Stephenson 2-18

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Stephenson 2-18
Stephenson 2-18 zoomed in, 2MASS survey, 2003.png
Stephenson 2 DFK 1 seen by PanSTARRS DR1.png
(Top) St2-18 together with its parent cluster Stephenson 2 (upper left), viewed by the Two-Micron All Sky Survey. (Below) St2-18 seen by PANSTARRS DR1.
Credit: Université de Strasbourg/CNRS (2003)
Observation data
Epoch J2000      Equinox J2000
Constellation Scutum
Right ascension 18h 39m 02.3709s[1]
Declination −06° 05′ 10.5357″[1]
Characteristics
Evolutionary stage Red supergiant, possible red hypergiant[2]
Spectral type ~M6[3]
Apparent magnitude (G) 15.2631±0.0092[1]
Apparent magnitude (J) 7.150[4]
Apparent magnitude (H) 4.698[4]
Apparent magnitude (K) 2.9[4]
Astrometry
Proper motion (μ) RA: −3.045±0.511[1] mas/yr
Dec.: −5.950±0.480[1] mas/yr
Parallax (π)−0.0081 ± 0.3120[1] mas
Distance18,900[5] ly
(5,800[5] pc)
Details
Radius~2,150[6][a] R☉
Luminosity437,000[6] (90,000[7]–630,000[5][b]) L☉
Temperature3,200[6] K
Other designations
Stephenson 2-18, Stephenson 2 DFK 1, RSGC2-18, 2MASS J18390238-0605106, IRAS 18363-0607
Database references
SIMBADdata

Coordinates: Sky map 18h 39m 02.3709s, −06° 05′ 10.5357″

Stephenson 2-18 (St2-18), also known as Stephenson 2 DFK 1 or RSGC2-18, is possibly a red supergiant or red hypergiant star in the constellation of Scutum, although some sources consider it to be a foreground object.[5] It lies near the open cluster Stephenson 2, which is located about 6,000 parsecs (20,000 ly) away from Earth in the Scutum–Centaurus Arm of the Milky Way galaxy, and is assumed to be one of a group of stars at a similar distance. It is possibly among the largest known stars, and one of the most luminous red supergiants, with a possible radius around 2,150 times that of the Sun (R☉), which would correspond to a volume nearly 10 billion times that of the Sun. If placed at the center of earth's Solar System, its photosphere could potentially engulf the orbit of Saturn.

Observation history[]

Comparison of the sizes of selected different stars. From left to right are Cygnus OB2 #12, V382 Carinae, V915 Scorpii, UY Scuti and Stephenson 2-18. Orbits of Saturn and Neptune are also shown for comparison.

The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey.[2][8] The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.[9]

The brightest star in the region of the cluster was given the identifier 1 in the first analysis of cluster member properties. However, it was not considered to be a member of Stephenson 2 due to its outlying position, abnormally high brightness, and slightly atypical proper motion.[2] The authors however note Stephenson 2-18 as having a significant infrared excess, possibly because of an extreme mass-losing episode. As a result, they state that the star may be a red hypergiant.[2]

In a later study, the same star was given the number 18 and assigned to an outlying group of stars called Stephenson 2 SW, assumed to be at a similar distance to the core cluster.[7] The designation St2-18 (short for Stephenson 2-18) is often used for the star, following the numbering from Deguchi (2010).[6][7] To avoid confusion from using the same number for different stars and different numbers for the same star, designations from Davis (2007) are often given a prefix of DFK or D,[9] for example Stephenson 2 DFK 1.[10]

Physical properties[]

St 2-18 shows the traits and properties of a highly luminous red supergiant, with a spectral type of M6, which is unusual for a supergiant star.[3] This places it at the top right corner of the Hertzsprung–Russell diagram, a region characterized for exceptionally large and luminous low-temperature stars.

One calculation for finding the bolometric luminosity by fitting the Spectral Energy Distribution (SED) gives the star a luminosity of nearly 440,000 L☉, with an effective temperature of 3,200 K, which corresponds to a very large radius of 2,150 R☉ (1.50×109 km; 10.0 au; 930,000,000 mi),[a] which would be considerably larger and more luminous than theoretical models of the largest, and most luminous red supergiants possible (roughly 1,500 R☉ and 105.5 L☉ respectively).[11][6] An alternate but older calculation from 2010, still assuming membership of the Stephenson 2 cluster at 5.5 kpc but based on 12 and 25 μm fluxes, gives a much lower and relatively modest luminosity of 90,000 L☉.[7] A newer calculation, based on SED integration and assuming a distance of 5.8 kpc, gives a bolometric luminosity of 630,000 L☉. However, it has been noted that its SED is somewhat peculiar, with the fluxes that couldn't fit with the accepted range of appropriate temperatures for an RSG. This would suggest a higher extinction, which would make it be even more luminous. Because of this unusually high luminosity, the star's membership to the Stephenson 2 cluster has been considered doubtful, though it is also stated that it cannot necessarily be ruled out yet.[5][b]

In 2013, an article describing the red supergiants in Stephenson 2 stated that Stephenson 2-18 (referred to as D1) and D2 (another member of Stephenson 2) have maser emissions, indicating that they have the highest mass loss in the cluster.[3]

Membership[]

It has been debated for a while if this star is actually part of its supposed cluster. Due to its radial velocity being below the other cluster stars, some sources state that the star is unlikely to be a foreground giant;[2][3] however, more recent papers considered the star an unlikely member due to its extreme properties.[5]

Uncertainty[]

The distance of Stephenson 2-18 has been stated to have an uncertainty greater than 50%,[6] and the radius of 2,150 R☉ is very likely an overestimation because it greatly exceeds the theoretical limit of 1,500 R☉.[11] Another estimate of the luminosity gave a value of 90,000 L☉.[7]


See also[]

Notes[]

  1. ^ a b Applying the Stefan-Boltzmann Law with a nominal solar effective temperature of 5,772 K:
    .
  2. ^ a b Mistakenly referred to as RSGC1-F01.

References[]

  1. ^ a b c d e f 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.
  2. ^ a b c d e Davies, B.; Figer, D. F.; Kudritzki, R. P.; MacKenty, J.; Najarro, F.; Herrero, A. (2007). "A Massive Cluster of Red Supergiants at the Base of the Scutum‐Crux Arm". The Astrophysical Journal. 671 (1): 781–801. arXiv:0708.0821. Bibcode:2007ApJ...671..781D. doi:10.1086/522224. S2CID 1447781.
  3. ^ a b c d Negueruela, I.; González-Fernández, C.; Dorda, R.; Marco, A.; Clark, J. S. (2013). "The population of M-type supergiants in the starburst cluster Stephenson 2". Eas Publications Series. 60: 279. arXiv:1303.1837. Bibcode:2013EAS....60..279N. doi:10.1051/eas/1360032. S2CID 119232033.
  4. ^ a b c 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.
  5. ^ a b c d e f Humphreys, Roberta M.; Helmel, Greta; Jones, Terry J.; Gordon, Michael S. (2020). "Exploring the Mass Loss Histories of the Red Supergiants". The Astronomical Journal. 160 (3): 145. arXiv:2008.01108. Bibcode:2020AJ....160..145H. doi:10.3847/1538-3881/abab15. S2CID 220961677.
  6. ^ a b c d e f Fok, Thomas K. T; Nakashima, Jun-ichi; Yung, Bosco H. K; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv:1209.6427. Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65. S2CID 53393926.
  7. ^ a b c d e Deguchi, Shuji; Nakashima, Jun-Ichi; Zhang, Yong; Chong, Selina S. N.; Koike, Kazutaka; Kwok, Sun (2010). "SiO and H2O Maser Observations of Red Supergiants in Star Clusters Embedded in the Galactic Disk". Publications of the Astronomical Society of Japan. 62 (2): 391–407. arXiv:1002.2492. Bibcode:2010PASJ...62..391D. doi:10.1093/pasj/62.2.391. S2CID 24396370.
  8. ^ Stephenson, C. B. (1990). "A possible new and very remote galactic cluster". The Astronomical Journal. 99: 1867. Bibcode:1990AJ.....99.1867S. doi:10.1086/115464.
  9. ^ a b Negueruela, I.; González-Fernández, C.; Marco, A.; Clark, J. S.; Martínez-Núñez, S. (2010). "Another cluster of red supergiants close to RSGC1". Astronomy and Astrophysics. 513: A74. arXiv:1002.1823. Bibcode:2010A&A...513A..74N. doi:10.1051/0004-6361/200913373. S2CID 118531372.
  10. ^ Verheyen, L.; Messineo, M.; Menten, K. M. (2012). "SiO maser emission from red supergiants across the Galaxy . I. Targets in massive star clusters". Astronomy & Astrophysics. 541: A36. arXiv:1203.4727. Bibcode:2012A&A...541A..36V. doi:10.1051/0004-6361/201118265. S2CID 55630819.
  11. ^ a b Emily M. Levesque; Philip Massey; K. A. G. Olsen; Bertrand Plez; et al. (August 2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought". The Astrophysical Journal. 628 (2): 973–985. arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID 15109583.
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