238:
104:(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
20:
1068:
1088:
1002:
250:
1078:
262:
76:
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
96:
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.
88:
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
509:
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)".
226:, 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.
195:, 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.
57:(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
77:
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
637:
Geoffrey C. Clayton; et al. (November 21, 2011). "The circumstellar environment of R Coronae
Borealis: white dwarf merger of final-helium-shell flash?".
611:
Galaxies and Their
Constituents at the Highest Angular Resolutions, Proceedings of IAU Symposium #205, Held 15–18 August 2000 at Manchester, United Kingdom
152:-dominated or dust-dominated. The distinction can be made in terms of the formula for the "dynamic ionization balance within a photoevaporation flow", F
100:
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
682:
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".
172:
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", n
237:
744:
215:
merger or final helium shell flash that periodically dims due to a build-up of carbon dust surrounding it, acting as a 'natural
1029:
892:
457:
Huggins, Patrick J.; Forveille, Thierry; Bachiller, Rafael; Cox, Pierre; Ageorges, Nancy; Walsh, Jeremy R. (2002-07-01).
828:
294:
120:
flow, which is characteristically associated with planetary nebulae, but several other types of photoevaporation flows (
846:
1118:
1113:
1035:
129:
1023:
1018:
1013:
101:
249:
882:
877:
872:
836:
814:
737:
609:
Henney, W. J. (2001). R. T. Schilizzi (ed.). "Title: Classification of ionized photoevaporation flows".
90:
1087:
701:
656:
618:
581:
529:
358:
317:
149:
948:
343:
1081:
931:
910:
691:
646:
519:
474:
208:
905:
804:
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545:
105:
34:
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1005:
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730:
709:
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484:
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204:
125:
117:
19:
974:
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936:
809:
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46:
261:
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533:
362:
969:
772:
713:
541:
133:
1107:
668:
54:
50:
867:
851:
792:
570:"The global motions of the cometary knots in the Helix planetary nebula (NGC 7293)"
141:
58:
38:
24:
915:
841:
797:
767:
216:
212:
137:
42:
1091:
953:
594:
569:
176:
is the "peak ionized density in the flow", and h, which is approximately 0.1 r
66:
549:
145:
289:
180:, is the "effective thickness of the flow". In advection-dominated flows,
479:
223:
93:
85:, but comets are solid bodies and much smaller in overall size and mass.
121:
379:
753:
33:
also referred as globules, are structures observed in several nearby
211:, which is a peculiar star described as potentially the result of a
489:
458:
370:
696:
651:
568:
Meaburn, J.; Clayton, C. A.; Bryce, M. & Walsh, J. R. (1996).
524:
78:
70:
62:
18:
463:
Molecular Line
Imaging of a Cometary Globule in the Helix Nebula"
299:
82:
726:
73:. There are about 40,000 cometary knots in the Helix Nebula.
203:
Several structures have been described as cometary knots or
722:
392:
would be more like those of our solar system planets (
1045:
999:
983:
962:
924:
891:
860:
827:
760:
574:Monthly Notices of the Royal Astronomical Society
288:Nemiroff, R.; Bonnell, J., eds. (13 April 2008).
342:O'dell, C. R.; Handron, Kerry D. (April 1996).
148:-dominated than the other varieties, which are
738:
8:
325:Revista Mexicana de Astronomía y Astrofísica
290:"Curious Cometary Knots in the Helix Nebula"
65:), with masses of around 0.00001 times the
1001:
745:
731:
723:
695:
650:
593:
563:
561:
559:
523:
488:
478:
378:
69:, which is comparable to the mass of the
144:. Cometary knots are described as more
112:Relation to other photoevaporation flows
434:) than of our largest observed comets (
311:
309:
277:
233:
116:Cometary knots are one type of ionized
283:
281:
7:
1077:
504:
502:
500:
452:
450:
344:"Cometary Knots in the Helix Nebula"
108:of low to intermediate mass stars.
16:Structure in some planetary nebulae
14:
316:C.R. O'Dell; et al. (2003).
1086:
1076:
1067:
1066:
1000:
260:
248:
236:
222:Three-dimensional modelling of
199:Reports in more distant objects
385:Their masses of about 10
1:
714:10.1088/0004-637X/699/2/1633
542:10.1088/0004-637X/700/2/1067
318:"Knots in planetary nebulae"
295:Astronomy Picture of the Day
255:Knots in the Dumbbell Nebula
847:Evaporating gaseous globule
1135:
669:10.1088/0004-637X/743/1/44
1061:
684:The Astrophysical Journal
512:The Astrophysical Journal
467:The Astrophysical Journal
459:"High-Resolution CO and H
81:that faces away from its
595:10.1093/mnras/281.3.l57
102:Asymptotic Giant Branch
883:Integrated Flux Nebula
27:
23:Cometary knots in the
873:Protoplanetary nebula
837:Giant molecular cloud
815:Protoplanetary nebula
639:Astrophysical Journal
37:(PNe), including the
22:
893:Post-stellar nebulae
351:Astronomical Journal
53:(NGC 2392), and the
984:Intergalactic blobs
949:High-velocity cloud
829:Pre-stellar nebulae
706:2009ApJ...699.1633G
661:2011ApJ...743...44C
623:2001IAUS..205..272H
586:1996MNRAS.281L..57M
534:2009ApJ...700.1067M
363:1996AJ....111.1630O
61:(i.e. the orbit of
1036:Supernova remnants
932:Interstellar cloud
911:Pulsar wind nebula
209:R Coronae Borealis
28:
1119:Stellar astronomy
1114:Planetary nebulae
1101:
1100:
906:Supernova remnant
878:Wolf–Rayet nebula
805:Reflection nebula
788:Supernova remnant
205:cometary globules
136:) are known from
126:cometary globules
106:stellar evolution
35:planetary nebulae
1126:
1090:
1080:
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1004:
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991:Lyman-alpha blob
901:Planetary nebula
783:Planetary nebula
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718:
717:
699:
690:(2): 1633–1638.
679:
673:
672:
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628:
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518:(2): 1067–1077.
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480:astro-ph/0205516
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187:is greater than
118:photoevaporation
49:(NGC 6853), the
45:(NGC 6720), the
41:(NGC 7293), the
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1103:
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1019:Largest Nebulae
995:
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975:Pinwheel nebula
958:
942:Infrared cirrus
937:Molecular cloud
920:
887:
856:
823:
810:Variable nebula
778:Emission nebula
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134:champagne flows
130:elephant trunks
114:
67:mass of the Sun
47:Dumbbell Nebula
31:Cometary knots,
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12:
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1030:Protoplanetary
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970:Bipolar nebula
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861:Stellar nebula
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773:Diffuse nebula
770:
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761:Visible nebula
758:
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749:
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629:
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580:(3): L57–L61.
555:
496:
490:10.1086/342021
473:(1): L55–L58.
460:
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371:10.1086/117902
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1053:Cometary knot
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267:Eskimo Nebula
263:
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229:
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150:recombination
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107:
103:
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80:
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72:
68:
64:
60:
56:
55:Retina Nebula
52:
51:Eskimo Nebula
48:
44:
40:
36:
32:
26:
21:
1052:
868:Nova remnant
852:Solar nebula
793:Nova remnant
687:
683:
677:
642:
638:
632:
614:
610:
604:
577:
573:
515:
511:
470:
466:
427:
406:
384:
354:
350:
337:
328:
324:
293:
221:
202:
188:
181:
157:
142:Orion Nebula
140:such as the
138:H II regions
115:
99:
87:
75:
59:Solar System
39:Helix Nebula
30:
29:
25:Helix Nebula
842:Bok globule
798:H II region
768:Dark nebula
440:10 gms
243:Ring Nebula
217:coronograph
213:white dwarf
43:Ring Nebula
1108:Categories
1092:Wiktionary
963:Morphology
954:H I region
916:Supershell
380:1911/17047
273:References
1024:Planetary
697:0905.1166
652:1110.3235
645:(1): 44.
617:: 272–3.
550:0004-637X
525:0906.2870
168:. Here F
146:advection
1072:Category
431:☉
426:10
410:☉
405:10
390:☉
357:: 1630.
331:: 29–33.
224:NGC 6337
122:proplyds
94:Angstrom
1082:Commons
1046:Related
1014:Diffuse
754:Nebulae
702:Bibcode
657:Bibcode
619:Bibcode
582:Bibcode
530:Bibcode
359:Bibcode
230:Gallery
1038:(SNRs)
1032:(PPNe)
925:Clouds
548:
132:, and
1026:(PNe)
1006:Lists
692:arXiv
647:arXiv
520:arXiv
475:arXiv
347:(PDF)
321:(PDF)
92:5007
79:comet
71:Earth
63:Pluto
546:ISSN
300:NASA
162:+ αn
83:star
710:doi
688:699
665:doi
643:743
615:205
590:doi
578:281
538:doi
516:700
485:doi
471:573
422:9.6
375:hdl
367:doi
355:111
219:'.
1110::
708:.
700:.
686:.
663:.
655:.
641:.
613:.
588:.
576:.
572:.
558:^
544:.
536:.
528:.
514:.
499:^
483:.
469:.
465:.
449:^
442:).
436:10
420:=
413:,
399:=
383:.
373:.
365:.
353:.
349:.
329:15
327:.
323:.
308:^
298:.
292:.
280:^
189:αn
182:μn
158:μn
156:≈
128:,
124:,
746:e
739:t
732:v
716:.
712::
704::
694::
671:.
667::
659::
649::
627:.
625:.
621::
598:.
592::
584::
552:.
540::
532::
522::
493:.
487::
477::
461:2
438:×
428:M
424:×
418:J
415:M
407:M
403:×
401:3
397:E
394:M
387:M
377::
369::
361::
302:.
193:h
191:0
184:0
178:0
174:0
170:*
166:h
164:0
160:0
154:*
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