HD 179821: Difference between revisions

HD 179821
	; HD 179821; Credit: Hubble Legacy Archive
Observation data; Epoch J2000 Equinox J2000
Constellation	Aquila
Right ascension	19h 13m 58.61s
Declination	+00° 07′ 31.9″
Apparent magnitude (V)	8.19 (7.83 - 8.23)
Characteristics
Evolutionary stage	YHG or post-AGB
Spectral type	G4 0-Ia (G5Ia - K4I)
Apparent magnitude (U)	10.81
Apparent magnitude (B)	9.694
Apparent magnitude (R)	8.2
Apparent magnitude (J)	5.371
Apparent magnitude (H)	4.998
Apparent magnitude (K)	4.728
B−V color index	+1.504
Variable type	SRd
Astrometry
Radial velocity (Rv)	+81.8±3.7 km/s
Proper motion (μ)	RA: −0.090 mas/yr ; Dec.: −3.494 mas/yr
Parallax (π)	0.1893 ± 0.0206 mas
Distance	approx. 17,000 ly ; (approx. 5,300 pc)
Absolute magnitude (MV)	−8.9 or −5.7
Details
if a hypergiant
Mass	19 - 30 M☉
Radius	400 - 450 R☉
Luminosity	126,000 - 295,000 L☉
Surface gravity (log g)	0.5 cgs
Temperature	4,898 - 6,761 K
Metallicity [Fe/H]	0.0 dex
if post-AGB
Mass	0.8 M☉
Radius	147 R☉
Luminosity	20,000+12,000; −7,400 L☉
Surface gravity (log g)	0.5 cgs
Temperature	5,660 K
Metallicity [Fe/H]	0.0 dex
Other designations
	HD 179821, V1427 Aql, AFGL 2343, BD−00°3679, HIP 94496, SAO 124414, IRAS 19114+0002, 2MASS J19135861+0007319
Database references
SIMBAD	data

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Latest revision as of 07:06, 7 October 2024

HD 179821 or V1427 Aquilae is either a post-red supergiant yellow hypergiant or a post-AGB yellow supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable nearing the end of its life.

Discovery

HD 179821 was first catalogued as an unremarkable 8th magnitude star at the start of the 20th century.^[12] It was later listed as a spectral standard G4 0-Ia, indicating a highly luminous star type now known as a hypergiant.^[13]

It was first considered notable for its infrared excess and double-peaked spectral energy distribution in the infrared.^[14] These were considered to be indicators of surrounding dust and HD 179821 was identified as a possible protoplanetary nebula.^[15] Variability was also detected.^[16]

High resolution spectroscopic studies and modern space-based observations have revealed an unusual chemical makeup and a hollow spherical dust shell, but haven't fully resolved whether HD 179821 is a highly luminous yellow hypergiant or a dimmer, lower-mass post-AGB star.^[10]

Observations

HD 179821 has a cold detached dust shell that has been studied with the help of the Hubble Space Telescope. The shell is approximately circular in shape, has an inner diameter of ~3".3 corresponding to 20,000 AU at 6,000 pc, and an outer diameter of 5".7 or more, with the star 0".35 from the centre of the shell. The current mass loss is low, but during the formation of the shell it is estimated to have been 4×10⁻⁴ M_☉/yr, an exceptionally high rate being comparable to that of the archetypical OH/IR red supergiant, VY Canis Majoris.^[17] Like its constellation neighbor and also hypergiant star IRC +10420, it is surrounded by an extended reflection nebula. Discovered at near-IR wavelength, this indicates a massive star^[18] and, as with the reflection nebula around IRC +10420, it may be masking a star hotter than the given G5 spectral type.^[19]

It is that which contributes to a double-peaked spectral energy distribution.^[20] It is estimated the star has lost about 10% of its initial mass after being a red supergiant star just 1,600 years ago,^[17] and is a likely supernova candidate.^[21]

The distance was once estimated to be around 6,000 parsecs. It has a high luminosity of between 1.26×10⁵ and 2.95×10⁵ L_⊙ and a radius of between 400 and 450 R_☉.^[11]^[9] It has a high radial velocity of +100 km/s.^[22] According to the studies of Jura et al (2001), the star may explode as a supernova in the next 100,000 years.

Variability

The blue band light curve of HD 179821, adapted from Arkhipova *et al.* (2001)^[23]

HD 179821 is a semiregular variable star with the variable star designation V1427 Aquilae. Between 1899 and 1989, its photographic apparent magnitude varied erratically between about magnitudes 9 and 10, although coverage is not complete and some larger variations may have been missed. It then varied by no more than 0.1 magnitudes until 2009, at a visual magnitude around 8.1.^[11]

The colour of the star changed noticeably so that the variability at different wavelengths is not consistent. In general, the star became bluer from 1899 until 1990, and then redder again. The colour changes most likely reflect changes in the effective temperature, and probably underlying evolutionary trends with the star performing a blue loop between temperatures of 4,000 K and 8,000 K.^[11] Pulsations for much of this time occurred with an approximate period of 100 to 150 days, although this increased to 250 days between 2010 and 2017, which is expected for stars which are decreasing in temperature.^[11] At its coolest, the spectral type has been recorded as K4,^[3] while near its hottest in 2007 it was classified as F7.^[24]

Chemical composition

The chemical composition of this star differs from that of other yellow supergiant stars. The star is moderately metal-deficient^[18] and the main elements present in the star (apart from hydrogen and helium) are oxygen, carbon and nitrogen. Molecules such as hydrogen isocyanide, sulfur monoxide and HCO⁺ have been detected in the circumstellar envelope of the star. These molecules may result from an active photochemistry, generated by UV photons emitted by the central star as it warms up, or can be produced in shocks.^[20]

Controversy

While most authors consider HD 179821 to be a warm hypergiant star,^[25] others think it is actually a protoplanetary nebula or a smaller post-AGB star at a distance of 1 kiloparsec (3,200 light years).^[26] In that case the star's luminosity and radius would be much lower, around 16,000 times that of the Sun and 60 to 80 R_☉, and its initial mass would be equal to the current mass of the Sun.^[11]^[10]

This discrepancy arises because its distance was too great to be measured by parallax before the Gaia mission and it has some properties of both a yellow hypergiant and a protoplanetary nebula/Post-AGB star.^[8]^[10] Gaia Data Release 3 gives a parallax of 0.19 mas implying a distance around 5,300 pc.^[1]

Notes

^ Applying the Stefan-Boltzmann Law with a nominal solar effective temperature of 5,772 K:
${\sqrt {(5772/5660)^{4}*20,000}}=147.07\ R\odot$

References

^ ^a ^b ^c ^d ^e ^f Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b ^c Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862. ISBN 0333750888.
^ ^a ^b ^c ^d Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
^ Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373. S2CID 123149047.
^ Nordhaus, J.; Minchev, I.; Sargent, B.; Forrest, W.; Blackman, E. G.; De Marco, O.; Kastner, J.; Balick, B.; Frank, A. (2008). "Towards a spectral technique for determining material geometry around evolved stars: Application to HD 179821". Monthly Notices of the Royal Astronomical Society. 388 (2): 716. arXiv:0801.2978. Bibcode:2008MNRAS.388..716N. doi:10.1111/j.1365-2966.2008.13428.x. S2CID 17468700.
^ ^a ^b ^c ^d Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
^ ^a ^b ^c ^d ^e ^f ^g Parthasarathy, M.; Jasniewicz, G.; Thěvenin, F. (2019). "Correction to: The puzzling high velocity G5 supergiant star HD 179821: New insight from Gaia DR2 data". Astrophysics and Space Science. 364 (2): 25. arXiv:1901.08995. Bibcode:2019Ap&SS.364...25P. doi:10.1007/s10509-019-3512-5. S2CID 119427229.
^ ^a ^b ^c ^d ^e ^f Reddy, B. E.; Hrivnak, Bruce J. (April 1999). "Spectroscopic Study of HD 179821 (IRAS 19114+0002): Proto–Planetary Nebula or Supergiant?". The Astronomical Journal. 117 (4): 1834–1844. Bibcode:1999AJ....117.1834R. doi:10.1086/300815.
^ ^a ^b ^c Klochkova, V. G. (2019). "Unity and Diversity of Yellow Hypergiants Family". Astrophysical Bulletin. 74 (4): 475–489. arXiv:1911.09387. Bibcode:2019AstBu..74..475K. doi:10.1134/S1990341319040138. S2CID 208202411.
^ ^a ^b ^c ^d Ferguson, Brian A.; Ueta, Toshiya (March 2010). "Differential Proper-motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821". The Astrophysical Journal. 711 (2): 613–618. arXiv:1001.3135. Bibcode:2010ApJ...711..613F. doi:10.1088/0004-637X/711/2/613. S2CID 119199893.
^ ^a ^b ^c ^d ^e ^f ^g ^h van Genderen, A. M.; Lobel, A.; Nieuwenhuijzen, H.; Henry, G. W.; De Jager, C.; Blown, E.; Di Scala, G.; Van Ballegoij, E. J. (2019). "Pulsations, eruptions, and evolution of four yellow hypergiants". Astronomy and Astrophysics. 631: A48. arXiv:1910.02460. Bibcode:2019A&A...631A..48V. doi:10.1051/0004-6361/201834358. S2CID 203836020.
^ Cannon, Annie J.; Pickering, Edward C. (1918). "The Henry Draper catalogue 0h, 1h, 2h, and 3h". Annals of Harvard College Observatory. 91: 1. Bibcode:1918AnHar..91....1C.
^ Keenan, P. C.; Yorka, S. B. (1988). "1988 Revised MK Spectral Standards for Stars GO and Later". Bulletin d'Information du Centre de Données Stellaires. 35: 37. Bibcode:1988BICDS..35...37K.
^ Pottasch, S. R.; Parthasarathy, M. (1988). "The far-infrared (IRAS) excess in luminous F-G stars". Astronomy and Astrophysics. 192: 182. Bibcode:1988A&A...192..182P.
^ Hrivnak, Bruce J.; Kwok, Sun; Volk, Kevin M. (1989). "A study of several F and G supergiant-like stars with infrared excesses as candidates for proto-planetary nebulae". Astrophysical Journal. 346: 265. Bibcode:1989ApJ...346..265H. doi:10.1086/168007.
^ Arkhipova, V. P.; Ikonnikova, N. P.; Noskova, R. I. (1993). "The variability of four yellow supergiants - Possible protoplanetary objects". Astronomy Letters. 19: 169. Bibcode:1993AstL...19..169A.
^ ^a ^b Jura, M.; Werner, M. W. (10 November 1999). "The Detached Dust Shell around the Massive Star HD 179821". The Astrophysical Journal. 525 (2): L113 – L116. Bibcode:1999ApJ...525L.113J. doi:10.1086/312344. PMID 10525467. S2CID 8625560.
^ ^a ^b Szczerba, R.; Górny, S.K. (31 August 2001). Post-AGB Objects as a Phase of Stellar Evolution. Springer. pp. 315–. ISBN 978-0-7923-7145-8.
^ Nordhaus, J.; Minchev, I.; Sargent, B.; Forrest, W.; Blackman, E. G.; De Marco, O.; Kastner, J.; Balick, B.; Frank, A. (August 2008). "Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821". Monthly Notices of the Royal Astronomical Society. 388 (2): 716–722. arXiv:0801.2978. Bibcode:2008MNRAS.388..716N. doi:10.1111/j.1365-2966.2008.13428.x. S2CID 17468700.
^ ^a ^b Josselin, E.; Lèbre, A. (2001). "Probing the post-AGB nature of HD 179821". Astronomy and Astrophysics. 367 (3): 826. Bibcode:2001A&A...367..826J. doi:10.1051/0004-6361:20000496.
^ Jura, M.; Velusamy, T.; Werner, M. W. (20 July 2001). "What Next for the Likely Presupernova HD 179821?". The Astrophysical Journal. 556 (1): 408–416. arXiv:astro-ph/0103282. Bibcode:2001ApJ...556..408J. doi:10.1086/321553. S2CID 18053762.
^ Wing, Robert F. (31 July 2000). The Carbon Star Phenomenon. Springer. pp. 231–. ISBN 978-0-7923-6346-0.
^ Arkhipova, V. P.; Ikonnikova, N. P.; Noskova, R. I.; Sokol, G. V.; Shugarov, S. Yu. (March 2001). "Light variations in the candidate for protoplanetary objects HD 179821=V1427 Aql in 1899–1999". Astronomy Letters. 27 (3): 156–162. Bibcode:2001AstL...27..156A. doi:10.1134/1.1351559. S2CID 119759129. Retrieved 22 October 2021.
^ Suárez, O.; García-Lario, P.; Manchado, A.; Manteiga, M.; Ulla, A.; Pottasch, S. R. (2006). "A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS point source catalogue". Astronomy and Astrophysics. 458 (1): 173. arXiv:astro-ph/0608080. Bibcode:2006A&A...458..173S. doi:10.1051/0004-6361:20054108. S2CID 16719800.
^ Teyssier, D.; Quintana-Lacaci, G.; Marston, A. P.; Bujarrabal, V.; Alcolea, J.; Cernicharo, J.; Decin, L.; Dominik, C.; Justtanont, K.; De Koter, A.; Melnick, G.; Menten, K. M.; Neufeld, D. A.; Olofsson, H.; Planesas, P.; Schmidt, M.; Soria-Ruiz, R.; Schöier, F. L.; Szczerba, R.; Waters, L. B. F. M. (September 2012). "Herschel/HIFI observations of red supergiants and yellow hypergiants. I. Molecular inventory". Astronomy and Astrophysics. 545: A99. arXiv:1208.3143. Bibcode:2012A&A...545A..99T. doi:10.1051/0004-6361/201219545. S2CID 6602462. A99.
^ Kipper, Tõnu (2008). "Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql)". Baltic Astronomy. 17: 87–102. Bibcode:2008BaltA..17...87K.

Sources

Juraj Zverko; Jozef Ziznovsky; Saul J. Adelman; Werner W. Weiss (25 April 2005). The A-Star Puzzle (IAU S224). Cambridge University Press. pp. 390–. ISBN 978-0-521-85018-6.
Oudmaijer, R. D.; Davies, B.; De Wit, W.-J.; Patel, M. (2009). "Post-Red Supergiants". In Donald D. Luttermoser; Beverly J. Smith; Robert E. Stencil (eds.). Biggest, Baddest, Coolest Stars. Astronomical Society of the Pacific Conference Series. Vol. 412. p. 17. arXiv:0801.2315. Bibcode:2009ASPC..412...17O.

[radius-12] Applying the Stefan-Boltzmann Law with a nominal solar effective temperature of 5,772 K:
${\sqrt {(5772/5660)^{4}*20,000}}=147.07\ R\odot$

[dr3-1] ^ ^a ^b ^c ^d ^e ^f Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.

[tycho2-2] Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862. ISBN 0333750888.

[gcvs-3] Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.

[perkins1989-4] Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373. S2CID 123149047.

[nordhaus-5] Nordhaus, J.; Minchev, I.; Sargent, B.; Forrest, W.; Blackman, E. G.; De Marco, O.; Kastner, J.; Balick, B.; Frank, A. (2008). "Towards a spectral technique for determining material geometry around evolved stars: Application to HD 179821". Monthly Notices of the Royal Astronomical Society. 388 (2): 716. arXiv:0801.2978. Bibcode:2008MNRAS.388..716N. doi:10.1111/j.1365-2966.2008.13428.x. S2CID 17468700.

[2mass-6] Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.

[parthasarathy-7] ^ ^a ^b ^c ^d ^e ^f ^g Parthasarathy, M.; Jasniewicz, G.; Thěvenin, F. (2019). "Correction to: The puzzling high velocity G5 supergiant star HD 179821: New insight from Gaia DR2 data". Astrophysics and Space Science. 364 (2): 25. arXiv:1901.08995. Bibcode:2019Ap&SS.364...25P. doi:10.1007/s10509-019-3512-5. S2CID 119427229.

[reddy-8] ^ ^a ^b ^c ^d ^e ^f Reddy, B. E.; Hrivnak, Bruce J. (April 1999). "Spectroscopic Study of HD 179821 (IRAS 19114+0002): Proto–Planetary Nebula or Supergiant?". The Astronomical Journal. 117 (4): 1834–1844. Bibcode:1999AJ....117.1834R. doi:10.1086/300815.

[klochkova2019-9] Klochkova, V. G. (2019). "Unity and Diversity of Yellow Hypergiants Family". Astrophysical Bulletin. 74 (4): 475–489. arXiv:1911.09387. Bibcode:2019AstBu..74..475K. doi:10.1134/S1990341319040138. S2CID 208202411.

[Ferguson2010-10] Ferguson, Brian A.; Ueta, Toshiya (March 2010). "Differential Proper-motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821". The Astrophysical Journal. 711 (2): 613–618. arXiv:1001.3135. Bibcode:2010ApJ...711..613F. doi:10.1088/0004-637X/711/2/613. S2CID 119199893.

[van-11] ^ ^a ^b ^c ^d ^e ^f ^g ^h van Genderen, A. M.; Lobel, A.; Nieuwenhuijzen, H.; Henry, G. W.; De Jager, C.; Blown, E.; Di Scala, G.; Van Ballegoij, E. J. (2019). "Pulsations, eruptions, and evolution of four yellow hypergiants". Astronomy and Astrophysics. 631: A48. arXiv:1910.02460. Bibcode:2019A&A...631A..48V. doi:10.1051/0004-6361/201834358. S2CID 203836020.

[hd-13] Cannon, Annie J.; Pickering, Edward C. (1918). "The Henry Draper catalogue 0h, 1h, 2h, and 3h". Annals of Harvard College Observatory. 91: 1. Bibcode:1918AnHar..91....1C.

[mk-14] Keenan, P. C.; Yorka, S. B. (1988). "1988 Revised MK Spectral Standards for Stars GO and Later". Bulletin d'Information du Centre de Données Stellaires. 35: 37. Bibcode:1988BICDS..35...37K.

[15] Pottasch, S. R.; Parthasarathy, M. (1988). "The far-infrared (IRAS) excess in luminous F-G stars". Astronomy and Astrophysics. 192: 182. Bibcode:1988A&A...192..182P.

[16] Hrivnak, Bruce J.; Kwok, Sun; Volk, Kevin M. (1989). "A study of several F and G supergiant-like stars with infrared excesses as candidates for proto-planetary nebulae". Astrophysical Journal. 346: 265. Bibcode:1989ApJ...346..265H. doi:10.1086/168007.

[17] Arkhipova, V. P.; Ikonnikova, N. P.; Noskova, R. I. (1993). "The variability of four yellow supergiants - Possible protoplanetary objects". Astronomy Letters. 19: 169. Bibcode:1993AstL...19..169A.

[The_Detached_Dust_Shell_around_the_Massive_Star_HD_179821-18] Jura, M.; Werner, M. W. (10 November 1999). "The Detached Dust Shell around the Massive Star HD 179821". The Astrophysical Journal. 525 (2): L113 – L116. Bibcode:1999ApJ...525L.113J. doi:10.1086/312344. PMID 10525467. S2CID 8625560.

[SzczerbaGórny2001-19] Szczerba, R.; Górny, S.K. (31 August 2001). Post-AGB Objects as a Phase of Stellar Evolution. Springer. pp. 315–. ISBN 978-0-7923-7145-8.

[Nordhaus2008-20] Nordhaus, J.; Minchev, I.; Sargent, B.; Forrest, W.; Blackman, E. G.; De Marco, O.; Kastner, J.; Balick, B.; Frank, A. (August 2008). "Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821". Monthly Notices of the Royal Astronomical Society. 388 (2): 716–722. arXiv:0801.2978. Bibcode:2008MNRAS.388..716N. doi:10.1111/j.1365-2966.2008.13428.x. S2CID 17468700.

[josselin-21] Josselin, E.; Lèbre, A. (2001). "Probing the post-AGB nature of HD 179821". Astronomy and Astrophysics. 367 (3): 826. Bibcode:2001A&A...367..826J. doi:10.1051/0004-6361:20000496.

[What_Next_for_the_Likely_Presupernova_HD_179821?-22] Jura, M.; Velusamy, T.; Werner, M. W. (20 July 2001). "What Next for the Likely Presupernova HD 179821?". The Astrophysical Journal. 556 (1): 408–416. arXiv:astro-ph/0103282. Bibcode:2001ApJ...556..408J. doi:10.1086/321553. S2CID 18053762.

[Wing2000-23] Wing, Robert F. (31 July 2000). The Carbon Star Phenomenon. Springer. pp. 231–. ISBN 978-0-7923-6346-0.

[Arkhipova-24] Arkhipova, V. P.; Ikonnikova, N. P.; Noskova, R. I.; Sokol, G. V.; Shugarov, S. Yu. (March 2001). "Light variations in the candidate for protoplanetary objects HD 179821=V1427 Aql in 1899–1999". Astronomy Letters. 27 (3): 156–162. Bibcode:2001AstL...27..156A. doi:10.1134/1.1351559. S2CID 119759129. Retrieved 22 October 2021.

[suarez2006-25] Suárez, O.; García-Lario, P.; Manchado, A.; Manteiga, M.; Ulla, A.; Pottasch, S. R. (2006). "A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS point source catalogue". Astronomy and Astrophysics. 458 (1): 173. arXiv:astro-ph/0608080. Bibcode:2006A&A...458..173S. doi:10.1051/0004-6361:20054108. S2CID 16719800.

[Herschel2012-26] Teyssier, D.; Quintana-Lacaci, G.; Marston, A. P.; Bujarrabal, V.; Alcolea, J.; Cernicharo, J.; Decin, L.; Dominik, C.; Justtanont, K.; De Koter, A.; Melnick, G.; Menten, K. M.; Neufeld, D. A.; Olofsson, H.; Planesas, P.; Schmidt, M.; Soria-Ruiz, R.; Schöier, F. L.; Szczerba, R.; Waters, L. B. F. M. (September 2012). "Herschel/HIFI observations of red supergiants and yellow hypergiants. I. Molecular inventory". Astronomy and Astrophysics. 545: A99. arXiv:1208.3143. Bibcode:2012A&A...545A..99T. doi:10.1051/0004-6361/201219545. S2CID 6602462. A99.

[Kipper2008-27] Kipper, Tõnu (2008). "Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql)". Baltic Astronomy. 17: 87–102. Bibcode:2008BaltA..17...87K.

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@@ Line 1: / Line 1: @@
+{{Short description|Star in the constellation Aquila}}
 {{Starbox begin
 | name = HD&nbsp;179821}}{{Starbox image
@@ Line 9: / Line 10: @@
 | equinox =J2000
 | constell = [[Aquila (constellation)|Aquila]]
+| ra = {{RA|19|13|58.61}}<ref name=dr3/>
-| ra = {{RA|19|13|58.61}}<ref name=hipparcos>{{cite journal|bibcode=2007A&A...474..653V|title=Validation of the new Hipparcos reduction|journal=Astronomy and Astrophysics|volume=474|issue=2|pages=653–664|last1=Van Leeuwen|first1=F.|year=2007|doi=10.1051/0004-6361:20078357|arxiv = 0708.1752 |s2cid=18759600}}</ref>
-| dec = {{DEC|+00|07|31.9}}<ref name=hipparcos/>
+| dec = {{DEC|+00|07|31.9}}<ref name=dr3/>
 | appmag_v=8.19<ref name=tycho2>{{cite journal|bibcode=2000A&A...355L..27H|title=The Tycho-2 catalogue of the 2.5 million brightest stars|journal=Astronomy and Astrophysics|volume=355|pages=L27|last1=Høg|first1=E.|last2=Fabricius|first2=C.|last3=Makarov|first3=V. V.|last4=Urban|first4=S.|last5=Corbin|first5=T.|last6=Wycoff|first6=G.|last7=Bastian|first7=U.|last8=Schwekendiek|first8=P.|last9=Wicenec|first9=A.|year=2000|doi=10.1888/0333750888/2862|isbn=0333750888}}</ref> {{nowrap|(7.83 - 8.23<ref name=gcvs/>)}}
 }}
 {{Starbox character
-| type = either a [[yellow hypergiant]] or a [[post-AGB star|post-AGB]] [[yellow supergiant]]
+| type = [[yellow hypergiant|YHG]] or [[post-AGB star|post-AGB]]
 | class = G4 0-Ia<ref name=perkins1989/> {{nowrap|(G5Ia - K4I<ref name=gcvs/>)}}
 | b-v = +1.504<ref name=tycho2/>
 |appmag_1_passband=U
-|appmag_1=10.81<ref name=nordhaus>{{cite journal|arxiv=0801.2978|bibcode=2008MNRAS.388..716N|title=Towards a spectral technique for determining material geometry around evolved stars: Application to HD 179821|journal=Monthly Notices of the Royal Astronomical Society|volume=388|issue=2|pages=716|last1=Nordhaus|first1=J.|last2=Minchev|first2=I.|last3=Sargent|first3=B.|last4=Forrest|first4=W.|last5=Blackman|first5=E. G.|last6=De Marco|first6=O.|last7=Kastner|first7=J.|last8=Balick|first8=B.|last9=Frank|first9=A.|year=2008|doi=10.1111/j.1365-2966.2008.13428.x|s2cid=17468700}}</ref>
+|appmag_1=10.81<ref name=nordhaus>{{cite journal|arxiv=0801.2978|bibcode=2008MNRAS.388..716N|title=Towards a spectral technique for determining material geometry around evolved stars: Application to HD 179821|journal=Monthly Notices of the Royal Astronomical Society|volume=388|issue=2|pages=716|last1=Nordhaus|first1=J.|last2=Minchev|first2=I.|last3=Sargent|first3=B.|last4=Forrest|first4=W.|last5=Blackman|first5=E. G.|last6=De Marco|first6=O.|last7=Kastner|first7=J.|last8=Balick|first8=B.|last9=Frank|first9=A.|year=2008|doi=10.1111/j.1365-2966.2008.13428.x|doi-access=free |s2cid=17468700}}</ref>
 |appmag_2_passband=B
 |appmag_2=9.694<ref name=tycho2/>
@@ Line 24: / Line 25: @@
 |appmag_3=8.2<ref name=2mass/>
 |appmag_4_passband=J
-|appmag_4=5.371<ref name=2mass>{{cite journal|bibcode=2003yCat.2246....0C|title=VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)|journal=VizieR On-line Data Catalog: II/246. Originally Published in: 2003yCat.2246....0C|volume=2246|last1=Cutri|first1=R. M.|last2=Skrutskie|first2=M. F.|last3=Van Dyk|first3=S.|last4=Beichman|first4=C. A.|last5=Carpenter|first5=J. M.|last6=Chester|first6=T.|last7=Cambresy|first7=L.|last8=Evans|first8=T.|last9=Fowler|first9=J.|last10=Gizis|first10=J.|last11=Howard|first11=E.|last12=Huchra|first12=J.|last13=Jarrett|first13=T.|last14=Kopan|first14=E. L.|last15=Kirkpatrick|first15=J. D.|last16=Light|first16=R. M.|last17=Marsh|first17=K. A.|last18=McCallon|first18=H.|last19=Schneider|first19=S.|last20=Stiening|first20=R.|last21=Sykes|first21=M.|last22=Weinberg|first22=M.|last23=Wheaton|first23=W. A.|last24=Wheelock|first24=S.|last25=Zacarias|first25=N.|year=2003}}</ref>
+|appmag_4=5.371<ref name=2mass>{{cite journal |bibcode=2003yCat.2246....0C |title=VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003) |journal=CDS/ADC Collection of Electronic Catalogues |pages=II/246 |volume=2246 |last1=Cutri |first1=Roc M. |last2=Skrutskie |first2=Michael F. |last3=Van Dyk |first3=Schuyler D. |last4=Beichman |first4=Charles A. |last5=Carpenter |first5=John M. |last6=Chester |first6=Thomas |last7=Cambresy |first7=Laurent |last8=Evans |first8=Tracey E. |last9=Fowler |first9=John W. |last10=Gizis |first10=John E. |last11=Howard |first11=Elizabeth V. |last12=Huchra |first12=John P. |last13=Jarrett |first13=Thomas H. |last14=Kopan |first14=Eugene L. |last15=Kirkpatrick |first15=J. Davy |last16=Light |first16=Robert M. |last17=Marsh |first17=Kenneth A. |last18=McCallon |first18=Howard L. |last19=Schneider |first19=Stephen E. |last20=Stiening |first20=Rae |last21=Sykes |first21=Matthew J. |last22=Weinberg |first22=Martin D. |last23=Wheaton |first23=William A. |last24=Wheelock |first24=Sherry L. |last25=Zacarias |first25=N. |year=2003 |url=http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=II/246 }}</ref>
 |appmag_5_passband=H
 |appmag_5=4.998<ref name=2mass/>
@@ Line 32: / Line 33: @@
 }}
 {{Starbox astrometry
+| radial_v=+{{val|81.8|3.7}}<ref name=parthasarathy>{{cite journal|arxiv=1901.08995|doi=10.1007/s10509-019-3512-5|title=Correction to: The puzzling high velocity G5 supergiant star HD 179821: New insight from Gaia DR2 data|year=2019|last1=Parthasarathy|first1=M.|last2=Jasniewicz|first2=G.|last3=Thěvenin|first3=F.|journal=Astrophysics and Space Science|volume=364|issue=2|page=25 |s2cid=119427229|bibcode=2019Ap&SS.364...25P}}</ref>
-| radial_v=+{{val|81.8|3.7}}<ref name=parthasarathy/>
-| prop_mo_ra=–0.165<ref name=dr2/>
+| prop_mo_ra=−0.090<ref name=dr3/>
-| prop_mo_dec=–3.448<ref name=dr2/>
+| prop_mo_dec=−3.494<ref name=dr3/>
-| parallax=0.3102
+| parallax=0.1893
-| p_error=0.0512
+| p_error=0.0206
-| parallax_footnote=<ref name=dr2/>
+| parallax_footnote=<ref name=dr3/>
+| dist_ly=
-| dist_pc={{val|2,959|409|fmt=commas}}<ref name=parthasarathy>{{cite arXiv|eprint=1901.08995}}</ref>
+| dist_pc=
-| absmag_v=–8.9<ref name=reddy/><ref name=klochkova2019/> or –5.7<ref name=parthasarathy/>
+| absmag_v=−8.9<ref name=reddy/><ref name=klochkova2019/> or −5.7<ref name=parthasarathy/>
 }}
 {{Starbox detail
+| component1 = if a hypergiant
-| mass=30<ref name="Ferguson2010"/>
-| radius=400 - 450<ref name=van/> or 147<ref name=parthasarathy/>
+| mass=19 - 30<ref name=parthasarathy/><ref name="Ferguson2010"/>
+| radius=400 - 450<ref name=van/>
-| luminosity=126,000<ref name=van/> - 295,000<ref name=klochkova2019>{{cite journal |doi=10.1134/S1990341319040138 |bibcode=2019AstBu..74..475K |arxiv=1911.09387 |title=Unity and Diversity of Yellow Hypergiants Family |year=2019 |last1=Klochkova |first1=V. G. |journal=Astrophysical Bulletin |volume=74 |issue=4 |pages=475–489 |s2cid=208202411 }}</ref> or 20,000<ref name=parthasarathy/>
+| luminosity=126,000<ref name=van/> - 295,000<ref name=klochkova2019>{{cite journal |doi=10.1134/S1990341319040138 |bibcode=2019AstBu..74..475K |arxiv=1911.09387 |title=Unity and Diversity of Yellow Hypergiants Family |year=2019 |last1=Klochkova |first1=V. G. |journal=Astrophysical Bulletin |volume=74 |issue=4 |pages=475–489 |s2cid=208202411 }}</ref>
-| temperature=5,660<ref name=parthasarathy/> {{nowrap|(4,898 - 6,761)<ref name=van/>}}
+| temperature={{nowrap|4,898 - 6,761<ref name=van/>}}
 | metal_fe=0.0<ref name=reddy/>
-| gravity=0.5<ref name=reddy>{{cite journal|last1=Reddy|first1=B. E.|last2=Hrivnak|first2=Bruce J.|title=Spectroscopic Study of HD 179821 (IRAS 19114+0002): Proto–Planetary Nebula or Supergiant?|journal=The Astronomical Journal|date=April 1999|volume=117|issue=4|pages=1834–1844|doi=10.1086/300815|bibcode = 1999AJ....117.1834R }}</ref>
+| gravity=0.5<ref name=reddy>{{cite journal|last1=Reddy|first1=B. E.|last2=Hrivnak|first2=Bruce J.|title=Spectroscopic Study of HD 179821 (IRAS 19114+0002): Proto–Planetary Nebula or Supergiant?|journal=The Astronomical Journal|date=April 1999|volume=117|issue=4|pages=1834–1844|doi=10.1086/300815|bibcode = 1999AJ....117.1834R |doi-access=free}}</ref>
 | rotational_velocity=
 | age_gyr=
+| component2 = if post-AGB
+| mass2=0.8<ref name=parthasarathy/>
+| radius2=147<ref name=parthasarathy/>{{efn|name=radius}}
+| luminosity2={{val|20,000|12,000|7,400|fmt=commas}}<ref name=parthasarathy/>
+| temperature2=5,660<ref name=parthasarathy/>
+| metal_fe2=0.0<ref name=reddy/>
+| gravity2=0.5<ref name=reddy/>
+| rotational_velocity2=
+| age_gyr2=
 }}
 {{Starbox catalog
@@ Line 58: / Line 70: @@
 }}
 {{Starbox end}}
-'''HD 179821''' is either a post-[[red supergiant]] [[yellow hypergiant]] or a [[post-AGB star|post-AGB]] [[yellow supergiant]] star in the constellation of [[Aquila (constellation)|Aquila]], surrounded by a detached dust shell.  It is a [[semi-regular variable]] nearing the end of its life.
+'''HD 179821''' or '''V1427 Aquilae''' is either a post-[[red supergiant]] [[yellow hypergiant]] or a [[post-AGB star|post-AGB]] [[yellow supergiant]] star in the constellation of [[Aquila (constellation)|Aquila]], surrounded by a detached dust shell.  It is a [[semi-regular variable]] nearing the end of its life.
 ==Discovery==
 HD 179821 was first catalogued as an unremarkable 8th magnitude star at the start of the 20th century.<ref name=hd>{{cite journal|bibcode=1918AnHar..91....1C|title=The Henry Draper catalogue 0h, 1h, 2h, and 3h|journal=Annals of Harvard College Observatory|volume=91|pages=1|last1=Cannon|first1=Annie J.|last2=Pickering|first2=Edward C.|year=1918}}</ref>  It was later listed as a spectral standard G4 0-Ia, indicating a highly luminous star type now known as a hypergiant.<ref name=mk>{{cite journal|bibcode=1988BICDS..35...37K|title=1988 Revised MK Spectral Standards for Stars GO and Later|journal=Bulletin d'Information du Centre de Données Stellaires|volume=35|pages=37|last1=Keenan|first1=P. C.|last2=Yorka|first2=S. B.|year=1988}}</ref>
-It was first considered notable for its infrared excess and double-peaked [[spectral energy distribution]] in the infrared.<ref>{{cite journal|bibcode=1988A&A...192..182P|title=The far-infrared (IRAS) excess in luminous F-G stars|journal=Astronomy and Astrophysics|volume=192|pages=182|last1=Pottasch|first1=S. R.|last2=Parthasarathy|first2=M.|year=1988}}</ref>  These were considered to be indicators of surrounding dust and HD 179821 was identified as a possible [[proto-planetary nebulae|proto-planetary nebula]].<ref>{{cite journal|bibcode=1989ApJ...346..265H|title=A study of several F and G supergiant-like stars with infrared excesses as candidates for proto-planetary nebulae|journal=Astrophysical Journal|volume=346|pages=265|last1=Hrivnak|first1=Bruce J.|last2=Kwok|first2=Sun|last3=Volk|first3=Kevin M.|year=1989|doi=10.1086/168007}}</ref>  Variability was also detected.<ref>{{cite journal|bibcode=1993AstL...19..169A|title=The variability of four yellow supergiants - Possible protoplanetary objects|journal=Astronomy Letters|volume=19|pages=169|last1=Arkhipova|first1=V. P.|last2=Ikonnikova|first2=N. P.|last3=Noskova|first3=R. I.|year=1993}}</ref>
+It was first considered notable for its infrared excess and double-peaked [[spectral energy distribution]] in the infrared.<ref>{{cite journal|bibcode=1988A&A...192..182P|title=The far-infrared (IRAS) excess in luminous F-G stars|journal=Astronomy and Astrophysics|volume=192|pages=182|last1=Pottasch|first1=S. R.|last2=Parthasarathy|first2=M.|year=1988}}</ref>  These were considered to be indicators of surrounding dust and HD 179821 was identified as a possible [[protoplanetary nebula]].<ref>{{cite journal|bibcode=1989ApJ...346..265H|title=A study of several F and G supergiant-like stars with infrared excesses as candidates for proto-planetary nebulae|journal=Astrophysical Journal|volume=346|pages=265|last1=Hrivnak|first1=Bruce J.|last2=Kwok|first2=Sun|last3=Volk|first3=Kevin M.|year=1989|doi=10.1086/168007}}</ref>  Variability was also detected.<ref>{{cite journal|bibcode=1993AstL...19..169A|title=The variability of four yellow supergiants - Possible protoplanetary objects|journal=Astronomy Letters|volume=19|pages=169|last1=Arkhipova|first1=V. P.|last2=Ikonnikova|first2=N. P.|last3=Noskova|first3=R. I.|year=1993}}</ref>
-High resolution spectroscopic studies and modern space-based observations have revealed an unusual chemical makeup and a hollow spherical dust shell, but haven't fully resolved whether HD 179821 is a highly luminous [[yellow hypergiant]] or a more modest dying star.<ref name="Ferguson2010">{{Cite journal |last1=Ferguson|first1=Brian A. |last2=Ueta|first2=Toshiya | date = March 2010| journal = The Astrophysical Journal | volume = 711 | issue=2  | pages=613–618 | title = Differential Proper-motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821 | bibcode = 2010ApJ...711..613F | doi = 10.1088/0004-637X/711/2/613|arxiv = 1001.3135 |s2cid=119199893 }}</ref>
+High resolution spectroscopic studies and modern space-based observations have revealed an unusual chemical makeup and a hollow spherical dust shell, but haven't fully resolved whether HD 179821 is a highly luminous [[yellow hypergiant]] or a dimmer, lower-mass [[post-AGB star]].<ref name="Ferguson2010">{{Cite journal |last1=Ferguson|first1=Brian A. |last2=Ueta|first2=Toshiya | date = March 2010| journal = The Astrophysical Journal | volume = 711 | issue=2  | pages=613–618 | title = Differential Proper-motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821 | bibcode = 2010ApJ...711..613F | doi = 10.1088/0004-637X/711/2/613|arxiv = 1001.3135 |s2cid=119199893 }}</ref>
 ==Observations==
-HD 179821 has a cold detached dust shell that has been studied with the help of the [[Hubble Space Telescope]].  The shell is approximately circular in shape, has an inner diameter of ~3".3 corresponding to 20,000 [[Astronomical unit|AU]] at 6,000 pc, and an outer diameter of 5".7 or more, with the star 0".35 from the centre of the shell.  The current mass loss is low, but during the formation of the shell it is estimated to have been {{val|4|e=-4|u=''M''<sub>⊙</sub>}}, an exceptionally high rate.<ref name="The Detached Dust Shell around the Massive Star HD 179821" />  Like its constellation neighbor and also hypergiant star [[IRC +10420]], it is surrounded by an extended [[reflection nebula|reflection]] [[nebula]]. Discovered at near-IR wavelength, this indicates a massive star<ref name="SzczerbaGórny2001">{{cite book|last1=Szczerba|first1=R.|last2=Górny|first2=S.K.|title=Post-AGB Objects as a Phase of Stellar Evolution|date=31 August 2001|publisher=Springer|isbn=978-0-7923-7145-8|pages=315–}}</ref> and, as with the reflection nebula around IRC+10420, it may be masking a star hotter than the given G5 spectral type.<ref name="Nordhaus2008">{{Cite journal |last1=Nordhaus|first1=J.|last2=Minchev|first2=I. |last3=Sargent|first3=B. |last4=Forrest|first4=W. |last5=Blackman|first5=E. G. |last6=De Marco|first6=O. |last7=Kastner|first7=J. |last8=Balick|first8=B. |last9=Frank|first9=A. | date = August 2008 | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 388 | issue=2 | pages=716–722 | title = Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821 | bibcode = 2008MNRAS.388..716N | doi = 10.1111/j.1365-2966.2008.13428.x |arxiv = 0801.2978 |s2cid=17468700}}</ref>
+HD 179821 has a cold detached dust shell that has been studied with the help of the [[Hubble Space Telescope]].  The shell is approximately circular in shape, has an inner diameter of ~3".3 corresponding to 20,000 [[Astronomical unit|AU]] at 6,000 pc, and an outer diameter of 5".7 or more, with the star 0".35 from the centre of the shell.  The current mass loss is low, but during the formation of the shell it is estimated to have been {{val|4|e=-4|u={{solar mass}}/yr}}, an exceptionally high rate being comparable to that of the archetypical [[OH/IR star|OH/IR]] red supergiant, [[VY Canis Majoris]].<ref name="The Detached Dust Shell around the Massive Star HD 179821" />  Like its constellation neighbor and also hypergiant star [[IRC +10420]], it is surrounded by an extended [[reflection nebula|reflection]] [[nebula]]. Discovered at near-IR wavelength, this indicates a massive star<ref name="SzczerbaGórny2001">{{cite book|last1=Szczerba|first1=R.|last2=Górny|first2=S.K.|title=Post-AGB Objects as a Phase of Stellar Evolution|date=31 August 2001|publisher=Springer|isbn=978-0-7923-7145-8|pages=315–}}</ref> and, as with the reflection nebula around IRC +10420, it may be masking a star hotter than the given G5 spectral type.<ref name="Nordhaus2008">{{Cite journal |last1=Nordhaus|first1=J.|last2=Minchev|first2=I. |last3=Sargent|first3=B. |last4=Forrest|first4=W. |last5=Blackman|first5=E. G. |last6=De Marco|first6=O. |last7=Kastner|first7=J. |last8=Balick|first8=B. |last9=Frank|first9=A. | date = August 2008 | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 388 | issue=2 | pages=716–722 | title = Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821 | bibcode = 2008MNRAS.388..716N | doi = 10.1111/j.1365-2966.2008.13428.x |doi-access=free |arxiv = 0801.2978 |s2cid=17468700}}</ref>
-It is that which contributes to a double-peaked spectral energy distribution.<ref name=josselin>{{cite journal|doi=10.1051/0004-6361:20000496|bibcode = 2001A&A...367..826J|title = Probing the post-AGB nature of HD 179821|journal = Astronomy and Astrophysics|volume = 367|issue = 3|pages = 826|year = 2001|last1 = Josselin|first1 = E.|last2 = Lèbre|first2 = A.|doi-access = free}}</ref> It is estimated the star has lost about 10% of its initial mass after being a [[red supergiant]] star just 1,600 years ago,<ref name="The Detached Dust Shell around the Massive Star HD 179821">{{cite journal|last1=Jura|first1=M.|last2=Werner|first2=M. W.|title=The Detached Dust Shell around the Massive Star HD 179821|journal=The Astrophysical Journal|date=10 November 1999|volume=525|issue=2|pages=L113–L116|doi=10.1086/312344|pmid=10525467|bibcode = 1999ApJ...525L.113J }}</ref> and is a likely supernova candidate.<ref name="What Next for the Likely Presupernova HD 179821?">{{cite journal|last1=Jura|first1=M.|last2=Velusamy|first2=T. |last3=Werner|first3=M. W. |title=What Next for the Likely Presupernova HD 179821?|journal=The Astrophysical Journal|date=20 July 2001|volume=556|issue=1|pages=408–416|doi=10.1086/321553|arxiv = astro-ph/0103282 |bibcode = 2001ApJ...556..408J |s2cid=18053762}}</ref>
+It is that which contributes to a double-peaked spectral energy distribution.<ref name=josselin>{{cite journal|doi=10.1051/0004-6361:20000496|bibcode = 2001A&A...367..826J|title = Probing the post-AGB nature of HD 179821|journal = Astronomy and Astrophysics|volume = 367|issue = 3|pages = 826|year = 2001|last1 = Josselin|first1 = E.|last2 = Lèbre|first2 = A.|doi-access = free}}</ref> It is estimated the star has lost about 10% of its initial mass after being a [[red supergiant]] star just 1,600 years ago,<ref name="The Detached Dust Shell around the Massive Star HD 179821">{{cite journal|last1=Jura|first1=M.|last2=Werner|first2=M. W.|title=The Detached Dust Shell around the Massive Star HD 179821|journal=The Astrophysical Journal|date=10 November 1999|volume=525|issue=2|pages=L113–L116|doi=10.1086/312344|pmid=10525467|bibcode = 1999ApJ...525L.113J |s2cid=8625560 |doi-access=free}}</ref> and is a likely supernova candidate.<ref name="What Next for the Likely Presupernova HD 179821?">{{cite journal|last1=Jura|first1=M.|last2=Velusamy|first2=T. |last3=Werner|first3=M. W. |title=What Next for the Likely Presupernova HD 179821?|journal=The Astrophysical Journal|date=20 July 2001|volume=556|issue=1|pages=408–416|doi=10.1086/321553|arxiv = astro-ph/0103282 |bibcode = 2001ApJ...556..408J |s2cid=18053762}}</ref>
-The distance is estimated to be around 6,000 parsecs and has a high luminosity of between {{val|1.26|e=5}} and {{val|2.95|e=5|u=''L''<sub>⊙</sub>}} and a radius of between 400 and {{solar radius|450}}.<ref name=van/><ref name=klochkova2019/> It has a high radial velocity of {{val|p=+|100|u=km/s}}.<ref name="Wing2000">{{cite book|last1=Wing|first1=Robert F. |title=The Carbon Star Phenomenon|date=31 July 2000|publisher=Springer|isbn=978-0-7923-6346-0|pages=231–}}</ref> According to the studies of Jura ''et al'' (2001), the star may explode as a supernova in the next 100,000 years.
+The distance was once estimated to be around 6,000 parsecs. It has a high luminosity of between {{val|1.26|e=5}} and {{val|2.95|e=5|u=''L''<sub>⊙</sub>}} and a radius of between 400 and {{solar radius|450}}.<ref name=van/><ref name=klochkova2019/> It has a high radial velocity of {{val|p=+|100|u=km/s}}.<ref name="Wing2000">{{cite book|last1=Wing|first1=Robert F. |title=The Carbon Star Phenomenon|date=31 July 2000|publisher=Springer|isbn=978-0-7923-6346-0|pages=231–}}</ref> According to the studies of Jura ''et al'' (2001), the star may explode as a supernova in the next 100,000 years.
 ===Variability===
+[[File:HD179821LightCurve.png|thumb|left|The [[Photometric_system#Photometric_letters|blue band]] [[light curve]] of HD 179821, adapted from Arkhipova ''et al.'' (2001)<ref name="Arkhipova"/>]]
 HD 179821 is a semiregular variable star with the [[variable star designation]] V1427 Aquilae.  Between 1899 and 1989, its photographic [[apparent magnitude]] varied erratically between about magnitudes 9 and 10, although coverage is not complete and some larger variations may have been missed.  It then varied by no more than 0.1 magnitudes until 2009, at a [[visual magnitude]] around 8.1.<ref name=van/>
-The colour of the star changed noticeably so that the variability at different wavelengths is not consistent.  In general, the star became bluer from 1899 until 1990, and then redder again.  The colour changes most likely reflect changes in the [[effective temperature]], and probably underlying [[stellar evolution|evolutionary]] trends with the star performing a blue loop between temperatures of {{val|4,000|fmt=commas|ul=K}} and {{val|8,000|fmt=commas|u=K}}.<ref name=van/>  Pulsations for much of this time occurred with an approximate period of 100 to 150 days, although this increased to 250 days between 2010 and 2017, which is expected for stars which are decreasing in temperature.<ref name=van/>  At its coolest, the spectral type has been recorded as K4,<ref name=gcvs/> while near its hottest in 2007 it was classifies F7.<ref name=suarez2006/>
+The colour of the star changed noticeably so that the variability at different wavelengths is not consistent.  In general, the star became bluer from 1899 until 1990, and then redder again.  The colour changes most likely reflect changes in the [[effective temperature]], and probably underlying [[stellar evolution|evolutionary]] trends with the star performing a blue loop between temperatures of {{val|4,000|fmt=commas|ul=K}} and {{val|8,000|fmt=commas|u=K}}.<ref name=van/>  Pulsations for much of this time occurred with an approximate period of 100 to 150 days, although this increased to 250 days between 2010 and 2017, which is expected for stars which are decreasing in temperature.<ref name=van/>  At its coolest, the spectral type has been recorded as K4,<ref name=gcvs/> while near its hottest in 2007 it was classified as F7.<ref name=suarez2006/>
 ===Chemical composition===
@@ Line 83: / Line 96: @@
 ==Controversy==
-While most authors consider HD 179821 to be a warm supergiant or hypergiant star,<ref name="Herschel2012">{{Cite journal | date = September 2012| journal = Astronomy and Astrophysics | volume = 545 | pages = A99| id=A99 | title = Herschel/HIFI observations of red supergiants and yellow hypergiants. I. Molecular inventory | bibcode = 2012A&A...545A..99T | doi = 10.1051/0004-6361/201219545|arxiv = 1208.3143 | last1 = Teyssier | first1 = D. | last2 = Quintana-Lacaci | first2 = G. | last3 = Marston | first3 = A. P. | last4 = Bujarrabal | first4 = V. | last5 = Alcolea | first5 = J. | last6 = Cernicharo | first6 = J. | last7 = Decin | first7 = L. | last8 = Dominik | first8 = C. | last9 = Justtanont | first9 = K. | last10 = De Koter | first10 = A. | last11 = Melnick | first11 = G. | last12 = Menten | first12 = K. M. | last13 = Neufeld | first13 = D. A. | last14 = Olofsson | first14 = H. | last15 = Planesas | first15 = P. | last16 = Schmidt | first16 = M. | last17 = Soria-Ruiz | first17 = R. | last18 = Schöier | first18 = F. L. | last19 = Szczerba | first19 = R. | last20 = Waters | first20 = L. B. F. M. | s2cid = 6602462 }}</ref> others think it is actually a [[protoplanetary nebula]] or a post-[[Asymptotic giant branch|AGB]] star at a distance of 1 kiloparsec (3,200 light years).<ref name="Kipper2008">{{Cite journal |date = 2008 | journal = Baltic Astronomy | volume = 17 | pages=87–102 | title = Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql) | bibcode = 2008BaltA..17...87K| last1 = Kipper | first1 = Tõnu }}</ref> In that case the star's luminosity and radius would be much lower, around 16,000 times that of the Sun and 60 to {{solar radius|80}}, and its initial mass would be equal to the current mass of the Sun.<ref name=van/><ref name="Ferguson2010" />
+While most authors consider HD 179821 to be a warm hypergiant star,<ref name="Herschel2012">{{Cite journal | date = September 2012| journal = Astronomy and Astrophysics | volume = 545 | pages = A99| id=A99 | title = Herschel/HIFI observations of red supergiants and yellow hypergiants. I. Molecular inventory | bibcode = 2012A&A...545A..99T | doi = 10.1051/0004-6361/201219545|arxiv = 1208.3143 | last1 = Teyssier | first1 = D. | last2 = Quintana-Lacaci | first2 = G. | last3 = Marston | first3 = A. P. | last4 = Bujarrabal | first4 = V. | last5 = Alcolea | first5 = J. | last6 = Cernicharo | first6 = J. | last7 = Decin | first7 = L. | last8 = Dominik | first8 = C. | last9 = Justtanont | first9 = K. | last10 = De Koter | first10 = A. | last11 = Melnick | first11 = G. | last12 = Menten | first12 = K. M. | last13 = Neufeld | first13 = D. A. | last14 = Olofsson | first14 = H. | last15 = Planesas | first15 = P. | last16 = Schmidt | first16 = M. | last17 = Soria-Ruiz | first17 = R. | last18 = Schöier | first18 = F. L. | last19 = Szczerba | first19 = R. | last20 = Waters | first20 = L. B. F. M. | s2cid = 6602462 }}</ref> others think it is actually a [[protoplanetary nebula]] or a smaller post-[[Asymptotic giant branch|AGB]] star at a distance of 1 kiloparsec (3,200 light years).<ref name="Kipper2008">{{Cite journal |date = 2008 | journal = Baltic Astronomy | volume = 17 | pages=87–102 | title = Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql) | bibcode = 2008BaltA..17...87K| last1 = Kipper | first1 = Tõnu }}</ref> In that case the star's luminosity and radius would be much lower, around 16,000 times that of the Sun and 60 to {{solar radius|80}}, and its initial mass would be equal to the current mass of the Sun.<ref name=van/><ref name="Ferguson2010" />
-This discrepancy arises because its distance was too great to be measured by [[parallax]] before the [[Gaia mission]] and it has some properties of both a yellow hypergiant and a protoplanetary nebula/Post-AGB star.<ref name=reddy/><ref name="Ferguson2010" />  [[Gaia Data Release 2]] gives a parallax of {{val|0.31|ul=mas}} implying a distance around {{val|3,000|ul=pc|fmt=commas}}.<ref name=dr2>{{cite DR2|4264026012336768000}}</ref>
+This discrepancy arises because its distance was too great to be measured by [[parallax]] before the [[Gaia mission]] and it has some properties of both a yellow hypergiant and a protoplanetary nebula/Post-AGB star.<ref name=reddy/><ref name="Ferguson2010" />  [[Gaia Data Release 3]] gives a parallax of {{val|0.19|ul=mas}} implying a distance around {{val|5,300|ul=pc|fmt=commas}}.<ref name=dr3/>
 ==See also==
 * [[List of supernova candidates]]
+== Notes ==
+{{notelist|refs=
+{{efn | name=radius | Applying the [[Stefan-Boltzmann Law]] with a nominal [[sun|solar]] [[effective temperature]] of 5,772&nbsp;[[Kelvin|K]]:
+:<math>\sqrt{(5772/5660)^4 * 20,000} = 147.07\ R\odot</math>}}
+}}
 ==References==
-{{reflist|30em|refs=
+{{Reflist|30em|refs=
+<ref name=dr3>{{cite Gaia DR3|4264026012336768000}}</ref>
+<ref name="Arkhipova">{{cite journal |last1=Arkhipova |first1=V. P. |last2=Ikonnikova |first2=N. P. |last3=Noskova |first3=R. I. |last4=Sokol |first4=G. V. |last5=Shugarov |first5=S. Yu. |title=Light variations in the candidate for protoplanetary objects HD 179821=V1427 Aql in 1899–1999 |journal=Astronomy Letters |date=March 2001 |volume=27 |issue=3 |pages=156–162 |doi=10.1134/1.1351559 |bibcode=2001AstL...27..156A |s2cid=119759129 |url=https://ui.adsabs.harvard.edu/abs/2001AstL...27..156A |access-date=22 October 2021}}</ref>
 <ref name=van>{{cite journal|bibcode=2019A&A...631A..48V|title=Pulsations, eruptions, and evolution of four yellow hypergiants|last1=van Genderen|first1=A. M.|last2=Lobel|first2=A.|last3=Nieuwenhuijzen|first3=H.|last4=Henry|first4=G. W.|last5=De Jager|first5=C.|last6=Blown|first6=E.|last7=Di Scala|first7=G.|last8=Van Ballegoij|first8=E. J.|s2cid=203836020|journal=Astronomy and Astrophysics|year=2019|volume=631|pages=A48|doi=10.1051/0004-6361/201834358|arxiv=1910.02460}}</ref>
@@ Line 100: / Line 125: @@
  | journal=Astrophysical Journal Supplement Series
  | volume=71 | pages=245 | year=1989
- | doi=10.1086/191373 | bibcode=1989ApJS...71..245K }}</ref>
+ | doi=10.1086/191373 | bibcode=1989ApJS...71..245K | s2cid=123149047 }}</ref>
 <ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|display-authors=etal|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>
@@ Line 109: / Line 134: @@
 ==Sources==
-*{{cite book|author1=Juraj Zverko|author2=Jozef Ziznovsky|author3=[[Saul Adelman|Saul J. Adelman]]|author4=Werner W. Weiss|title=The A-Star Puzzle (IAU S224)|date=25 April 2005|publisher=[[Cambridge University Press]]|isbn=978-0-521-85018-6|pages=390–}}
+*{{cite book|author1=Juraj Zverko|author2=Jozef Ziznovsky|author3=Saul J. Adelman|author3-link=Saul Adelman|author4=Werner W. Weiss|title=The A-Star Puzzle (IAU S224)|date=25 April 2005|publisher=[[Cambridge University Press]]|isbn=978-0-521-85018-6|pages=390–}}
-*{{cite journal|arxiv=0801.2315|bibcode=2009ASPC..412...17O|title=Post-Red Supergiants|journal=The Biggest|volume=412|pages=17|last1=Oudmaijer|first1=R. D.|last2=Davies|first2=B.|last3=De Wit|first3=W.-J.|last4=Patel|first4=M.|year=2009}}
+*{{cite conference|arxiv=0801.2315|bibcode=2009ASPC..412...17O|chapter=Post-Red Supergiants|title=Biggest, Baddest, Coolest Stars|journal=[[Astronomical Society of the Pacific Conference Series]]|volume=412|pages=17|last1=Oudmaijer|first1=R. D.|last2=Davies|first2=B.|last3=De Wit|first3=W.-J.|last4=Patel|first4=M.|year=2009|editor1=Donald D. Luttermoser|editor2=Beverly J. Smith|editor3=Robert E. Stencil}}
 {{Stars of Aquila}}
+{{DEFAULTSORT:HD 179821}}
 [[Category:Aquila (constellation)]]
 [[Category:Henry Draper Catalogue objects|179821]]
 [[Category:G-type hypergiants]]
 [[Category:G-type supergiants]]
+[[Category:K-type supergiants]]
+[[Category:Post-asymptotic-giant-branch stars]]
 [[Category:Protoplanetary nebulae]]
 [[Category:Objects with variable star designations|Aquilae, V1427]]
 [[Category:Hipparcos objects|094496]]
-[[Category:Durchmusterung objects]]
+[[Category:Durchmusterung objects|BD-00 3679]]
 [[Category:Semiregular variable stars]]
-[[Category:2MASS objects]]
+[[Category:2MASS objects|J19135861+0007319]]

Observation data Epoch J2000 Equinox J2000
HD 179821 Credit: Hubble Legacy Archive
Constellation	Aquila
Right ascension	19^h 13^m 58.61^s^[1]
Declination	+00° 07′ 31.9″^[1]
Apparent magnitude (V)	8.19^[2] (7.83 - 8.23^[3])
Characteristics
Evolutionary stage	YHG or post-AGB
Spectral type	G4 0-Ia^[4] (G5Ia - K4I^[3])
Apparent magnitude (U)	10.81^[5]
Apparent magnitude (B)	9.694^[2]
Apparent magnitude (R)	8.2^[6]
Apparent magnitude (J)	5.371^[6]
Apparent magnitude (H)	4.998^[6]
Apparent magnitude (K)	4.728^[6]
B−V color index	+1.504^[2]
Variable type	SRd^[3]
Astrometry

Radial velocity (R_v)	+81.8±3.7^[7] km/s
Proper motion (μ)	RA: −0.090^[1] mas/yr Dec.: −3.494^[1] mas/yr
Parallax (π)	0.1893 ± 0.0206 mas^[1]
Distance	approx. 17,000 ly (approx. 5,300 pc)
Absolute magnitude (M_V)	−8.9^[8]^[9] or −5.7^[7]

Details
if a hypergiant
Mass	19 - 30^[7]^[10] `M`_☉
Radius	400 - 450^[11] `R`_☉
Luminosity	126,000^[11] - 295,000^[9] `L`_☉
Surface gravity (log g)	0.5^[8] cgs
Temperature	4,898 - 6,761^[11] K
Metallicity [Fe/H]	0.0^[8] dex
if post-AGB
Mass	0.8^[7] `M`_☉
Radius	147^[7]^[a] `R`_☉
Luminosity	20,000+12,000 −7,400^[7] `L`_☉
Surface gravity (log g)	0.5^[8] cgs
Temperature	5,660^[7] K
Metallicity [Fe/H]	0.0^[8] dex
Other designations
HD 179821, V1427 Aql, AFGL 2343, BD−00°3679, HIP 94496, SAO 124414, IRAS 19114+0002, 2MASS J19135861+0007319
Database references
SIMBAD	data