| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| 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]yellow hypergiant[4] |
| Apparent magnitude (K) | 6.849[5] |
| Apparent magnitude (R) | 15.69[6] |
| Apparent magnitude (G) | 15.0971[1] |
| Apparent magnitude (I) | 12.795[7] |
| Apparent magnitude (J) | 9.252[5] |
| Apparent magnitude (H) | 7.745[5] |
| Variable type | Slow irregular variable +symbiotic[4] |
| B | |
| Spectral type | B[4] |
| Astrometry | |
| Radial velocity (Rv) | +285±2[4] km/s |
| Proper motion (μ) | RA: +1.689mas/yr[8] Dec.: −0.013mas/yr[8] |
| Distance | 160,000 ly (50,000[3] pc) |
| Absolute magnitude (MV) | −6.00[3] |
| Details | |
| A | |
| Mass | 28 (initial mass)[4] M☉ |
| Radius | ~800[4] R☉ |
| Surface gravity (log g) | 0.0[4] cgs |
| Temperature | 4,700[4] K |
| Age | ≤5[9] Myr |
| Other designations | |
| WOH G064,2MASS J04551048−6820298,IRAS 04553−6825, MSX LMC 1182 | |
| Database references | |
| SIMBAD | data |
WOH G64 (IRAS 04553−6825) is asymbiotic binary in theLarge Magellanic Cloud (LMC), roughly 160,000light-years fromEarth. The main component of this system was once recognized as the best candidate for thelargest known star when it was ared supergiant,[3] until it gradually became ayellow hypergiant with half of its original size. The secondary is aB-type star. This system also exhibits features ofB(e) stars.[4]
WOH G64 is surrounded by an optically thick dust envelope of roughly a light year in diameter, containing 3 to 9 times theSun's mass of expelled material that was created by the strongstellar wind.[10]
WOH G64 was discovered in the 1970s byBengt Westerlund, N. Olander and B. Hedin. LikeNML 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 massivesuper star clusterWesterlund 1 in the constellationAra.[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 theVery Large Telescope (VLT) showed that WOH G64 is surrounded by a torus-shaped cloud.[10] 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]
As a red supergiant, WOH G64 A varies regularly in brightness by over a magnitude at visual wavelengths with a primary period of around 800 days.[6] 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-richMira orlong-period variable, which would necessarily be anasymptotic-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.[3] It is now classified as anirregular variable.[4]
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The spectral type of WOH G64 A in its red supergiant stage was 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.[9][15][13]
WOH G64 was likely to be the largest star and the most luminous andcoolest red supergiant in the LMC.[3] The combination of the star's temperature and luminosity placed it toward the upper right corner of theHertzsprung–Russell diagram. It had an average mass loss rate of 3.1 to5.8×10−4 M☉ per year, among the highest known and unusually high even for a red supergiant.[16][17]
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 of3,500 K, based on optical and infraredphotometry and assuming spherically-symmetric radiation from the surrounding dust. This would suggest a radius of 1,788 R☉.[19][a]
The dust surrounding WOH G64 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 of282,000+40,000
−30,000 L☉ was derived based on radiative transfer modelling of the surrounding torus, suggesting an initial mass of25±5 M☉ and a radius around 1,730 R☉ for aneffective temperature of3,200 K.[10] In 2009,Emily Levesque calculated an effective temperature of3,400±25 K by spectral fitting of the optical and near-UVSED. Adopting the Ohnaka luminosity with this new temperature gives a radius of1,540±77 R☉.[3] Those physical parameters are consistent with the largest galactic red supergiants and hypergiants found elsewhere such asVY Canis Majoris and with theoretical models of the coolest, most luminous and largest possible cool supergiants (e.g. theHayashi limit or theHumphreys–Davidson limit).[3][10][15]
WOH G64 was discovered to be a prominent source ofOH,H
2O, andSiOmasers emission, which is typical of anOH/IR supergiant star.[3] It shows an unusualspectrum of nebular emission; the hot gas is rich in nitrogen and has aradial velocity considerably more positive than that of the star.[3] The stellar atmosphere is producing a strong silicateabsorption band in mid-infrared wavelengths, accompanied a line emission due to highly excitedcarbon monoxide.[20]
WOH G64 has shrunk in size since 2014, and has become a smalleryellow hypergiant. It is now about half of its size in the red supergiant phase, at 800 R☉. Thevariability of WOH G64 also changed, fromsemiregular toirregular. Its change would only be recognized in a 2024preprint, which also discovered WOH G64 is asymbiotic star with a smallerB-type companion.[4]
Since 2016, the spectrum of WOH G64 exhibits features of bothB[e] stars and yellow stars, which is interpreted as the spectral signature of a massivesymbiotic binary consisting of a yellow hypergiant losing material to anaccreting B-type star companion.[4] 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.[4][14] The interacting binary systemHR 5171 is considered an analog to WOH G64, as it also contains a yellow hypergiant with a B-type star companion.[4] The presence of a hot stellar companion of WOH G64 was first suspected by Levesque et al. in 2009, who proposed that a lateO-type main-sequence star companion could be ionizing the nebula surrounding WOH G64 in order to explain the50 km/s shift between the nebular emission lines and WOH G64's spectral features.[4][3]
