revista Mexicana de Astronomía y Astrofísica. 15: 29–33.
  3. ↑ O'dell, C. R.; Handron, Kerry D. (April 1996). "Cometary Knots in the Helix Nebula" (PDF). Astronomical Journal. 111: 1630. Bibcode:1996AJ....111.1630O. doi:10.1086/117902. hdl: 1911/17047 . Their masses of about 10−5 M☉ would be more like those of our solar system planets (MEarth = 3×10−6 M☉, MJ = 9.6×10−4 M☉) than of our largest observed comets (10×1018 gms).
  4. 1 2 Huggins, Patrick J.; Forveille, Thierry; Bachiller, Rafael; Cox, Pierre; Ageorges, Nancy; Walsh, Jeremy R. (2002-07-01). "High-Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula". The Astrophysical Journal. 573 (1): L55–L58. arXiv: astro-ph/0205516 . doi: 10.1086/342021 .
  5. 1 2 3 Matsuura, M.; Speck, A. K.; McHunu, B. M.; Tanaka, I.; Wright, N. J.; Smith, M. D.; Zijlstra, A. A.; Viti, S.; Wesson, R. (2009-08-01). "A "firework" of H2Knots in the Planetary Nebula NGC 7293 (The Helix Nebula)". The Astrophysical Journal. 700 (2): 1067–1077. arXiv: 0906.2870 . Bibcode:2009ApJ...700.1067M. doi:10.1088/0004-637X/700/2/1067. ISSN   0004-637X.
  6. 1 2 3 Meaburn, J.; Clayton, C. A.; Bryce, M. & Walsh, J. R. (1996). "The global motions of the cometary knots in the Helix planetary nebula (NGC 7293)". Monthly Notices of the Royal Astronomical Society. 281 (3): L57–L61. Bibcode:1996MNRAS.281L..57M. doi: 10.1093/mnras/281.3.l57 .
  7. ↑ Henney, W. J. (2001). R. T. Schilizzi (ed.). "Title: Classification of ionized photoevaporation flows". Galaxies and Their Constituents at the Highest Angular Resolutions, Proceedings of IAU Symposium #205, Held 15–18 August 2000 at Manchester, United Kingdom. 205: 272–3. Bibcode:2001IAUS..205..272H..
  8. ↑ Geoffrey C. Clayton; et al. (November 21, 2011). "The circumstellar environment of R Coronae Borealis: white dwarf merger of final-helium-shell flash?". Astrophysical Journal. 743 (1): 44. arXiv: 1110.3235 . Bibcode:2011ApJ...743...44C. doi:10.1088/0004-637X/743/1/44.
  9. ↑ Ma. T. García-Díaz; D. M. Clark; J. A. López; W. Steffen; M. G. Richer (June 24, 2009). "The outflows and three dimensional structure of NGC 6337". The Astrophysical Journal. 699 (2): 1633–1638. arXiv: 0905.1166 . Bibcode:2009ApJ...699.1633G. doi:10.1088/0004-637X/699/2/1633.
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