WOH G64

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[2]
Characteristics
Evolutionary stage Red Hypergiant
Spectral type M5 I[3]
Apparent magnitude (K) 6.85[2]
Variable type Mira?[4]
Astrometry
Radial velocity (Rv)294±2[3] 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
Distance163,000 ly
(50,000[3] pc)
Absolute magnitude (MV)−6.00[3]
Details
Mass<25[5] M☉
Radius1,540 ± 77[3], 1,730[5] - 2,575[6] R☉
Luminosity280,000[3][5]-490,000[6] L☉
Surface gravity (log g)–0.5[3] cgs
Temperature3,400 ± 25[3], 3,008[6] - 3,200[5] K
Other designations
WOH G64, 2MASS J04551048-6820298, IRAS 04553-6825
Database references
SIMBADdata

WOH G64, also known as IRAS 04553-6825, is an unusual late-type red hypergiant (RHG) or 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, with a radius of 1,540 to 2,575 solar radii (1.071×109 to 1.791×109 km; 7.16 to 11.97 au), corresponding to a volume some 3.65 to 17 billion times bigger than the Sun. It is also one of the most luminous and massive red supergiants.

WOH G64 is surrounded by an optically thick dust envelope of roughly a light year in diameter containing 3-9 M☉ of expelled material that was created by the strong stellar wind.[5] If placed at the center of the Solar System, the star's surface would engulf Jupiter or possibly Saturn.

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.[7] Westerlund also discovered another notable cool hypergiant star, Westerlund 1-26, found in the massive super star cluster Westerlund 1 in the constellation Ara.[8]

WOH G64 was also discovered to be a well-known source of OH, H
2O, and SiO masers emission, which is typical of an OH/IR supergiant star.[3] It also 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]

Distance

The distance of WOH G64 is assumed to be 50,000 parsecs (160,000 ly) away from Earth, since it appears to be in the LMC.[3] The Gaia Data Release 2 parallax for WOH G64 is –0.2280±0.0625 mas but this value is negative and not meaningful.[1]

Variability

WOH G64 varies regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days.[9] 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.[4] 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]

Characteristics

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

WOH G64 is classified as a luminous M class supergiant and is likely to be the largest star and the most luminous and coolest red supergiant in the LMC.[3] 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.

WOH G64 was originally calculated to be around 500,000 - 600,000 L☉ based on spectroscopic measurements assuming spherical shells, suggesting an initial mass of 40 M☉ and consequently larger values for the radius, such as an estimate of 3,000 R☉.[10][6][11] In 2007 Ohnaka et al. using the Very Large Telescope (VLT) showed that the star is rather surrounded by a torus-shaped cloud and therefore calculated that the star has a bolometric luminosity of 282,000 L☉, suggesting an initial mass of 25 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.[5] 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%.[3] However, other 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.[3]

The physical parameters found by the most recent research are consistent with the largest galactic red supergiants found elsewhere such as VY Canis Majoris, NML Cygni and UY Scuti and with theoretical models of the coolest, most luminous and largest possible cool supergiants (e.g. the Hayashi limit).[3][5][6]

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.[3]

References

  1. 1 2 3 4 5 6 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. 1 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. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 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.
  4. 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.
  5. 1 2 3 4 5 6 7 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. Bibcode:2009IAUS..256..454O. doi:10.1017/S1743921308028858.
  6. 1 2 3 4 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: 273–289. arXiv:astro-ph/0504379. Bibcode:2005A&A...438..273V. doi:10.1051/0004-6361:20042555.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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.
  11. ↑ 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.
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