Astronomy:NML Cygni

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Short description: Star in the constellation Cygnus

Coordinates: Sky map 20h 46m 25.54s, +40° 06′ 59.40″

NML Cygni
256px
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
Equinox J2000.0]] (ICRS)
Constellation Cygnus
Right ascension  20h 46m 25.54s[1]
Declination +40° 06′ 59.5″[1]
Apparent magnitude (V) 16.60 (variable)[2]
Characteristics
Evolutionary stage OH/IR[3] RHG[4]
Spectral type M4.5–M7.9 Ia–III[5]
Apparent magnitude (K) 0.791±0.204[1]
Apparent magnitude (G) 11.148[1]
Apparent magnitude (J) 4.877±0.037[1]
Apparent magnitude (H) 2.389±0.2[1]
B−V color index +2.04[2]
Variable type SR[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
Mass25 (initial)[4] M☉
Radius<1,350+195
−229[7][lower-alpha 1] R☉
Luminosity229,000+40,000
−41,000,[9] 270,000+50,000
−50,000[4] L☉
Temperature3,375[8] K
Age8[4] Myr
Other designations
NML Cyg, V1489 Cyg, 2MASS J20462554+4006594
Database references
SIMBADdata

NML Cygni or V1489 Cygni (abbreviated to NML Cyg or V1489 Cyg) is a red hypergiant[4] or red supergiant (RHG or RSG) in the constellation Cygnus. It is one of the largest known stars currently known, 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.

The distance of NML Cygni from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.[10] 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.[11] Based on the estimated distance and an upper limit of its angular diameter of 7.8±0.64 milliarcseconds,[8] NML Cygni's physical radius is estimated to be no more than 1,350 R☉. If placed at the center of the Solar System, its surface would potentially extend past the orbit of Jupiter.

Observational history

A near infrared (3.5 micron) light curve for V1489 Cygni, plotted from data published by Strecker (1975)[12]

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.[13] The name NML comes from the names of these three discoverers.[14] The second star was briefly referred to as NML Tauri[15] but is now known as IK Tauri,[16] an M9 Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations,[17] 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.[18] H2O, SiO, CO, HCN, CS, SO, SO2, and H2S molecules have also been detected.[19]

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.[11][5] 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.

Due to its similarity to VY CMa, NML Cygni has been suggested in 2025 to be a possible candidate for a star in a second red supergiant phase; similar to less massive AGB stars, it may have once evolved blueward into a post-RSG warm hypergiant and then redward into an extreme red supergiant in a very short and final high mass loss state following a blue loop, before eventually exploding into a supernova or directly collapsing to a black hole.[20]

Size, luminosity, and temperature

NML Cygni compared to the Sun and Earth's orbit.

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.[21][22][23] 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.[24] 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 2][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.[25]

NML Cygni's uniform disk angular diameter was measured by interferometry, leading to an apparent size of 7.8±0.64 milliarcseconds.[8] Assuming the distance measured by Zhang et al. (2012) (1610+130
−110 parsecs),[4] it leads to a physical radius of 1,350 R☉.[7] If placed in the center of the Solar System, its photosphere would past the orbit of Jupiter. NML Cygni is covered by a complex dust shell, which is causing interference in the angular diameter, therefore this radius is only an upper limit.[8]

Mass and mass loss

NML Cygni lies close to the expected position that a 25 M☉ star would evolve to after eight million years.[4]

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][11] and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers.[26]

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.[27] These dust envelopes are formed due to the strong post-main-sequence wind, which has a velocity 23 km/s.[11]

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.[11]

See also

Notes

  1. ↑ Surrounding dusty region is very complex, making the radius hard to determine.[8]
  2. ↑ Applying the Stefan-Boltzmann Law with a nominal solar effective temperature of 5,772 K:
    (5772/3250)4*270,000=1638.96 R⊙
    (5772/2500)4*270,000=2769.84 R⊙

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "NML Cyg". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NML+Cyg. 
  2. ↑ 2.0 2.1 Johnson, Harold L.; Eugenio E., Mendoza V.; Wisniewski, Weislaw Z. (October 1965). "Observations of "Infrared Stars."" (in en). The Astrophysical Journal 142: 1249. doi:10.1086/148393. ISSN 0004-637X. Bibcode: 1965ApJ...142.1249J. 
  3. ↑ 3.0 3.1 Gordon, Michael S.; Humphreys, Roberta M.; Jones, Terry J.; Shenoy, Dinesh; Gehrz, Robert D.; Helton, L. Andrew; Marengo, Massimo; Hinz, Philip M. et al. (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. doi:10.3847/1538-3881/aab961. Bibcode: 2018AJ....155..212G. 
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 Zhang, B.; Reid, M. J.; Menten, K. M.; Zheng, X. W.; Brunthaler, A. (August 2012). "The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry". Astronomy & Astrophysics 544: A42. doi:10.1051/0004-6361/201219587. ISSN 0004-6361. Bibcode: 2012A&A...544A..42Z. 
  5. ↑ 5.0 5.1 "GCVS Query=V1489 Cyg". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. http://www.sai.msu.su/gcvs/cgi-bin/search.cgi?search=V1489+Cyg. 
  6. ↑ 6.0 6.1 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. doi:10.1051/0004-6361/200913771. Bibcode: 2010A&A...523A..18D. 
  7. ↑ 7.0 7.1 Lang, Kenneth R. (2006). Astrophysical formulae. Astronomy and astrophysics library. 1 (3rd ed.). Berlin: Springer. ISBN 978-3-540-29692-8. https://books.google.com/books?id=OvTjLcQ4MCQC&pg=PA41. . The radius (R*) is given by:
    2⋅R*=(10−3⋅1,610⋅7.8) AU0.0046491 AU/R⨀2,701⋅R⨀
  8. ↑ 8.0 8.1 8.2 8.3 8.4 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. et al. (2004-04-10). "High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal 605 (1): 436–461. doi:10.1086/382218. ISSN 0004-637X. 
  9. ↑ Davies, Ben; Beasor, Emma R. (March 2020). "The 'red supergiant problem': the upper luminosity boundary of Type II supernova progenitors" (in en). MNRAS 493 (1): 468–476. doi:10.1093/mnras/staa174. Bibcode: 2020MNRAS.493..468D. 
  10. ↑ Schuster, Michael Thomas (2007). Investigating the Circumstellar Environments of the Cool Hypergiants. University of Minnesota. ISBN 978-0-549-32782-0. https://books.google.com/books?id=LrbvS-1punoC. 
  11. ↑ 11.0 11.1 11.2 11.3 11.4 Schuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A. et al. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics". The Astrophysical Journal 699 (2): 1423–1432. doi:10.1088/0004-637X/699/2/1423. Bibcode: 2009ApJ...699.1423S. 
  12. ↑ Strecker, D. W. (June 1975). "Variability of R CrB and NML Cyg at 3.5 μ". The Astronomical Journal 80 (6): 451–453. doi:10.1086/111763. Bibcode: 1975AJ.....80..451S. 
  13. ↑ Neugebauer, G.; Martz, D. E.; Leighton, R. B. (July 1965). "Observations of Extremely Cool Stars". Astrophysical Journal 142: 399–401. doi:10.1086/148300. Bibcode: 1965ApJ...142..399N. https://authors.library.caltech.edu/74594/1/1965ApJ___142__399N.pdf. 
  14. ↑ 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. https://books.google.com/books?id=Kp7G4IqK7woC&pg=PA278. Retrieved 23 August 2012. 
  15. ↑ Pesch, P. (1967). "Objective-Prism Spectra of Some Very Red Stars". The Astrophysical Journal 147: 381. doi:10.1086/149015. Bibcode: 1967ApJ...147..381P. 
  16. ↑ 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. 
  17. ↑ 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. 
  18. ↑ 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. doi:10.1093/mnras/225.3.491. PMID 11540900. Bibcode: 1987MNRAS.225..491C. 
  19. ↑ Marvel, Kevin (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. https://books.google.com/books?id=wwx1Gj5wR5QC&pg=PR182. Retrieved 23 August 2012. 
  20. ↑ Humphreys, Roberta M. (2025). "Red Supergiants—The Other Side of the H-R Diagram". Galaxies 13 (2): 25. doi:10.3390/galaxies13020025. Bibcode: 2025Galax..13...25H. 
  21. ↑ 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. doi:10.1086/421700. Bibcode: 2004ApJ...610..427Z. 
  22. ↑ Monnier, J. D; Bester, M; Danchi, W. C; Johnson, M. A; Lipman, E. A; Townes, C. H; Tuthill, P. G; Geballe, T. R et al. (1997). "Nonuniform Dust Outflow Observed around Infrared Object NML Cygni". The Astrophysical Journal 481 (1): 420. doi:10.1086/304050. Bibcode: 1997ApJ...481..420M. 
  23. ↑ 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. et al. (2004). "High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer". The Astrophysical Journal 605 (1): 436–461. doi:10.1086/382218. Bibcode: 2004ApJ...605..436M. 
  24. ↑ 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. doi:10.1086/505025. Bibcode: 2006ApJ...646.1203M. 
  25. ↑ 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. doi:10.3847/1538-4357/ab0e83. Bibcode: 2019ApJ...875..114X. 
  26. ↑ Schuster, Michael T.; Humphreys, Roberta M.; Marengo, Massimo (January 2006). "The Circumstellar Environments of NML Cygni and the Cool Hypergiants" (in en). The Astronomical Journal 131 (1): 603–611. doi:10.1086/498395. ISSN 0004-6256. Bibcode: 2006AJ....131..603S. 
  27. ↑ 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. doi:10.1086/322237. Bibcode: 2001ApJ...555..405D. 



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