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HD 179821 is a yellow supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable and either a moderate-mass post-AGB star or distant massive hypergiant.
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Discovery
HD 179821 was first catalogued as an anonymous 8th magnitude star at the start of the 20th century. It was later listed as a spectral standard G4 0-Ia, indicating a highly luminous star type now known as a hypergiant.
It was first considered notable for its infrared excess and double-peaked Spectral energy distribution in the infrared. These were considered to be indicators of surrounding dust and HD 179821 was identified as a possible proto-planetary nebula. Variability was also detected.
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.
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 6996400000000000000♠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.
It is that which 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.
The distance is estimated to be around 6,000 parsecs and has a high luminosity of between 7005310000000000000♠3.1×105 and 7005600000000000000♠6×105 L⊙. It has a high radial velocity of 7005100000000000000♠+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.
Chemical composition
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.
Controversy
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 the Sun, and its initial mass would be equal to the current mass of the Sun.
This discrepancy arises because its distance is too great to be measured by parallax and it has some properties of both a yellow hypergiant and a protoplanetary nebula/Post-AGB star.