Stephenson 2-18

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 or an unrelated red supergiant.[5][8] 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.

Stephenson 2-18


(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 mas[1]
Distance18,900[5] ly
(5,800[5] pc)
Details
Radius~2,150[6][lower-alpha 1] R☉
Luminosity437,000[6] (90,000[7]–630,000[5][lower-alpha 2]) L☉
Temperature3,200[6] K
Other designations
Stephenson 2-18, Stephenson 2 DFK 1, RSGC2-18, 2MASS J18390238-0605106, IRAS 18363-0607, DENIS J183902.4-060510, MSX6C G026.1044-00.0283
Database references
SIMBADdata

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][9] The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.[10]

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, being categorized as a unrelated red supergiant.[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 Davies (2007) are often given a prefix of DFK or D,[10] for example Stephenson 2 DFK 1.[8]

In 2012, Stephenson 2-18, along with 56 other red supergiants, was observed in a study regarding the maser emissions from red supergiants across the galaxy. The study derived the properties of those red supergiants, including Stephenson 2-18.[6]

That same year, it was observed yet again for a study regarding the types of masers on red supergiant stars in clusters. The study identified Stephenson 2-18 as an unrelated star to Stephenson 2, based on its radial velocity.[8] During 2013, in a study regarding the red supergiants in Stephenson 2, Stephenson 2-18 (referred to as D1) was observed and had its spectrum taken, with its spectral type identified.[3] In several later studies, the star was described as being a "very late-type red supergiant".[11][5]

It was also mentioned in Humphreys (2020), which denoted its extreme properties and doubtful membership.[5]

Physical properties

St2-18 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.

Stephenson 2-18 is usually classified as a red supergiant, partly due to its broad line profile.[7][2] However, its significant infrared excess (which suggests a possible extreme mass-losing episode) has led the authors of Davies (2007) to state that the star might be a red hypergiant, like VY Canis Majoris. It is also stated that Stephenson 2-18 is on the brink of ejecting its outer layers and evolving into a Luminous blue variable (LBV) or Wolf Rayet star (WR star). [2]

One calculation for finding the bolometric luminosity by fitting the Spectral Energy Distribution (SED) using the DUSTY model 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),[lower-alpha 1] 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).[12][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 flux densities, gives a much lower and relatively modest luminosity of 90,000 L☉.[7] A newer calculation, based on SED integration (based on published fluxes) 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 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][lower-alpha 2] As stated in Negueruela et al 2012, the stellar association is spread over a large area.[13][5]

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. Only the stars with the highest bolometric luminosity in the cluster seem to present maser emissions. [3] Stephenson 2-18 displays strong silicate emission, especially at wavelengths of 10 μm and 18 μm.[7]

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 but with some spectrum-derived indications showing signs of membership, 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] Using radial velocities determined from SiO maser emission and IR CO absorption, a study of red supergiant masers in massive clusters considered Stephenson 2-18 as a field red supergiant, unrelated to Stephenson 2 due to its lower radial velocity that is significantly different compared to other stars from Stephenson 2.[8] Unfortunately, Stephenson 2-18's membership cannot be ruled out yet.[5]

Another possibility is that Stephenson 2-18 is actually a member, because its radial velocity is offset by an expanding optically thick envelope. After all, the velocity difference between this star’s radial velocity and Stephenson 2 itself (20 kilometers per second) is a typical outflow speed for red supergiants.[2] Another study says that Stephenson 2-18 is part of a cluster related to Stephenson 2, Stephenson 2 SW, which is assumed to be at the same distance as the core cluster itself and also contains several massive stars and red supergiants.[7]

Uncertainty

The distance of Stephenson 2-18 has been stated to have a relative 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☉.[12] Another estimate of the luminosity gave a value of 90,000 L☉.[7]


See also

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

References
  1. 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. 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. 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. 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. 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. 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. 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. 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.
  9. 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.
  10. 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.
  11. Negueruela, Ignacio (2016). "Clusters rich in red supergiants". Astronomy in Focus, as presented at the IAU XXIX General Assembly, 2015. 29B: 461–463. doi:10.1017/S1743921316005858. Retrieved April 6, 2022.
  12. 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.
  13. Negueruela, I.; Marco, A.; González-Fernández, C.; Jiménez-Esteban, F.; Clark, J. S.; Garcia, M.; Solano, E. (2012). "Red supergiants around the obscured open cluster Stephenson 2". Astronomy & Astrophysics. 547: A15. arXiv:1208.3282. Bibcode:2012A&A...547A..15N. doi:10.1051/0004-6361/201219540. S2CID 42961348.
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