HD 179821

Last updated
HD 179821
HD 179821.jpg
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
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 (Rv)+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  (MV)−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] [lower-alpha 1]   R
Luminosity20,000+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

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.

Contents

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 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) HD179821LightCurve.png
The blue band light curve of HD 179821, adapted from Arkhipova et al. (2001)

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]

See also

Notes

  1. Applying the Stefan-Boltzmann Law with a nominal solar effective temperature of 5,772  K:

Related Research Articles

<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

<span class="mw-page-title-main">Red supergiant</span> Stars with a supergiant luminosity class with a spectral type of K or M

Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

<span class="mw-page-title-main">V509 Cassiopeiae</span> Star in the constellation Cassiopeia

V509 Cassiopeiae is one of two yellow hypergiant stars found in the constellation Cassiopeia, which also contains Rho Cassiopeiae.

<span class="mw-page-title-main">Yellow hypergiant</span> Class of massive star with a spectral type of A to K

A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.

<span class="mw-page-title-main">Yellow supergiant</span> Star that has a supergiant luminosity class, with a spectral type of F or G

A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.

<span class="mw-page-title-main">V1401 Aquilae</span> Star in the constellation Aquila

V1401 Aquilae is a single, semi-regular pulsating star in the equatorial constellation of Aquila. It has the designation HD 190390 from the Henry Draper Catalogue, and was formerly designated 64 Sagittarii. The evolutionary status of the star is unclear, and it has been classified as a post-AGB object, a UU Herculis variable, or belonging to the W Virginis variable subclass of the type II Cepheids. It is dimly visible to the naked eye with an apparent visual magnitude that fluctuates around 6.38. Based on parallax measurements, it is located at a distance of approximately 2,380 light years. It lies 21.5° from the galactic plane.

<span class="mw-page-title-main">S Persei</span> Red supergiant or hypergiant variable star in the constellation Perseus

S Persei is a red supergiant or hypergiant located near the Double Cluster in Perseus, north of the cluster NGC 869. It is a member of the Perseus OB1 association and one of the largest known stars. If placed in the Solar System, its photosphere would engulf the orbit of Jupiter. It is also a semiregular variable, a star whose variations are less regular than those of Mira variables.

<span class="mw-page-title-main">6 Cassiopeiae</span> Star in the constellation Cassiopeia

6 Cassiopeiae is a white hypergiant in the constellation Cassiopeia, and a small-amplitude variable star.

<span class="mw-page-title-main">Hypergiant</span> Rare star with tremendous luminosity and high rates of mass loss by stellar winds

A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae. Notable examples of hypergiants include the Pistol Star, a blue hypergiant located close to the Galactic Center and one of the most luminous stars known; Rho Cassiopeiae, a yellow hypergiant that is one of the brightest to the naked eye; and Mu Cephei (Herschel's "Garnet Star"), one of the largest and brightest stars known.

<span class="mw-page-title-main">V4381 Sagittarii</span> Star in the constellation Sagittarius

V4381 Sagittarii is a variable star in the constellation Sagittarius. A white supergiant of spectral type A2/A3Iab, it is an Alpha Cygni variable that varies between apparent photographic magnitudes 6.57 and 6.62. Its visual apparent magnitude is about 6.54.

<span class="mw-page-title-main">IRAS 17163−3907</span> Yellow hypergiant in the constellation Scorpius

IRAS 17163−3907, also known as Hen 3-1379, is a yellow hypergiant star located 13,000 light years from Earth in the constellation of Scorpius. The star is embedded in thick shells of expelled gases and dust, and owing to its appearance has been nicknamed by astronomers the "Fried Egg Nebula". Yellow hypergiants are in an extremely active phase of their evolution.

<span class="mw-page-title-main">HD 33579</span> Star in the constellation Dorado

HD 33579 is a white/yellow hypergiant and one of the brightest stars in the Large Magellanic Cloud (LMC). It is a suspected variable star.

<span class="mw-page-title-main">IRC +10420</span> Hypergiant star in the constellation Aquila

IRC +10420, also known as V1302 Aquilae, is a yellow hypergiant star located in the constellation of Aquila at a distance of 4-6 kiloparsecs of the Sun.

<span class="mw-page-title-main">HD 168607</span> Star in the constellation Sagittarius

HD 168607 is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and possible luminous blue variable, that can be seen at the south-east of M17, the Omega Nebula.

<span class="mw-page-title-main">Westerlund 1 W26</span> Star in the constellation Ara

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<span class="mw-page-title-main">IRAS 19475+3119</span> Nebula in the constellation Cygnus

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<span class="mw-page-title-main">V915 Scorpii</span> Variable star in the constellation Scorpius

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<span class="mw-page-title-main">HD 143183</span> Star in the constellation Norma

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<span class="mw-page-title-main">V1027 Cygni</span> Star in the constellation Cygnus

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Sources