WOH G64

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WOH G64
WOH G64 VLTI.jpg
VLTI image of the dusty torus around the star.
Credit: ESO
Observation data
Epoch J2000.0        Equinox J2000.0
Constellation Dorado
Right ascension 04h 55m 10.5252s [1]
Declination −68° 20′ 29.998″ [1]
Apparent magnitude  (V)17.7 - 18.8 [2]
Characteristics
A
Evolutionary stage OH/IR [3] red supergiant [4]
Spectral type M5 I [3] – M7.5e [5] [6]
Apparent magnitude  (K)6.849 [7]
Apparent magnitude  (R)15.69 [8]
Apparent magnitude  (G)15.0971 [1]
Apparent magnitude  (I)12.795 [9]
Apparent magnitude  (J)9.252 [7]
Apparent magnitude  (H)7.745 [7]
Variable type symbiotic [10] [4]
B
Spectral type B [10]
Astrometry
Radial velocity (Rv)+285±2 [10] km/s
Proper motion (μ)RA: +1.689 mas/yr [11]
Dec.: −0.013 mas/yr [11]
Distance 160,000  ly
(50,000 [3]   pc)
Absolute magnitude  (MV)−6.00 [3]
Details
A
Mass 25±5 (initial mass) [3]   M☉
Radius 1,540±77 [3] [12] [13]   R☉
Luminosity 282,000+34,400
−30,700 [3]   L☉
Surface gravity (log g)−0.5 [3]   cgs
Temperature 3,400±25 [3]   K
Age ≤5 [14]   Myr
Other designations
WOH G064, 2MASS  J04551048−6820298, IRAS  04553−6825, MSX LMC 1182
Database references
SIMBAD data

WOH G64 (IRAS 04553-6825) is a symbiotic binary in the Large Magellanic Cloud (LMC), roughly 160,000 light-years from Earth. The primary component is the largest known star with a well-defined radius. [3] [15] It is also one of the most luminous and massive red supergiants, with a radius calculated to be around 1,540 times that of the Sun (R☉) and a luminosity around 282,000 times the solar luminosity (L☉). If placed at the center of the Solar System, the star's photosphere would engulf the orbit of Jupiter.

Contents

WOH G64 is surrounded by an optically thick dust envelope of roughly a light year in diameter, containing 3 to 9 times the Sun's mass of expelled material that was created by the strong stellar wind of the red supergiant primary. [16]

Observational history

WOH G64 was discovered in the 1970s by Bengt Westerlund, N. Olander and B. Hedin. Like NML Cygni, the "WOH" in the star's name comes from the last names of its three discoverers, but in this case refers to a whole catalogue of giant and supergiant stars in the LMC. [17] Westerlund also discovered another notable red supergiant star, Westerlund 1 W26, found in the massive super star cluster Westerlund 1 in the constellation Ara. [18] In 1986, infrared observations showed that it was a highly luminous supergiant surrounded by gas and dust which absorbed around three quarters of its radiation. [6]

In 2007, observers using the Very Large Telescope (VLT) showed that WOH G64 is surrounded by a torus-shaped cloud. [16] In 2024, the dusty torus around WOH G64 was directly imaged by VLTI, showing the elongated and compact emission around the hypergiant. This is also the first interferometric imaging of a star outside the Milky Way. [19]

Physical properties

Artist's impression of the dusty torus and elliptical cocoon of dust surrounding WOH G64 (European Southern Observatory) Artist's impression of the dying star WOH G64 (eso2417c).jpg
Artist's impression of the dusty torus and elliptical cocoon of dust surrounding WOH G64 (European Southern Observatory)

The spectral type of WOH G64 A is given as M5, [3] but it is usually found to have a much cooler spectral type of M7.5, highly unusual for a supergiant star. [14] [5] [6]

Based on spectroscopic measurements assuming spherical shells, the star was originally calculated to have luminosity around between 490,000 and 600,000 L☉, suggesting an initial mass of at least 40 M☉ and consequently larger values for the radius between 2,575 and 3,000 R☉. [6] [5] [20] One measurement from 2018 gives a luminosity of 432,000 L☉ and a higher effective temperature of 3,500  K , based on optical and infrared photometry and assuming spherically-symmetric radiation from the surrounding dust. This would suggest a radius of 1,788 R☉. [21] [a]

The dust surrounding WOH G64 A was revealed in 2007 to have a torus-like shape which was being viewed pole-on, meaning that the previous radius and luminosity estimates which assumed spherical dust shells were overestimated, as the radiation escape through the cavity (i.e. toward us). A much lower luminosity of 282,000+40,000
−30,000  L☉ was derived based on radiative transfer modelling of the surrounding torus, suggesting an initial mass of 25±5  M☉ and a radius around 1,730 R☉ for an effective temperature of 3,200  K . [16] In 2009, Emily Levesque calculated an effective temperature of 3,400±25 K by spectral fitting of the optical and near-UV SED. Adopting the Ohnaka luminosity with this new temperature gives a radius of 1,540±77  R☉ . [3] Those physical parameters are consistent with the largest galactic red supergiants and hypergiants found elsewhere such as VY Canis Majoris and with theoretical models of the coolest, most luminous and largest possible cool supergiants (e.g. the Hayashi limit or the Humphreys–Davidson limit). [3] [16] [5]

WOH G64 A is possibly the largest known star and the most luminous and coolest red supergiant in the LMC. [3] The combination of the star's temperature and luminosity placed it toward the upper right corner of the Hertzsprung–Russell diagram. It has an average mass loss rate of 3.1 to 5.8×10−4 M☉ per year, among the highest known and unusually high even for a red supergiant. [22] [23]

WOH G64 A was discovered to be a prominent source of OH, H
2O , and SiO masers emission, which is typical of an OH/IR supergiant star. [3] It shows an unusual spectrum of nebular emission; the hot gas is rich in nitrogen and has a radial velocity considerably more positive than that of the star. [3] The stellar atmosphere is producing a strong silicate absorption band in mid-infrared wavelengths, accompanied a line emission due to highly excited carbon monoxide. [24]

Variability

WOH G64 A used to vary regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days. [8] The star suffers from over six magnitudes of extinction at visual wavelengths, and the variation at infra-red wavelengths is much smaller. [3] It has been described as a carbon-rich Mira or long-period variable, which would necessarily be an asymptotic-giant-branch star (AGB star) rather than a supergiant. [9] Brightness variability has been confirmed by other researchers in some spectral bands, but it is unclear what the actual variable type is. No significant spectral variation has been found. [3] The variability has since been observed to transition from semi-regular to irregular circa 2014. [10]

Supposed yellow supergiant transition

Photometric measurements spanning from 30 years showed a transition from semi-regular to irregular variability during around 2014, which together with the absence or strength of certain spectral lines in spectroscopic observations that would be inconsistent with a red supergiant, led to Munoz-Sanchez et al. (2024) to conclude that WOH G64 A transitioned from a red supergiant to a yellow supergiant. The lack of a violent outburst and smooth transition would be explained by the presence of a B-type companion forming a symbiotic binary. As a red supergiant, WOH G64 A would be half of its original size, at 800 R☉, and have a hotter effective temperature of 4,700  K. [10]

This interpretation was subsequently challenged by van Loon & Ohnaka (2026), which detected molecular absorption bands of titanium oxide in spectroscopic data taken between 2024 and 2025, implying that the central component is still a red supergiant. The anomalies observed by Munoz-Sanchez et al. (2024) were explained instead by the periastron passage of the companion: Its tidal forces stretched the outer layers of the primary's atmosphere, causing the atmospheric layer of optical depth 1 to be an inner layer that has a hotter temperature, instead of a cooler layer as it used to be. This, in turn, changes the nature of the variability. Since then, the star has returned to its original state. [4]

Companion

WOH G64 was historically considered to be a solitary red supergiant star, with no known companions. However, since 2016, its spectrum exhibits features of B[e] stars, which is interpreted as the spectral signature of a massive symbiotic binary consisting of a supergiant losing material to an accreting B-type star companion. [10] [4] The persistent presence of surrounding hot dust, elongated emission in interferometric imaging, [10] [19] and the decline in optical brightness in the 2010s further supports the binary nature of WOH G64. [4] The orbital period of such a companion is no less than a century. [4] The presence of a hot stellar companion of WOH G64 was first suspected by Levesque et al. in 2009, who proposed that a late O-type main-sequence star companion could be ionizing the nebula surrounding WOH G64 in order to explain the 50 km/s shift between the nebular emission lines and WOH G64's spectral features. [10] [3]

See also

Notes

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

References

  1. 1 2 3 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. ↑ Bhardwaj, Anupam; Kanbur, Shashi; He, Shiyuan; Rejkuba, Marina; Matsunaga, Noriyuki; De Grijs, Richard; Sharma, Kaushal; Singh, Harinder P.; Baug, Tapas; Ngeow, Chow-Choong; Ou, Jia-Yu (2019). "Multiwavelength Period-Luminosity and Period-Luminosity-Color Relations at Maximum Light for Mira Variables in the Magellanic Clouds". The Astrophysical Journal. 884 (1): 20. arXiv: 1908.01795 . Bibcode:2019ApJ...884...20B. doi: 10.3847/1538-4357/ab38c2 . S2CID   199452754.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Levesque, E. M.; Massey, P.; Plez, B.; Olsen, K. A. G. (2009). "The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?". The Astronomical Journal. 137 (6): 4744. arXiv: 0903.2260 . Bibcode:2009AJ....137.4744L. doi:10.1088/0004-6256/137/6/4744. S2CID   18074349.
  4. 1 2 3 4 5 6 van Loon, Jacco Th; Ohnaka, Keiichi (2026-01-05). "A phoenix rises from the ashes: WOH G64 is still a red supergiant, for now". Monthly Notices of the Royal Astronomical Society. arXiv: 2601.02057 .
  5. 1 2 3 4 Van Loon, J. Th.; Cioni, M.-R. L.; Zijlstra, A. A.; Loup, C. (2005). "An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars". Astronomy and Astrophysics . 438 (1): 273–289. arXiv: astro-ph/0504379 . Bibcode:2005A&A...438..273V. doi:10.1051/0004-6361:20042555. S2CID   16724272.
  6. 1 2 3 4 Elias, J. H.; Frogel, J. A.; Schwering, P. B. W. (March 1986). "Two supergiants in the Large Magellanic Cloud with thick dust shells". The Astrophysical Journal . 302: 675. Bibcode:1986ApJ...302..675E. doi:10.1086/164028. hdl: 1887/6514 . ISSN   0004-637X.
  7. 1 2 3 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.
  8. 1 2 Fraser, Oliver J.; Hawley, Suzanne L.; Cook, Kem H. (2008). "The Properties of Long-Period Variables in the Large Magellanic Cloud from MACHO". The Astronomical Journal. 136 (3): 1242–1258. arXiv: 0808.1737 . Bibcode:2008AJ....136.1242F. doi:10.1088/0004-6256/136/3/1242. S2CID   2754884.
  9. 1 2 Soszyñski, I.; Udalski, A.; Szymañski, M. K.; Kubiak, M.; Pietrzyñski, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (2009). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud". Acta Astronomica. 59 (3): 239. arXiv: 0910.1354 . Bibcode:2009AcA....59..239S.
  10. 1 2 3 4 5 6 7 8 Munoz-Sanchez, G.; et al. (28 November 2024). "The dramatic transition of the extreme Red Supergiant WOH G64 to a Yellow Hypergiant". arXiv: 2411.19329 [astro-ph.SR].
  11. 1 2 Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv: 2208.00211 . Bibcode:2023A&A...674A...1G. doi: 10.1051/0004-6361/202243940 . S2CID   244398875. Gaia DR3 record for this source at VizieR.
  12. ↑ Levesque, E. M. (June 2010). The Physical Properties of Red Supergiants. Hot and Cool: Bridging Gaps in Massive Star Evolution ASP Conference Series. Vol. 425. p. 103. arXiv: 0911.4720 . Bibcode:2010ASPC..425..103L. S2CID   8921166.
  13. ↑ Beasor, Emma R.; Smith, Nathan (2022-05-01). "The Extreme Scarcity of Dust-enshrouded Red Supergiants: Consequences for Producing Stripped Stars via Winds". The Astrophysical Journal. 933 (1): 41. arXiv: 2205.02207 . Bibcode:2022ApJ...933...41B. doi: 10.3847/1538-4357/ac6dcf . S2CID   248512934.
  14. 1 2 Davies, Ben; Crowther, Paul A.; Beasor, Emma R. (2018). "The luminosities of cool supergiants in the Magellanic Clouds, and the Humphreys–Davidson limit revisited". Monthly Notices of the Royal Astronomical Society. 478 (3): 3138–3148. arXiv: 1804.06417 . Bibcode:2018MNRAS.478.3138D. doi: 10.1093/mnras/sty1302 . S2CID   59459492.
  15. ↑ Jones, Olivia; Woods, Paul; Kemper, Franziska; Kraemer, Elena; Sloan, G.; Srinivasan, Sivakrishnan; Oliveira, Joana; van Loon, Jacco; Boyer, Martha; Sargent, Benjamin; Mc Donald, I.; Meixner, Margaret; Zijlstra, A.; Ruffel, Paul; Lagadec, Eric; Pauly, Tyler (May 7, 2017). "The SAGE-Spec Spitzer Legacy program: the life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification – III". Monthly Notices of the Royal Astronomical Society. 470 (3): 3250–3282. arXiv: 1705.02709 . doi: 10.1093/mnras/stx1101 . Retrieved 23 June 2022.
  16. 1 2 3 4 Ohnaka, K.; Driebe, T.; Hofmann, K. H.; Weigelt, G.; Wittkowski, M. (July 2008). "Resolving the dusty torus and the mystery surrounding LMC red supergiant WOH G64". Proceedings of the International Astronomical Union. 4: 454–458. Bibcode:2009IAUS..256..454O. doi: 10.1017/S1743921308028858 .
  17. ↑ Westerlund, B. E.; Olander, N.; Hedin, B. (1981). "Supergiant and giant M type stars in the Large Magellanic Cloud". Astronomy and Astrophysics Supplement Series . 43: 267–295. Bibcode:1981A&AS...43..267W. ISSN   0365-0138.
  18. ↑ Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA". Astronomy & Astrophysics . Supplement. 70 (3): 311–324. Bibcode:1987A&AS...70..311W. ISSN   0365-0138.
  19. 1 2 Ohnaka, K.; Hofmann, K.-H.; Weigelt, G.; van Loon, J. Th.; Schertl, D.; Goldman, S. R. (November 2024). "Imaging the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud". Astronomy & Astrophysics. 691: L15. arXiv: 2412.01921 . Bibcode:2024A&A...691L..15O. doi: 10.1051/0004-6361/202451820 . ISSN   0004-6361.
  20. ↑ Monnier, J. D; Millan-Gabet, R; Tuthill, P. G; Traub, W. A; Carleton, N. P; Coudé 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. ↑ Groenewegen, Martin A. T.; Sloan, Greg C. (2018). "Luminosities and mass-loss rates of Local Group AGB stars and Red Supergiants". Astronomy & Astrophysics . 609: A114. arXiv: 1711.07803 . Bibcode:2018A&A...609A.114G. doi:10.1051/0004-6361/201731089. ISSN   0004-6361. S2CID   59327105.
  22. ↑ Goldman, Steven R.; van Loon, Jacco Th.; Zijlstra, Albert A.; et al. (February 2017). "The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity". Monthly Notices of the Royal Astronomical Society . 465 (1): 403–433. arXiv: 1610.05761 . Bibcode:2017MNRAS.465..403G. doi: 10.1093/mnras/stw2708 . ISSN   0035-8711. S2CID   11352637.
  23. ↑ de Wit, S.; Bonanos, A.Z.; Tramper, F.; Yang, M.; Maravelias, G.; Boutsia, K.; Britavskiy, N.; Zapartas, E. (2023). "Properties of luminous red supergiant stars in the Magellanic Clouds". Astronomy and Astrophysics . 669: 17. arXiv: 2209.11239 . Bibcode:2023A&A...669A..86D. doi:10.1051/0004-6361/202243394. S2CID   252519285.
  24. ↑ Matsuura, Mikako; Sargent, B.; Swinyard, Bruce; Yates, Jeremy; Royer, P.; Barlow, M. J.; Boyer, Martha; Decin, L.; Khouri, Theo; Meixner, Margaret; van Loon, Jacco Th.; Woods, Paul M. (1 November 2016). "The mass-loss rates of red supergiants at low metallicity: detection of rotational CO emission from two red supergiants in the Large Magellanic Cloud". Monthly Notices of the Royal Astronomical Society. 462 (3): 2995–3005. arXiv: 1608.01729 . doi: 10.1093/mnras/stw1853 . ISSN   0035-8711.
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