Westerlund 1

Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster[5]) is a compact young super star cluster in the Milky Way galaxy, about 2.6 kpc away from Earth. It is one of the most massive young star clusters in the Milky Way,[4] and was discovered by Bengt Westerlund in 1961[6] 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.[7]

Westerlund 1
Credit: ESO/VPHAS+ Survey/N. Wright
Observation data (J2000 epoch)
Constellation	Ara
Right ascension	16^h 47^m 04.0^s[1]
Declination	−45° 51′ 04.9″[1]
Distance	3.78+0.56 −0.46[2] (2.6[3] – 3.87[2]) kpc
Physical characteristics
Mass	63,000[4] M☉
Radius	3.26 ly[4]
Estimated age	3.50 Myr[4]
Other designations	Ara Cluster, Westerlund 1, ESO 277-12, C 1644-457, VDBH 197

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.[8] 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.[9] 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

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 infra red. Stars in the cluster are generally named using a classification introduced by Westerlund,[10] although a separate naming convention is often used for the Wolf-Rayet stars.[11]

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. 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.[12]

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.[13]

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[14] and radial velocity[15] 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[11] and in excess of 40% for the OB supergiants are currently estimated, although both may be incomplete.[15]

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.[16]

Designation	Right ascension	Declination	Object type	Spectral type	Luminosity (L☉)	Temperature (K)	Radius (R☉)
W2a[8]	16^h 46^m 59.7^s	−45° 50′ 51.1″	Blue supergiant	O9.5 Ia - B0.5 Ia
W4[8]	16^h 47^m 01.42^s	−45° 50′ 37.1″	Yellow hypergiant	G0 Ia^+ - F2 Ia^+
W6a[8]	16^h 47^m 04.0^s	−45° 50′ 21.0″	Blue supergiant
W7[8]	16^h 46^m 03.62^s	−45° 50′ 14.2″	Blue hypergiant
W8a[8]	16^h 47^m 04.79^s	−45° 50′ 24.9″	Yellow hypergiant
W8b[17]	16^h 47^m 04.95^s	−45° 50′ 26.7″	Blue supergiant	B1-5Ia
W9[8]	16^h 47^m 04.14^s	−45° 50′ 31.1″	B[e] star	sgB[e]
W12a[8]	16^h 47^m 02.21^s	−45° 50′ 58.8″	Yellow hypergiant
W13[18][15]	16^h 47^m 06.45^s	−45° 50′ 26.0″	Eclipsing binary	B0.5 Ia^+, OB
W16a[8]	16^h 47^m 06.61^s	−45° 50′ 42.1″	Yellow hypergiant
W20[8]	16^h 47^m 04.70^s	−45° 51′ 23.8″	Red supergiant		126,000[19]	3,500[19]	965[19]
W26[8]	16^h 47^m 05.40^s	−45° 50′ 36.5″	Red hypergiant	M2-6Ia[20]	380,000[21] - 1,100,000[19]	3,600[22] - 3,700[19]	1,530[21]-2,550[19]
W32[8]	16^h 47^m 03.67^s	−45° 50′ 43.5″	Yellow hypergiant
W33[8]	16^h 47^m 04.12^s	−45° 50′ 48.3″	Blue hypergiant
W75[23]	16^h 47^m 08.93^s	−45° 49′ 58.4″	Red supergiant		68,000[19]	3,600[19]	668[19]
W237[8]	16^h 47^m 03.09^s	−45° 52′ 18.8″	Red supergiant		234,000[19]	3,600[19]	1,245[19]
W243[8]	16^h 47^m 07.55^s	−45° 52′ 28.5″	Luminous blue variable	LBV
W265[8]	16^h 47^m 06.26^s	−45° 49′ 23.7″	Yellow hypergiant
WR 77a[24]	16^h 46^m 55.4^s	−45° 51′ 34″	Wolf–Rayet star	WN6-7
WR 77aa[25]	16^h 46^m 46.3^s	−45° 47′ 58″	Wolf–Rayet star	WC9d
WR 77b[24]	16^h 46^m 59.9^s	−45° 55′ 26″	Wolf–Rayet star	WC8
WR 77c[24]	16^h 47^m 00.89^s	−45° 51′ 20.9″	Wolf–Rayet star	WNL
WR 77d, W57c[24]	16^h 47^m 01.5^s	−45° 51′ 45″	Wolf–Rayet star	WN8
WR 77e[24]	16^h 47^m 01.67^s	−45° 51′ 19.9″	Wolf–Rayet star	WN6-8
WR 77f, W5[24]	16^h 47^m 02.97^s	−45° 50′ 19.5″	Wolf–Rayet star	WNVL
WR 77g[24]	16^h 47^m 03.1^s	−45° 50′ 43″	Wolf–Rayet star	WC7
WR 77h, W66[24]	16^h 47^m 04.0^s	−45° 51′ 37.5″	Wolf–Rayet star	WC9
WR 77i[24]	16^h 47^m 04.02^s	−45° 51′ 25.2″	Wolf–Rayet star	WN6-8
WR 77j, W44[24]	16^h 47^m 04.20^s	−45° 51′ 07.0″	Wolf–Rayet star	WN9
WR 77k[24]	16^h 47^m 04.1^s	−45° 51′ 20.0″	Wolf–Rayet star	WC9
WR 77l[24]	16^h 47^m 04.40^s	−45° 51′ 03.8″	Wolf–Rayet star	WC8.5
WR 77m, W239[24]	16^h 47^m 05.21^s	−45° 52′ 25.0″	Wolf–Rayet star	WC9
WR 77n[24]	16^h 47^m 05.35^s	−45° 51′ 05.0″	Wolf–Rayet star	WN8 (uncertain)
WR 77o, W14c[24]	16^h 47^m 06.0^s	−45° 15′ 22″	Wolf–Rayet star	WN7o[26]
WR 77p, W241[24]	16^h 47^m 06.06^s	−45° 52′ 08.3″	Wolf–Rayet star	WC9
WR 77q[24]	16^h 47^m 06.24^s	−45° 51′ 26.5″	Wolf–Rayet star	WN6-8
WR 77r[24]	16^h 47^m 07.6^s	−45° 52′ 36″	Wolf–Rayet star	WN6
WR 77s, W72[24]	16^h 47^m 08.32^s	−45° 50′ 45.5″	Wolf–Rayet star	earlier than WN7
CXOU J164710.2-455216[27]	16^h 47^m 10.18^s	−45° 52′ 16.7″	anomalous X-ray pulsar

References

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12. "Comet-like stars". www.eso.org. Retrieved 3 December 2018.
13. Negueruela, Ignacio; Clark, J. Simon; Hadfield, Lucy J.; Crowther, Paul A.; et al. (2007). "Westerlund 1 as a Template for Massive Star Evolution". Proceedings of the International Astronomical Union. 3: 301–306. arXiv:0802.4168. Bibcode:2008IAUS..250..301N. doi:10.1017/S1743921308020620. S2CID 10747013.
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15. Ritchie, B. W.; Clark, J. S.; Negueruela, I.; Crowther, P. A.; et al. (2009). "A VLT/FLAMES survey for massive binaries in Westerlund 1: I. first observations of luminous evolved stars". Pre-Print. 507 (3): 1585. arXiv:0909.3815. Bibcode:2009A&A...507.1585R. doi:10.1051/0004-6361/200912686. S2CID 197460709.
16. 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.
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19. 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.
20. 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.
21. 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.
22. 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.
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24. 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.
25. 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.
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27. SIMBAD, CXOU J164710.2-455216

External links

• "Cl Westerlund 1". SIMBAD. Centre de données astronomiques de Strasbourg.
• Artist’s impression of a magnetar X-ray satellites catch magnetar in gigantic stellar ‘hiccup’, ESA website, 2007
• Image of Westerlund 1
• Neutron star found where black hole expected