Westerlund 1

Last updated
Westerlund 1
Surprise Cloud Around Vast Star.jpg

Credit: ESO/VPHAS+ Survey/N. Wright
Observation data (J2000 epoch)
Right ascension 16h 47m 04.0s [1]
Declination −45° 51′ 04.9″ [1]
Distance 3.78+0.56
−0.46 [2] (2.6 [3] – 4.23 [4] ) kpc
Physical characteristics
Mass63,000 [5]   M☉
Radius3.26 ly [5]
Estimated age3.50 Myr [5]
Other designationsAra Cluster, Westerlund 1, ESO 277-12, C 1644-457, VDBH 197
Associations
Constellation Ara
See also: Open cluster, List of open clusters

Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster [6] ) is a compact young super star cluster about 3.8 kpc (12,000 ly) away from Earth. It is thought to be the most massive young star cluster in the Milky Way, [4] and was discovered by Bengt Westerlund in 1961 [7] but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster. [8]

Contents

The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which has been proposed to be the remnant of a recent stellar merger. [9] In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.20-455217, a slow rotating neutron star that must have formed from a high-mass progenitor star. [10] Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions.

Aside from hosting some of the most massive and least-understood stars in our galaxy, Westerlund 1 is useful as a relatively nearby, easy to observe super star cluster that can help astronomers determine what occurs within extragalactic super star clusters.

Observations

Images of Westerlund 1: left is visible light, with all stars appearing red due to interstellar absorption; right is X-ray wavelengths, with the magnetar marked Westerlund1 optical xray.jpg
Images of Westerlund 1: left is visible light, with all stars appearing red due to interstellar absorption; right is X-ray wavelengths, with the magnetar marked

The brightest O7–8V main sequence stars in Wd1 have V-band photometric magnitudes around 20.5, and therefore at visual wavelengths Wd1 is dominated by highly luminous post-Main Sequence stars (V-band magnitudes of 14.5–18, absolute magnitudes −7 to −10), along with less-luminous post-Main Sequence stars of luminosity class Ib and II (V-band magnitudes of 18–20). Due to the extremely high interstellar reddening towards Wd1, it is very difficult to observe in the U- and B-bands, and most observations are made in the R- or I-bands at the red end of the spectrum or in the infrared. Stars in the cluster are generally named using a classification introduced by Westerlund, [11] although a separate naming convention is often used for the Wolf-Rayet stars. [12]

At X-ray wavelengths, Wd1 shows diffuse emission from interstellar gas and point emission from both high-mass, post-Main Sequence and low mass, pre-Main Sequence stars. The Westerlund 1 magnetar is the most luminous X-ray point source in the cluster, with the sgB[e] star W9, the (presumed) binary W30a and the Wolf–Rayet stars WR A and WR B all strong X-ray sources. Approximately 50 other X-ray point sources are associated with luminous optical counterparts. Finally, at radio wavelengths the sgB[e] star W9 and red supergiants W20 and W26 are strong radio sources, while the majority of the cool hypergiants and a few OB supergiants and Wolf–Rayet stars are also detected.

Age and evolutionary state

Artist's impression of the magnetar CXOU J164710.2-455216 in the star cluster Westerlund 1 (ESO/L. Calcada) Artist's impression of the magnetar in the star cluster Westerlund 1.jpg
Artist's impression of the magnetar CXOU J164710.2-455216 in the star cluster Westerlund 1 (ESO/L. Calçada)

The age of Wd1 is estimated at 4–5  Myr from comparison of the population of evolved stars with models of stellar evolution. The presence of significant numbers of both Wolf–Rayet stars and red and yellow supergiants in Wd1 represents a strong constraint on the age: theory suggests that red supergiants will not form until around 4 Myr as the most massive stars do not go through a red supergiant phase, while the Wolf–Rayet population declines sharply after 5 Myr. This range of ages is broadly consistent with infra-red observations of Wd1 that reveal the presence of late-O main sequence stars, although a lower age of around 3.5 Myr has been suggested from observations of lower-mass stars in Wd1. [1]

Comet-like stars in Westerlund 1 Comet-like stars Westerlund 1.tif
Comet-like stars in Westerlund 1

If Wd1 formed stars with a typical initial mass function then the cluster would have originally contained a significant number of very massive stars, such as those currently observed in the younger Arches cluster. Current estimates of the age of Wd1 are greater than the lifetimes of these stars, and stellar evolution models suggest that there would already have been 50–150 supernovae in Wd1, with a supernova rate of approximately one per 10,000 years over the last million years. However, to date only one definitive supernova remnant has been detected—the Westerlund 1 magnetar—and the lack of other compact objects and high-mass X-ray binaries is puzzling. A number of suggestions have been put forward, including high supernova kick velocities that disrupt binary systems, the formation of slowly accreting (and therefore undetectable) stellar mass black holes, or binary systems in which both objects are now compact objects, but the problem has yet to be resolved.

As the stars in Westerlund 1 have the same age, composition and distance, the cluster represents an ideal environment for understanding the evolution of massive stars. The simultaneous presence of stars evolving on to and off of the Main Sequence presents a robust test for stellar evolution models, which are also currently unable to correctly predict the observed distribution of Wolf–Rayet subtypes in Westerlund 1. [14]

Binary fraction

A number of lines of evidence point to a high binary fraction amongst the high-mass stars in Wd1. Some massive binaries are detected directly through photometry [15] and radial velocity [16] observations, while many others are inferred through secondary characteristics (such as high X-ray luminosity, non-thermal radio spectra and excess infra-red emission) that are typical of colliding-wind binaries or dust-forming Wolf–Rayet stars. Overall binary fractions of 70% for the Wolf–Rayet population [12] and in excess of 40% for the OB supergiants are currently estimated, although both may be incomplete. [16]

Members

As well as documented members of the cluster, the luminous blue variable MN44 is thought to be a runaway star ejected from Westerlund 1 four to five million years ago. [17]

Designation Right ascension Declination Object typeSpectral typeLuminosity (L☉)Temperature (K)Radius (R☉)
W2a [9] 16h 46m 59.7s−45° 50′ 51.1″ Blue supergiant O9.5 Ia – B0.5 Ia
W4 [9] 16h 47m 01.42s−45° 50′ 37.1″ Yellow hypergiant G0 Ia+ – F2 Ia+
W5 [18] 16h 47m 02.97s−45° 50′ 19.5″ Blue hypergiant
W6a [9] 16h 47m 04.0s−45° 50′ 21.0″ Blue supergiant
W7 [9] 16h 46m 03.62s−45° 50′ 14.2″ Blue hypergiant
W8a [9] 16h 47m 04.79s−45° 50′ 24.9″ Yellow hypergiant
W8b [18] 16h 47m 04.95s−45° 50′ 26.7″ Blue hypergiant B1-5Ia
W9 [9] 16h 47m 04.14s−45° 50′ 31.1″ B[e] star sgB[e]
W12a [9] 16h 47m 02.21s−45° 50′ 58.8″ Yellow hypergiant
W13 [19] [16] 16h 47m 06.45s−45° 50′ 26.0″ Eclipsing binary B0.5 Ia+ + OB
W16a [9] 16h 47m 06.61s−45° 50′ 42.1″ Yellow hypergiant
W20 [9] 16h 47m 04.70s−45° 51′ 23.8″ Red supergiant 126,000 [20] 3,500 [20] 965 [20]
W26 [9] 16h 47m 05.40s−45° 50′ 36.5″ Red supergiant M2-6Ia [21] 380,000 [22] – 1,100,000 [20] 3,600 [23] – 3,700 [20] 1,530 [22] −2,550 [20] ,1,165–1,221 [24]
W32 [9] 16h 47m 03.67s−45° 50′ 43.5″ Yellow hypergiant
W33 [9] 16h 47m 04.12s−45° 50′ 48.3″ Blue hypergiant
W36 [25] 16h 47m 05.08s−45° 50′ 55.1″ Eclipsing binary O6.5III + O9.5 IV275,000 + 89,00012.69 + 10.46
W75 [26] 16h 47m 08.93s−45° 49′ 58.4″ Red supergiant 68,000 [20] 3,600 [20] 668 [20]
W237 [9] 16h 47m 03.09s−45° 52′ 18.8″ Red supergiant, Possible foreground halo object234,000, [20] 1,000 [27] 3,605 [20] 1,245, [20] 216 [28]
W243 [9] 16h 47m 07.55s−45° 52′ 28.5″ Luminous blue variable LBV
W265 [9] 16h 47m 06.26s−45° 49′ 23.7″ Yellow hypergiant
WR 77a [29] 16h 46m 55.4s−45° 51′ 34″ Wolf–Rayet star WN6–7
WR 77aa [30] 16h 46m 46.3s−45° 47′ 58″ Wolf–Rayet star WC9d
WR 77b [29] 16h 46m 59.9s−45° 55′ 26″ Wolf–Rayet star WC8
WR 77c [29] 16h 47m 00.89s−45° 51′ 20.9″ Wolf–Rayet star WNL
WR 77d, W57c [29] 16h 47m 01.5s−45° 51′ 45″ Wolf–Rayet star WN8
WR 77e [29] 16h 47m 01.67s−45° 51′ 19.9″ Wolf–Rayet star WN6–8
WR 77f, W5 [29] 16h 47m 02.97s−45° 50′ 19.5″ Wolf–Rayet star WNVL
WR 77g [29] 16h 47m 03.1s−45° 50′ 43″ Wolf–Rayet star WC7
WR 77h, W66 [29] 16h 47m 04.0s−45° 51′ 37.5″ Wolf–Rayet star WC9
WR 77i [29] 16h 47m 04.02s−45° 51′ 25.2″ Wolf–Rayet star WN6–8
WR 77j, W44 [29] 16h 47m 04.20s−45° 51′ 07.0″ Wolf–Rayet star WN9
WR 77k [29] 16h 47m 04.1s−45° 51′ 20.0″ Wolf–Rayet star WC9
WR 77l [29] 16h 47m 04.40s−45° 51′ 03.8″ Wolf–Rayet star WC8.5
WR 77m, W239 [29] 16h 47m 05.21s−45° 52′ 25.0″ Wolf–Rayet star WC9
WR 77n [29] 16h 47m 05.35s−45° 51′ 05.0″ Wolf–Rayet star WN8 (uncertain)
WR 77o, W14c [29] 16h 47m 06.0s−45° 15′ 22″ Wolf–Rayet star WN7o [31]
WR 77p, W241 [29] 16h 47m 06.06s−45° 52′ 08.3″ Wolf–Rayet star WC9
WR 77q [29] 16h 47m 06.24s−45° 51′ 26.5″ Wolf–Rayet star WN6–8
WR 77r [29] 16h 47m 07.6s−45° 52′ 36″ Wolf–Rayet star WN6
WR 77s, W72 [29] 16h 47m 08.32s−45° 50′ 45.5″ Wolf–Rayet star WN6o [12]
WR 77sa [12] 16h 47m 07.58s−45° 49′ 22.2″ Wolf–Rayet star WN6h
WR 77sb [12] 16h 47m 07.66s−45° 52′ 35.9″ Wolf–Rayet star WN6o
WR 77sc, W72 [12] 16h 47m 08.32s−45° 50′ 45.5″ Wolf–Rayet star WN7b
WR 77sd [12] 16h 47m 14.1s−45° 48′ 32″ Wolf–Rayet star WN4–5
CXOU J164710.2-455216 [32] 16h 47m 10.18s−45° 52′ 16.7″ anomalous X-ray pulsar

Related Research Articles

<span class="mw-page-title-main">Westerlund 2</span> Open star cluster in the Carina constellation

Westerlund 2 is an obscured compact young star cluster in the Milky Way, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known. The cluster resides inside a stellar breeding ground known as Gum 29, located 20,000 light-years away in the constellation Carina. It is half a degree from the naked eye Cepheid variable V399 Carinae.

<span class="mw-page-title-main">Yellow hypergiant</span> Class of massive star with a spectral type of A to K

A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.

<span class="mw-page-title-main">R136</span> Super star cluster in the constellation Dorado, in the Large Magellanic Cloud

R136 is the central concentration of stars in the NGC 2070 star cluster, which lies at the centre of the Tarantula Nebula in the Large Magellanic Cloud. When originally named it was an unresolved stellar object but is now known to include 72 class O and Wolf–Rayet stars within 5 parsecs of the centre of the cluster. The extreme number and concentration of young massive stars in this part of the LMC qualifies it as a starburst region.

<span class="mw-page-title-main">Arches Cluster</span>

The Arches Cluster is the densest known star cluster in the Milky Way, about 100 light-years from its center in the constellation Sagittarius, 25,000 light-years from Earth. Its discovery was reported by Nagata et al. in 1995, and independently by Cotera et al. in 1996. Due to extremely heavy optical extinction by dust in this region, the cluster is obscured in the visual bands, and is observed in the X-ray, infrared and radio bands. It contains approximately 135 young, very hot stars that are many times larger and more massive than the Sun, plus many thousands of less massive stars.

<span class="mw-page-title-main">WR 136</span> Star in the constellation of Cygnus

WR 136 is a Wolf–Rayet star located in the constellation Cygnus. It is in the center of the Crescent Nebula. Its age is estimated to be around 4.7 million years and it is nearing the end of its life. Within a few hundred thousand years, it is expected to explode as a supernova.

<span class="mw-page-title-main">R136b</span> Star in the constellation Dorado

R136b is a blue supergiant star in the R136 cluster in the Large Magellanic Cloud. It is one of the most massive and most luminous stars known. It is found in the dense R136 open cluster at the centre of NGC 2070 in the Tarantula Nebula.

<span class="mw-page-title-main">Hypergiant</span> Rare star with tremendous luminosity and high rates of mass loss by stellar winds

A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae.

<span class="mw-page-title-main">Cygnus OB2</span> Cluster of massive and luminous stars

Cygnus OB2 is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni. The region is embedded within a wider one of star formation known as Cygnus X, which is one of the most luminous objects in the sky at radio wavelengths. The region is approximately 1,570 parsecs from Earth in the constellation of Cygnus.

<span class="mw-page-title-main">WR 25</span> Binary star system in the constellation Carina

WR 25 is a binary star system in the turbulent star-forming region the Carina Nebula, about 6,800 light-years from Earth. It contains a Wolf-Rayet star and a hot luminous companion and is a member of the Trumpler 16 cluster. The name comes from the Catalogue of Galactic Wolf–Rayet Stars.

<span class="mw-page-title-main">Westerlund 1 W26</span> Star in the constellation Ara

Westerlund 1 W26 or Westerlund 1 BKS AS is a red supergiant located at the outskirts of the Westerlund 1 super star cluster. It is one of the largest known stars and the most luminous supergiant stars discovered so far with radius calculated to be in excess of a thousand times the solar radius, and a luminosity of over 200,000 times the solar luminosity. If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter.

<span class="mw-page-title-main">Theta Muscae</span> Star in the constellation Musca

Theta Muscae is a multiple star system in the southern constellation Musca, containing a Wolf-Rayet star and two massive companions. With an apparent magnitude of 5.5, it is the second-brightest Wolf–Rayet star in the sky, although much of the visual brightness comes from the massive companions and it is not one of the closest of its type.

<span class="mw-page-title-main">WR 20a</span> Star in the constellation Carina

WR 20a is an eclipsing binary star belonging to or recently ejected from the young, massive cluster Westerlund 2. It was discovered in 2004 to be one of the most massive binary systems known, for which the masses of the components have been accurately measured.

<span class="mw-page-title-main">HM 1</span> Open cluster in the constellation Scorpius

HM 1, also known as Havlen-Moffat 1, is an open cluster located in the constellation of Scorpius, close to the galactic plane. It was first observed by R. J. Havlen and A. F. J. Moffat in 1976. HM 1 is thought to be 9,500 to 12,700 light-years away from the Earth, beyond the Carina–Sagittarius Arm. It is heavily reddened by interstellar extinction, so although it comprises mostly blue-colored stars, it appears brighter for longer-wavelength passbands. It is projected against the H II region known as RCW 121, and appears to be the source of ionization for the nearby regions RCW 122 and RCW 123.

<span class="mw-page-title-main">Westerlund 1-237</span> Possible red supergiant in the Westerlund 1 super star cluster

Westerlund 1-237 or Westerlund 1 BKS B is a possible red supergiant (RSG) in the constellation of Ara. It is one out of four known red supergiants in the Westerlund 1 super star cluster, although its outlying position, spectrum, and parallax, suggest it could be a foreground giant. As a red supergiant, it would be one of the largest known stars and one of the most luminous of its type.

<span class="mw-page-title-main">Westerlund 1-20</span> Red supergiant star located in the Westerlund 1 super star cluster

Westerlund 1-20 (abbreviated to Wd 1-20 or just W20) is a red supergiant (RSG) located in the Westerlund 1 super star cluster. Its radius was calculated to be around 965 solar radii (6.72 × 108 km, 4.48 au), making it one of the largest stars discovered so far. This corresponds to a volume 899 million times bigger than the Sun. If placed at the center of the Solar System, the photosphere of Westerlund 1-20 would almost reach the orbit of Jupiter.

<span class="mw-page-title-main">Westerlund 1-75</span> Red supergiant in the Westerlund 1 super star cluster

Westerlund 1 W75 or Wd 1-75 is a red supergiant (RSG) located in the Westerlund 1 super star cluster. Its radius is calculated to be around 668 solar radii (4.65 × 108 km, 3.10 au). This corresponds to a volume 298 million times bigger than the Sun. If placed at the center of the Solar System, Westerlund 1-75 would engulf the inner limits of the asteroid belt.

<span class="mw-page-title-main">Westerlund 1-243</span> Luminous blue variable star undergoing an eruptive phase in Westerlund 1

Westerlund 1-243 or Wd 1-243 is a luminous blue variable (LBV) star undergoing an eruptive phase located within the outskirts of the super star cluster Westerlund 1. Located about 13,400 ly (4,100 pc) from Earth, it has a luminosity of 0.73 million L☉ making it one of the most luminous stars known.

References

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  2. ↑ Parker, Richard J.; Crowther, Paul A.; Rate, Gemma (2020). "Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – II. Cluster and association membership". Monthly Notices of the Royal Astronomical Society. 495: 1209–1226. arXiv: 2005.02533 . doi:10.1093/mnras/staa1290. S2CID   218516882.
  3. ↑ Aghakhanloo, Mojgan; Murphy, Jeremiah W.; Smith, Nathan; Parejko, John; Díaz-Rodríguez, Mariangelly; Drout, Maria R.; Groh, Jose H.; Guzman, Joseph; Stassun, Keivan G. (2020-02-21). "Inferring the parallax of Westerlund 1 from Gaia DR2". Monthly Notices of the Royal Astronomical Society. 492 (2): 2497–2509. arXiv: 1901.06582 . Bibcode:2020MNRAS.492.2497A. doi:10.1093/mnras/stz3628. ISSN   0035-8711. S2CID   119465620.
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  15. ↑ Bonanos, Alceste Z. (2007). "Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1". Astronomical Journal. 133 (6): 2696–2708. arXiv: astro-ph/0702614 . Bibcode:2007AJ....133.2696B. doi:10.1086/518093. S2CID   119074868.
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  17. ↑ Gvaramadze, V. V. (2018). "MN44: A Luminous Blue Variable Running Away from Westerlund 1". Research Notes of the AAS. 2 (4): 214. arXiv: 1811.07899 . Bibcode:2018RNAAS...2..214G. doi: 10.3847/2515-5172/aaf23d . S2CID   119237114.
  18. 1 2 Negueruela, Ignacio; Clark, J. Simon; Ritchie, Ben W. (2010). "The population of OB supergiants in the starburst cluster Westerlund 1". Astronomy and Astrophysics. 516 (78): A78. arXiv: 1003.5204 . Bibcode:2010A&A...516A..78N. doi:10.1051/0004-6361/201014032. S2CID   230718.
  19. ↑ Negueruela, I.; Clark, J. S.; Ritchie, B. W. (2010). "The population of OB supergiants in the starburst cluster Westerlund 1". Astronomy & Astrophysics. 516: 3. arXiv: 1003.5204 . Bibcode:2010A&A...516A..78N. doi:10.1051/0004-6361/201014032. S2CID   230718.
  20. 1 2 3 4 5 6 7 8 9 10 11 12 Fok, Thomas K. T.; Nakashima, Jun-Ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv: 1209.6427 . Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65. S2CID   53393926.
  21. ↑ Clark, J. S.; Ritchie, B. W.; Negueruela, I.; Crowther, P. A.; Damineli, A.; Jablonski, F. J.; Langer, N. (2011). "A VLT/FLAMES survey for massive binaries in Westerlund 1" (PDF). Astronomy & Astrophysics. 531: A28. arXiv: 1105.0776 . Bibcode:2011A&A...531A..28C. doi:10.1051/0004-6361/201116990. S2CID   119299122.
  22. 1 2 Wright, N. J.; Wesson, R.; Drew, J. E.; Barentsen, G.; Barlow, M. J.; Walsh, J. R.; Zijlstra, A.; Drake, J. J.; Eisloffel, J.; Farnhill, H. J. (16 October 2013). "The ionized nebula surrounding the red supergiant W26 in Westerlund 1". Monthly Notices of the Royal Astronomical Society: Letters. 437 (1): L1–L5. arXiv: 1309.4086 . Bibcode:2014MNRAS.437L...1W. doi:10.1093/mnrasl/slt127. S2CID   14889377.
  23. ↑ MacKey, Jonathan; Castro, Norberto; Fossati, Luca; Langer, Norbert (2015). "Cold gas in hot star clusters: The wind from the red supergiant W26 in Westerlund 1". Astronomy & Astrophysics. 582: A24. arXiv: 1508.07003 . Bibcode:2015A&A...582A..24M. doi:10.1051/0004-6361/201526159. S2CID   54683876.
  24. ↑ Arévalo, Aura (2019). The Red Supergiants in the Supermassive Stellar Cluster Westerlund 1 (Thesis). doi: 10.11606/D.14.2019.tde-12092018-161841 .
  25. ↑ Rocha, Danilo F.; Almeida, Leonardo A.; Damineli, Augusto; Navarete, Felipe; Abdul-Masih, Michael; Mace, Gregory N. (2022). "Distance and age of the massive stellar cluster Westerlund 1 – II. The eclipsing binary W36". Monthly Notices of the Royal Astronomical Society. 517 (3): 3749–3762. arXiv: 2210.04985 . Bibcode:2022MNRAS.517.3749R. doi:10.1093/mnras/stac2927.
  26. ↑ Clark, J. S; Ritchie, B. W; Negueruela, I (2010). "A serendipitous survey for variability amongst the massive stellar population of Westerlund 1". Astronomy and Astrophysics. 514: A87. arXiv: 1003.5107 . Bibcode:2010A&A...514A..87C. doi:10.1051/0004-6361/200913820. S2CID   14780809.
  27. ↑ Fok, Thomas K. T.; Nakashima, Jun-Ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv: 1209.6427 . Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65. S2CID   53393926.
  28. ↑ Messineo, M.; Brown, A. G. A. (2019). "A Catalog of Known Galactic K-M Stars of Class I Candidate Red Supergiants in Gaia DR2". The Astronomical Journal. 158 (1): 20. arXiv: 1905.03744 . Bibcode:2019AJ....158...20M. doi: 10.3847/1538-3881/ab1cbd . S2CID   148571616.
  29. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Negueruela, I.; Clark, J. S. (2005). "Further Wolf–Rayet stars in the starburst cluster Westerlund 1". Astronomy & Astrophysics. 436 (2): 541. arXiv: astro-ph/0503303 . Bibcode:2005A&A...436..541N. doi:10.1051/0004-6361:20052699. S2CID   1755956.
  30. ↑ Crowther, Paul A.; Hadfield, L. J.; Clark, J. S.; Negueruela, I.; Vacca, W. D. (2006-09-29). "A census of the Wolf–Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy". Monthly Notices of the Royal Astronomical Society. 372 (3): 1411. arXiv: astro-ph/0608356 . Bibcode:2006MNRAS.372.1407C. doi:10.1111/j.1365-2966.2006.10952.x. S2CID   10505573.
  31. ↑ Koumpia, E.; Bonanos, A. Z. (2012). "The Fundamental Parameters of Four Massive Eclipsing Binaries in Westerlund 1". Astronomy & Astrophysics. 547: A30. arXiv: 1205.1369 . Bibcode:2012A&A...547A..30K. doi:10.1051/0004-6361/201219465. S2CID   118604150.
  32. ↑ SIMBAD, CXOU J164710.2-455216
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