NML Cygni

NML Cygni or V1489 Cygni (abbreviated to NML Cyg or V1489 Cyg) is a red hypergiant[4] or red supergiant (RSG) in the constellation Cygnus. It is one of the largest stars currently known by radius, and is also one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way.

NML Cygni

NML Cygni, seen as the deep red star at the center, from the Sloan Digital Sky Survey DR9. Note the green circumstellar nebula surrounding the star.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cygnus
Right ascension 20h 46m 25.54s[1]
Declination +40° 06′ 59.4″[1]
Apparent magnitude (V) 16.60 (variable)[2]
Characteristics
Evolutionary stage OH/IR[3] red hypergiant[4]
Spectral type M4.5–M7.9 Ia–III[5]
Apparent magnitude (K) 12.3[6]
Apparent magnitude (B) 18.64
Apparent magnitude (V) 16.60
Apparent magnitude (G) 11.0330
Apparent magnitude (J) 4.877
Apparent magnitude (H) 2.389
B−V color index +2.04[2]
Variable type SRc[6]
Astrometry
Proper motion (μ) RA: −1.55[4] mas/yr
Dec.: −4.59[4] mas/yr
Parallax (π)0.620 ± 0.047[4] mas
Distance5,250+420
−360 ly
(1,610+130
−110[4] pc)
Details
Mass50[7] M☉
Radiuseither 1,639 or 2,770[4][lower-alpha 1], 1,183[6] R☉
Luminosity229,000+40,000
−41,000[8] L☉
Temperature3,300[3] K
Age8[4] Myr
Other designations
NML Cyg, V1489 Cyg, RAFGL 2650, IRC +40448, 2MASS J20462554+4006594, AAVSO 2042+39
Database references
SIMBADdata

The distance of NML Cygni from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.[9] It is a part of the Cygnus OB2 association, one of the closest massive associations to the Sun, spanning nearly 2° on the sky or ∼30 pc in radius at the distance of 1.74±0.2 kpc.[10]

The radius of NML Cyg is uncertain but calculated to be between 1,183 and 2,770 times that of the Sun (R☉). If placed at the center of the Solar System, its surface would extend past the orbit of Jupiter or Saturn.

Observational history

NML Cygni was discovered in 1965 by American astronomers Neugebauer, Martz, and Leighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of 1,000 K.[11] The name NML comes from the names of these three discoverers.[12] The second star was briefly referred to as NML Tauri[13] but is now known as IK Tauri,[14] an M9 Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations,[15] but is still most commonly referred to as NML Cygni. Its composition began to be revealed with the discovery of OH masers (1612 MHz) in 1968.[16] H
2O, SiO, CO, HCN, CS, SO, SO
2, and H
2S molecules have also been detected.[17]

Physical characteristics
H-alpha light image of Cygnus OB2, the stellar association in which NML Cygni is located

NML Cygni is an extremely large and luminous cool supergiant with parameters similar to that of another notable but more extreme cool hypergiant star, VY Canis Majoris, and is also known as a heavily mass-losing OH/IR supergiant. It is also a semiregular variable star with a period of either 1,280 or 940 days.[5][10] It occupies the upper-right hand corner of the Hertzsprung–Russell diagram although most of the properties of the star depend directly on its distance.

The bolometric luminosity (Lbol) for NML Cygni was originally calculated to be 500,000 L☉ at an assumed distance of 2 kpc and the radius was calculated to be 3,700 R☉ based on an 8.6 mas angular diameter and distance.[18][19][20] A 2006 study, similar to those conducted on VY Canis Majoris, suggests that NML Cygni is a normal red supergiant with consequently much lower luminosity and radius values.[21] More modern and accurate measurements give a distance around 1.6 kpc, which gives a luminosity around 200,000 L☉. A radio angular diameter of 44 mas was given based on the distance, suggesting the optical angular diameter may be around 22 mas.[4] This distance and a luminosity of 270,000 L☉ were combined with assumptions of the effective temperature of the star, giving a radius of 1,640 R☉ for a temperature of 3,250 K or possibly 2,770 R☉ for a temperature of 2,500 K.[lower-alpha 1][4] However, another paper gives a much lower radius of 1,183 R☉ based on an assumed effective temperature of 3,834 K and a lower distance of 1.22 kpc.[6] There is a Gaia Data Release 2 parallax for NML Cygni of 1.5259±0.5677 mas, but the underlying measurements show a considerable level of noise and the parallax is considered unreliable.[22]

NML Cygni lies close to the expected position that a 25 M☉ star would evolve to after eight million years.[4] Estimates of its current mass are difficult. One published measure is 50 M☉.[7]

NML Cygni is evolved and a number of heavy elements and molecules have been detected in its atmosphere, particularly oxygen, hydroxyl, and water. It is surrounded by dusty material[4][10] and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers.[23]

NML Cygni has an estimated mass loss rate of 4.2 to 4.8×10−4 M☉ per year,[3] one of the highest known for any star. The annual parallax of NML Cygni is measured to be around 0.62 milliarcseconds.[4] From the observations, it is estimated that NML Cygni has two discrete optically thick envelopes of dust and molecules. The optical depth of the inner shell is found to be 1.9, whereas that of the outer one is 0.33.[24] These dust envelopes are formed due to the strong post-main-sequence wind, which has a velocity 23 km/s.[10]

Because of the star's position on the outskirts of the massive Cygnus OB2 association, the detectable effects of NML Cygni's radiation on the surrounding dust and gas are limited to the region away from the central hot stars of the association.[10]

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

References
  1. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally Published in: 2003yCat.2246....0C. 2246: II/246. Bibcode:2003yCat.2246....0C.
  2. Johnson, Harold L.; Mendoza v., Eugenio E.; Wisniewski, Weislaw Z. (1965). "Observations of "Infrared Stars."". Astrophysical Journal. 142: 1249. Bibcode:1965ApJ...142.1249J. doi:10.1086/148393.
  3. Gordon, Michael S.; Humphreys, Roberta M.; Jones, Terry J.; Shenoy, Dinesh; Gehrz, Robert D.; Helton, L. Andrew; Marengo, Massimo; Hinz, Philip M.; Hoffmann, William F. (2018). "Searching for Cool Dust. II. Infrared Imaging of the OH/IR Supergiants, NML Cyg, VX SGR, S Per, and the Normal Red Supergiants RS per and T per". The Astronomical Journal. 155 (5): 212. arXiv:1708.00018. Bibcode:2018AJ....155..212G. doi:10.3847/1538-3881/aab961. S2CID 73650032.
  4. Zhang, B.; Reid, M. J.; Menten, K. M.; Zheng, X. W.; Brunthaler, A. (2012). "The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry" (PDF). Astronomy & Astrophysics. 544: A42. arXiv:1207.1850. Bibcode:2012A&A...544A..42Z. doi:10.1051/0004-6361/201219587. S2CID 55509287.
  5. "GCVS Query=V1489 Cyg". Sternberg Astronomical Institute. General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2018-09-21.
  6. De Beck, E.; Decin, L.; De Koter, A.; Justtanont, K.; Verhoelst, T.; Kemper, F.; Menten, K. M. (2010). "Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: Derivation of mass-loss rate formulae". Astronomy and Astrophysics. 523: A18. arXiv:1008.1083. Bibcode:2010A&A...523A..18D. doi:10.1051/0004-6361/200913771. S2CID 16131273.
  7. Morris, M.; Jura, M. (1983). "The nature of NML Cygnus". Astrophysical Journal. 267: 179. Bibcode:1983ApJ...267..179M. doi:10.1086/160856.
  8. Davies, Ben; Beasor, Emma R. (March 2020). "The 'red supergiant problem': the upper luminosity boundary of Type II supernova progenitors". MNRAS. 493 (1): 468–476. arXiv:2001.06020. Bibcode:2020MNRAS.493..468D. doi:10.1093/mnras/staa174. S2CID 210714093.
  9. Schuster, Michael Thomas (2007). Investigating the Circumstellar Environments of the Cool Hypergiants. p. 57. ISBN 978-0-549-32782-0. Retrieved 27 August 2012.
  10. Schuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A.; Hoffmann, W. F. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics". The Astrophysical Journal. 699 (2): 1423–1432. arXiv:0904.4690. Bibcode:2009ApJ...699.1423S. doi:10.1088/0004-637X/699/2/1423. S2CID 17699562.
  11. Neugebauer, G.; Martz, D. E.; Leighton, R. B. (July 1965). "Observations of Extremely Cool Stars" (PDF). Astrophysical Journal. 142: 399–401. Bibcode:1965ApJ...142..399N. doi:10.1086/148300.
  12. Hearnshaw, J. B. (2 May 1996). "New infrared sources and their interpretation". The Measurement of Starlight: Two Centuries of Astronomical Photometry. Cambridge University Press. p. 278. ISBN 978-0-521-40393-1. Retrieved 23 August 2012.
  13. Pesch, P. (1967). "Objective-Prism Spectra of Some Very Red Stars". The Astrophysical Journal. 147: 381. Bibcode:1967ApJ...147..381P. doi:10.1086/149015.
  14. Kukarkin, B. V.; Efremov, Yu. N.; Frolov, M. S.; Medvedeva, G. I.; et al. (8 November 1968). "Identification List of the New Variable Stars Nominated in 1968". Information Bulletin on Variable Stars. 311 (1): 1. Bibcode:1968IBVS..311....1K.
  15. Kukarkin, B. V.; Kholopov, P. N.; Kukarkina, N. P. (27 November 1975). "61st Name-List of Variable Stars". Information Bulletin on Variable Stars. 1068 (1): 1. Bibcode:1975IBVS.1068....1K.
  16. Cohen, R. J.; Downs, G.; Emerson, R.; Grimm, M.; et al. (1 April 1987). "Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH-IR sources". Monthly Notices of the Royal Astronomical Society. 225 (3): 491–498. Bibcode:1987MNRAS.225..491C. doi:10.1093/mnras/225.3.491. PMID 11540900.
  17. Kevin Marvel (19 December 1996). "NML Cygni". The Circumstellar Environment of Evolved Stars As Revealed by Studies of Circumstellar Water Masers. Universal-Publishers. pp. 182–212. ISBN 978-1-58112-061-5. Retrieved 23 August 2012.
  18. Zubko, Viktor; Li, Di; Lim, Tanya; Feuchtgruber, Helmut; Harwit, Martin (2004). "Observations of Water Vapor Outflow from NML Cygnus". The Astrophysical Journal. 610 (1): 427. arXiv:astro-ph/0405044. Bibcode:2004ApJ...610..427Z. doi:10.1086/421700. S2CID 14352419.
  19. Monnier, J. D; Bester, M; Danchi, W. C; Johnson, M. A; Lipman, E. A; Townes, C. H; Tuthill, P. G; Geballe, T. R; Nishimoto, D; Kervin, P. W (1997). "Nonuniform Dust Outflow Observed around Infrared Object NML Cygni". The Astrophysical Journal. 481 (1): 420. arXiv:astro-ph/9702103. Bibcode:1997ApJ...481..420M. doi:10.1086/304050. S2CID 9503967.
  20. Monnier, J. D.; Millan‐Gabet, R.; Tuthill, P. G.; Traub, W. A.; Carleton, N. P.; Coude Du Foresto, V.; Danchi, W. C.; Lacasse, M. G.; Morel, S.; Perrin, G.; Porro, I. L.; Schloerb, F. P.; Townes, C. H. (2004). "High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal. 605 (1): 436–461. arXiv:astro-ph/0401363. Bibcode:2004ApJ...605..436M. doi:10.1086/382218. S2CID 7851916.
  21. Massey, Philip; Levesque, Emily M.; Plez, Bertrand (1 August 2006). "Bringing VY Canis Majoris down to size: an improved determination of its effective temperature". The Astrophysical Journal. 646 (2): 1203–1208. arXiv:astro-ph/0604253. Bibcode:2006ApJ...646.1203M. doi:10.1086/505025. S2CID 14314968.
  22. Xu, Shuangjing; Zhang, Bo; Reid, Mark J.; Zheng, Xingwu; Wang, Guangli (2019). "Comparison of Gaia DR2 Parallaxes of Stars with VLBI Astrometry". The Astrophysical Journal. 875 (2): 114. arXiv:1903.04105. Bibcode:2019ApJ...875..114X. doi:10.3847/1538-4357/ab0e83. S2CID 119192180.
  23. Schuster, M. T.; Humphreys, R. M.; Marengo, M. (2006). "The Circumstellar Environments of NML Cygni and the Cool Hypergiants". The Astronomical Journal. 131 (1): 603–611. arXiv:astro-ph/0510010. Bibcode:2006AJ....131..603S. doi:10.1086/498395. S2CID 16723190.
  24. DanchiI, W. C.; Green, W. H.; Hale, D. D. S.; McEleroy, K.; et al. (July 2001). "Proper Motions of Dust Shells Surrounding NML Cygni". The Astrophysical Journal. 555 (1): 405. Bibcode:2001ApJ...555..405D. doi:10.1086/322237.
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