Epoch J2000 Equinox J2000
|Right ascension||19h 13m 58.6s|
|Declination||+00° 07′ 32″|
|Apparent magnitude (V)||8,19|
HD 179821 is a massive bright star in the constellation of Aquila, generally classified as a G-type yellow hypergiant of perhaps 30 solar masses at a distance from Earth of roughly 6,000 parsecs (20,000 light-years). It is surrounded by a dust shell that contributes to a double-peaked spectral energy distribution. It is estimated the star has lost about 10% of its initial mass after being a red supergiant star just 1,600 years ago, and is a likely supernova candidate.
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 6,000pc, 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. 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 and, as with the reflection nebula around IRC+10420, it may be masking a star hotter than the given G5 spectral type.
The distance is estimated to be around 6,000 parsecs and has a high luminosity of between 3.1×105 and 6×105 L⊙. It has a high radial velocity of +100 km/s. According to the studies of Jura et al (2001), the star may explode as a supernova in the next 100,000 years.
The chemical composition of this star differs from that of other yellow supergiant stars. The star is moderately metal-deficient 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.
While most authors consider HD 179821 to be a supergiant star, others think it is actually a protoplanetary nebula or a post-AGB star at a distance of 1 kiloparsec (3,200 light years). In that case the star's luminosity would be much lower, around 16,000 times that of our Sun and its initial mass would be equal to the current mass of our Sun.
- 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.
- 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. Retrieved 25 December 2012.
- Josselin, E.; L�bre, A. (1 March 2001). "Probing the post-AGB nature of HD 179821". Astronomy and Astrophysics 367 (3): 826–830. Bibcode:2001A&A...367..826J. doi:10.1051/0004-6361:20000496.
- 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.
- 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.
- "The Supernova". University of Antarctica. Retrieved 25 December 2012.
- 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.
- R. Szczerba; S.K. Górny (31 August 2001). Post-AGB Objects as a Phase of Stellar Evolution. Springer. pp. 315–. ISBN 978-0-7923-7145-8. Retrieved 25 December 2012.
- 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:10.1086/300815.
- 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. arXiv:1208.3143. Bibcode:2012A&A...545A..99T. doi:10.1051/0004-6361/201219545. A99.
- Robert F. Wing (31 July 2000). The Carbon Star Phenomenon. Springer. pp. 231–. ISBN 978-0-7923-6346-0. Retrieved 25 December 2012.
- Kipper, Tõnu (2008). "Optical Spectroscopy of a Post-AGB Star HD 179821 (V1427 Aql)". Baltic Astronomy 17: 87–102. Bibcode:2008BaltA..17...87K.