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WOH G64

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WOH G64

VLTI image of the dusty torus around the star.
Credit: ESO
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Dorado (LMC)
Right ascension 04h 55m 10.5252s[1]
Declination −68° 20′ 29.998″[1]
Apparent magnitude (V) 17.7 - 18.8[2]
Characteristics
Evolutionary stage Yellow hypergiant[3]
Spectral type K3 (G–K)[3]
Apparent magnitude (K) 6.849[4]
Apparent magnitude (R) 15.69[5]
Apparent magnitude (G) 15.0971[1]
Apparent magnitude (I) 12.795[6]
Apparent magnitude (J) 9.252[4]
Apparent magnitude (H) 7.745[4]
Variable type Carbon-rich LPV (Mira?)[6]
Astrometry
Radial velocity (Rv)294±2[7] km/s
Proper motion (μ) RA: 1.108[1] mas/yr
Dec.: −1.348[1] mas/yr
Parallax (π)−0.2280 ± 0.0625 mas[1]
Distance160,000 ly
(50,000[7] pc)
Absolute magnitude (MV)−6.00[7]
Details
Mass28 (initial mass)[3] M☉
Radius~800[3] R☉
Surface gravity (log g)0.0[3] cgs
Temperature4,700[3] K
Age≤5[8] Myr
Other designations
WOH G064, 2MASS J04551048-6820298, IRAS 04553-6825, MSX LMC 1182
Database references
SIMBADdata

WOH G64 (IRAS 04553-6825) is an unusual[7][3] yellow hypergiant star in the Large Magellanic Cloud (LMC) satellite galaxy in the southern constellation of Dorado.

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.[9] It was formerly considered to be the largest known star with a well-defined radius[7][10] until it rapidly transitioned into a yellow hypergiant after a possible 30 year outburst, reducing its radius to ~800 R☉ and its luminosity by 34%.[3]

Observational history

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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.[11] Westerlund also discovered another notable red supergiant star, Westerlund 1-26, found in the massive super star cluster Westerlund 1 in the constellation Ara.[12] 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.[13]

In 2007, observers using the Very Large Telescope (VLT) showed that WOH G64 is surrounded by a torus-shaped cloud.[9]

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.[14]

Distance

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The distance of WOH G64 is assumed to be around 50,000 parsecs (160,000 ly) away from Earth, since it appears to be in the LMC.[7] The Gaia Data Release 2 parallax for WOH G64 is −0.2280±0.0625 mas and the negative parallax does not provide a reliable distance.[1]

Variability

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WOH G64 varies regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days.[5] The star suffers from over six magnitudes of extinction at visual wavelengths, and the variation at infra-red wavelengths is much smaller.[7] 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.[6] 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.[7]

Physical properties

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Red supergiant stage

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

The spectral type of WOH G64 in its red supergiant stage was given as M5,[7] but it is usually found to have a much cooler spectral type of M7.5, highly unusual for a supergiant star.[8][15][13] Later observations showed that while the star used to be a M5–7.5 red supergiant, with a temperature of between 3200 K and 3400 K, it rapidly evolved, reaching a temperature of 4700 K and becoming a yellow hypergiant.[9][7][3]

WOH G64 was classified as an extremely luminous M class supergiant and was likely to be the largest star and the most luminous and coolest red supergiant in the LMC.[7] The combination of the star's temperature and luminosity placed it toward the upper right corner of the Hertzsprung–Russell diagram. The star's evolved state means that it can no longer hold on to its atmosphere due to low density, high radiation pressure, and the relatively opaque products of thermonuclear fusion.[citation needed] It had 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.[16][17]

The parameters of WOH G64 are uncertain. Based on spectroscopic measurements assuming spherical shells, the star was originally calculated to have luminosity around between 490,000 and 600,000 L☉, suggesting initial masses at least 40 M☉ and consequently larger values for the radius between 2,575 and 3,000 R☉.[13][15][18] One such of these measurements 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☉.[19][a]

WOH G64 compared to the sun.

2007 measurements using the Very Large Telescope (VLT) gave the star a bolometric luminosity of 282,000+40,000
−30,000 L☉ based on radiative transfer modelling of the surrounding torus, suggesting an initial mass of 25±5 M☉ and a radius around 1,730 R☉ based on the assumption of an effective temperature of 3,200 K.[9] In 2009, 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 R☉ but with a margin of error of 5% or 77 R☉.[7] 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).[7][9][15] Ignoring the effect of the dusty torus in redirecting infrared radiation, estimates of 1,970 - 1,990 R☉ based on a luminosity of 450,000+150,000
−120,000 L☉ and an effective temperature of 3,372 - 3,400 K have also been derived.[7]

WOH G64 was discovered to be a prominent source of OH, H
2O, and SiO masers emission, which is typical of an OH/IR supergiant star.[7] 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.[7] The stellar atmosphere is producing a strong silicate absorption band in mid-infrared wavelengths, accompanied a line emission due to highly excited carbon monoxide.[20]

Companion

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Since 2016, the spectrum of WOH G64 exhibits features of both B[e] stars and yellow stars, which is interpreted as the spectral signature of a massive symbiotic binary consisting of a yellow hypergiant losing material to an accreting B-type star companion.[3] The persistent presence of surrounding hot dust, elongated appearance of the hypergiant in interferometric imaging, and the lack of a violent outburst during WOH G64's transition out of the red supergiant stage further supports the binary nature of WOH G64.[3][14] The interacting binary system HR 5171 is considered an analog to WOH G64, as it also contains a yellow hypergiant with a B-type star companion.[3] 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.[3][7]

See also

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  • B90, another red supergiant in the Large Magellanic Cloud
  • VY Canis Majoris, considered to be possibly the largest star in the Milky Way
  • NML Cygni
  • R136a1, one of the most massive and luminous stars known
  • IRC +10420, a yellow hypergiant evolving bluewards

Notes

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

References

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  1. ^ a b c d e f g 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. ^ a b c d e f g h i j k l m 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.
  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 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.
  6. ^ a b c 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.
  7. ^ a b c d e f g h i j k l m n o p q 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.
  8. ^ a b 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.
  9. ^ a b c d e Ohnaka, K.; Driebe, T.; Hofmann, K. H.; Weigelt, G.; Wittkowski, M. (2009). "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.
  10. ^ 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.
  11. ^ 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.
  12. ^ 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.
  13. ^ a b c 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.
  14. ^ a b 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. doi:10.1051/0004-6361/202451820. ISSN 0004-6361.
  15. ^ a b c 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.
  16. ^ 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.
  17. ^ 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.
  18. ^ 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.
  19. ^ 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.
  20. ^ 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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