WOH G64

WOH G64 (IRAS 04553-6825) is an unusual[4] red supergiant (RSG) star in the Large Magellanic Cloud (LMC) satellite galaxy in the southern constellation of Dorado. It is one of the largest known stars and one of the most luminous and massive red supergiants, with a radius from up to nearly 1,800 to over 2,400 times that of the Sun (R☉) and a luminosity ranging from 300,000 to 600,000 times the solar luminosity (L☉) however, it is more likely to be around 1,530 to 1,730 times the size of the Sun (R☉).

WOH G64
Location of WOH G64 (circled) in the Large Magellanic Cloud
Credit: NASA/JPL-Caltech/M. Meixner (STScI) & the SAGE Legacy Team
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) 18.46 (variable)[2][3]
Characteristics
Evolutionary stage OH/IR red supergiant
Spectral type M5 I[4] – M7.5e[5][6]
Apparent magnitude (K) 6.849[2]
Apparent magnitude (R) 15.69[3]
Apparent magnitude (G) 15.0971[1]
Apparent magnitude (I) 12.795[7]
Apparent magnitude (J) 9.252[2]
Apparent magnitude (H) 7.745[2]
Variable type Carbon-rich LPV (Mira?)[7]
Astrometry
Radial velocity (Rv)294±2[4] km/s
Proper motion (μ) RA: 1.108[1] mas/yr
Dec.: –1.348[1] mas/yr
Parallax (π)−0.2280 ± 0.0625[1] mas
Distance160,000 ly
(50,000[4] pc)
Absolute magnitude (MV)−6.00[4]
Details
Radius1,540–1,730[8] R☉
Luminosity340,000–454,000,[8] 589,000+57,000
−52,000[9] L☉
Surface gravity (log g)+0.0[10]–−0.5[4] cgs
Temperature3,008–3,300[8] K
Age≤5[9] Myr
Other designations
WOH G064, 2MASS J04551048-6820298, IRAS 04553-6825
Database references
SIMBADdata

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.[11] If placed at the center of the Solar System, the star's photosphere would engulf the orbit of Jupiter.

Discovery

WOH G64 was discovered in the 1970s by Bengt Westerlund, Olander and Hedin. Like NML Cygni, the "WOH" in the star's name comes from the names of its three discoverers, but in this case refers to a whole catalogue of giant and supergiant stars in the LMC.[12] Westerlund also discovered another notable red supergiant star, Westerlund 1-26, found in the massive super star cluster Westerlund 1 in the constellation Ara.[13] 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.[11] WOH G64 was also discovered to be a prominent source of OH, H
2O, and SiO masers emission, which is typical of an OH/IR supergiant star.[4] 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.[4]

Distance

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.[4] 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

WOH G64 varies regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days.[3] The star suffers from over six magnitudes of extinction at visual wavelengths, and the variation at infra-red wavelengths is much smaller.[4] 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.[7] 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.[4]

Physical properties
Artist's impression of the dusty torus around WOH G64 (European Southern Observatory)

The spectral type of WOH G64 is given as M5,[4] but it is usually found to have a much cooler spectral type of M7.5, highly unusual for a supergiant star.[9][5][6]

WOH G64 is classified as an extremely luminous M class supergiant and is likely to be the largest star and the most luminous and coolest red supergiant in the LMC.[4] The combination of the star's temperature and luminosity places 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. 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.[8]

The parameters of WOH G64 are uncertain. The star was originally calculated to be around between 490,000 and 600,000 L☉ based on spectroscopic measurements assuming spherical shells, suggesting initial masses at least 40 M☉ and consequently larger values for the radius between 2,575 and 3,000 R☉.[14][5][15] 2007 measurements using the Very Large Telescope (VLT) gave the star a bolometric luminosity of 282,000+40,000
−30,000 L☉, 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 and radiative transfer modelling of the surrounding torus.[11] 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☉.[4] 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.[4]

Those physical parameters are consistent with the largest galactic red supergiants and hypergiants found elsewhere such as Westerlund 1-26, VY Canis Majoris and NML Cygni and with theoretical models of the coolest, most luminous and largest possible cool supergiants (e.g. the Hayashi limit or the Humphreys–Davidson limit).[4][11][5]

A 2018 paper 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 suggests a radius of 1,788 R☉.[10][lower-alpha 1]

Possible companion

WOH G64 has a possible late O-type dwarf companion of a bolometric magnitude of −7.5 or a luminosity of 100,000 L☉, which would make WOH G64 a binary star although there has been no confirmation of this observation and the intervening dust clouds makes the study of the star very difficult.[4]

See also

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

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. 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.
  3. 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.
  4. 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.
  5. 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. Elias, J.H. (March 1986). "Two Supergiants in the Large Magellanic Cloud with Thick Dust Shells". Astrophysical Journal. 302: 675. Bibcode:1986ApJ...302..675E. doi:10.1086/164028. hdl:1887/6514.
  7. 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.
  8. Steven R. Goldman; Jacco Th. van Loon (2016). "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. S2CID 11352637.
  9. 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.
  10. 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.
  11. 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.
  12. Westerlund, B. E.; Olander, N.; Hedin, B. (1981). "Supergiant and giant M type stars in the Large Magellanic Cloud". Astronomy & Astrophysics Supplement Series. 43: 267–295. Bibcode:1981A&AS...43..267W.
  13. Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA". Astronomy and Astrophysics. Supplement. 70 (3): 311–324. Bibcode:1987A&AS...70..311W. ISSN 0365-0138.
  14. Elias, J. H; Frogel, J. A; Schwering, P. B. W (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.
  15. 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.
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