HD 179821

HD 179821 is a yellow hypergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable nearing the end of its life.

HD 179821

HD 179821
Credit: Hubble Legacy Archive
Observation data
Epoch J2000      Equinox J2000
Constellation Aquila
Right ascension  19h 13m 58.61s[1]
Declination +00° 07′ 31.9″[1]
Apparent magnitude (V) 8.19[2] (7.83 - 8.23[3])
Characteristics
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 (Rv)+82.0[7] km/s
Proper motion (μ) RA: −0.165[8] mas/yr
Dec.: −3.448[8] mas/yr
Parallax (π)0.3102 ± 0.0512[8] mas
Distanceapprox. 11,000 ly
(approx. 3,200 pc)
Absolute magnitude (MV)−8.9[7][9]
Details
Mass30[10] M☉
Radius400 - 450[11] R☉
Luminosity126,000[11] - 295,000[9] L☉
Surface gravity (log g)0.5[7] cgs
Temperature4,898 - 6,761[11] K
Metallicity [Fe/H]0.0[7] dex
Other designations
HD 179821, V1427 Aql, AFGL 2343, BD−00°3679, HIP 94496, SAO 124414, IRAS 19114+0002, 2MASS J19135861+0007319
Database references
SIMBADdata

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 proto-planetary 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 more modest dying 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⊙, an exceptionally high rate.[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 is estimated to be around 6,000 parsecs and 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

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 classifies F7.[23]

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 supergiant or hypergiant star,[24] others think it is actually a protoplanetary nebula or a post-AGB star at a distance of 1 kiloparsec (3,200 light years).[25] 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.[7][10] Gaia Data Release 2 gives a parallax of 0.31 mas implying a distance around 3,000 pc.[8]

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See also

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  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.
  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.
  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.
  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.
  6. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally Published in: 2003yCat.2246....0C. 2246. Bibcode:2003yCat.2246....0C.
  7. 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.
  8. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  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.
  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.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
  16. 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.
  17. 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.
  18. 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.
  19. 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.
  20. 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.
  21. 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.
  22. Wing, Robert F. (31 July 2000). The Carbon Star Phenomenon. Springer. pp. 231–. ISBN 978-0-7923-6346-0.
  23. 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.
  24. 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. A99.
  25. Kipper, Tõnu (2008). "Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql)". Baltic Astronomy. 17: 87–102. Bibcode:2008BaltA..17...87K.

Sources
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