Westerlund 1 ( Altar Cluster , Ara Cluster , Wd1 ) is a compact young star supercluster in the Milky Way galaxy , located at a distance of 3.5-5 kpc from the Sun. It is one of the most massive open clusters in the Galaxy [3] . It was discovered by Bengt Westerlund in 1961 [5] , but for many years it remained unexplored due to the high interstellar extinction in this direction. Perhaps in the future, Westerlund 1 will become a globular star cluster [6] .
| Westerlund 1 | |
|---|---|
| Star cluster | |
 | |
| Research history | |
| Discoverer | Bengt westerlund |
| opening date | 1961 |
| Designations | and |
| Observational data ( Age J2000.0 ) | |
| Type of | Open cluster |
| Right ascension | |
| Declination | |
| Distance | 12 100 ± 2,000 St. years (3 700 ± 600 pc ) [2] |
| Constellation | Altar |
| physical characteristics | |
| Class | |
| Weight | 63,000 [3] |
| Radius | 3.26 St. years [3] |
| Age | 3.50 million years [3] |
The cluster contains a large number of rare massive stars in the late stages of evolution, including 6 yellow hypergiants , 4 red supergiants (including Westerlund 1-26 , one of the largest known stars ), 24 Wolf-Rayet stars , a bright blue variable , many OB giants and an unusual supergiant sgB [e], supposedly the result of a collision of stars [7] . Observations in the X-ray range of the spectrum showed the presence of the anomalous X-ray pulsar CXOU J164710.2-455216 , a slowly rotating neutron star , probably formed from a massive predecessor star [8] . It is believed that the cluster Westerlund 1 was formed as a result of one outburst of star formation, which suggests a close age and chemical composition in all stars of the cluster.
Content
- 1 Observations
- 2 Age and evolutionary status
- 3 The proportion of double stars
- 4 Distance and location
- 5 notes
- 6 References
Observations
The brightest O7-8V main sequence stars have visible stellar magnitudes in the V band of about 20.5, therefore, in the visible region of the spectrum, radiation is mainly observed from bright departing stars from the main sequence (visible stellar magnitudes in the V band of 14.5-18, absolute from −7 up to −10) and less bright stars of luminosity classes Ib and II (magnitudes in the band V 18-20). Due to the extremely high interstellar reddening, observations in the U and B bands are difficult and most of the observations are made in the R and I bands in the red and infrared parts of the spectrum. Cluster stars are usually named according to the classification introduced by Westerlund [9] , although a different naming system is used for Wolf-Rayet stars [10] .
In the X-ray range, diffuse radiation from interstellar gas and radiation from point sources of large and small masses are revealed. The magnetar in the cluster is the brightest X-ray point source in the cluster, along with the powerful sources W9 (sgB [e]), W30a, WR A, and WR B. About 50 other point X-ray sources were associated with bright objects in the optical range. In the radio range, the sgB [e] star W9 and the red supergiants W20 and W26 are strong radio sources; most cold hypergiants, several OB supergiants and Wolf-Rayet stars are also observed.
Age and evolutionary status
The age of Westerlund 1 is estimated at 4-5 Ma when comparing the properties of a population of evolved stars with stellar evolution models. The presence of a significant number of both Wolf-Rayet stars and red and yellow supergiants in the cluster represents a strict restriction on the age of the cluster: theoretical studies predict that red supergiants cannot form before 4 million years until the most massive stars enter the red supergiant stage, and the number of Wolf-Rayet stars decreases sharply after the age of 5 million years. The obtained age interval is generally consistent with infrared observations that revealed the presence of late O-stars of the main sequence, although observations of small-mass stars gave an estimate of the age of 3.5 Ma [1] .
Assuming that Westerlund 1 forms stars with the usual initial mass function , it is likely that the cluster initially contained a significant number of very massive stars, such as the currently observed stars in the Arch cluster . Current estimates of the age of the Westerlund 1 cluster exceed the lifetime of such stars. Models of stellar evolution show that the cluster should contain 50-150 remnants of supernova outbreaks, and the rate of supernova outbreaks over the past million years was about one outbreak per 10 thousand years. However, at the moment, only one remnant of a supernova explosion is known reliably - magnetar . The question of the presence of other compact objects and massive X-ray binaries remains open. There are a number of assumptions, including the hypothesis of high speeds during supernova explosions that destroy binary systems, the assumption of the formation of stellar black holes slowly accreting matter (and therefore difficult to detect).
Since the stars of the cluster have approximately the same age, chemical composition and distance to the Sun, the cluster can serve as a good medium for studying the evolution of massive stars.
Double Star Share
There is some evidence of a high proportion of massive binary stars in the cluster. Some high mass binary stars were discovered directly from photometric observations [11] and in the study of radial velocities [12] , other binary stars were discovered in the study of such characteristics as luminosity in the X-ray range, nonthermal radio emission spectrum, and excess infrared radiation typical of binary c colliding winds and some types of Wolf-Rayet stars. In general, the proportion of binary stars for the Wolf-Rayet star population reaches 70%, for OB supergiants - more than 40% [12] .
Distance and location
Westerlund 1 is too far away to measure its distance with parallax . The distance is estimated based on the expected absolute stellar magnitude of the cluster stars and estimates of light absorption in the cluster direction. With this method, distances were determined for populations of yellow hypergiants [7] and Wolf-Rayet stars [10] ; in both cases, the distance turned out to be close to 5 kpc; according to the stars of the main sequence, the distance turned out to be 3.6 kpc [1] .
Notes
- ↑ 1 2 3 4 Brandner, W .; Clark, JS; Stolte, A .; Waters, R .; Negueruela, I .; Goodwin, SP et al. Intermediate to low-mass stellar content of Westerlund 1 (Eng.) // Astronomy and Astrophysics : journal. - EDP Sciences 2008. - Vol. 478 , no. 1 . - P. 137—149 . - DOI : 10.1051 / 0004-6361: 20077579 . - . - arXiv : 0711.1624 .
- ↑ E .; Koumpia; Bonanos, AZ Fundamental parameters of four massive eclipsing binaries in Westerlund 1 (Eng.) // Astronomy and Astrophysics : journal. - EDP Sciences 2012. - Vol. 547 . - P. A30 . - DOI : 10.1051 / 0004-6361 / 201219465 . - . - arXiv : 1108.4453 .
- ↑ 1 2 3 4 Portegies Zwart, Simon F .; McMillan, Stephen LW; Gieles, Mark. Young Massive Star Clusters (Eng.) // Annual Review of Astronomy and Astrophysics . - Annual Reviews , 2010. - Vol. 48 . - P. 431 . - DOI : 10.1146 / annurev-astro-081309-130834 . - . - arXiv : 1002.1961 .
- ↑ SIMBAD Astronomical Database
- ↑ Westerlund, B. A Heavily Reddened Cluster in Ara // The Astronomical Journal . - IOP Publishing , 1961. - Vol. 70 . - P. 57 . - DOI : 10.1086 / 108585 . - .
- ↑ Gallagher; Grebel Extragalactic Star Clusters: Speculations on the Future (Eng.) // Extragalactic Star Clusters, IAU Symposium: journal. - 2002. - Vol. 207 . - P. 207 . - . - arXiv : astro-ph / 0109052 .
- ↑ 1 2 Clark, JS; Negueruela, I .; Crowther, PA; Goodwin, SP et al. On the massive stellar population of the super star cluster Westerlund 1 (Eng.) // Astronomy and Astrophysics : journal. - EDP Sciences 2005. - Vol. 434 , no. 3 . - P. 949-969 . - DOI : 10.1051 / 0004-6361: 20042413 . - . - arXiv : astro-ph / 0504342 .
- ↑ Muno, Michael P .; Clark, J. Simon; Crowther, Paul A .; Dougherty, Sean M .; De Grijs, Richard; Law, Casey; McMillan, Stephen LW; Morris, Mark R .; Negueruela, Ignacio; Pooley, David; Portegies Zwart, Simon; Yusef-Zadeh, Farhad et al. A Neutron Star with a Massive Progenitor in Westerlund 1 (Eng.) // The Astrophysical Journal : journal. - IOP Publishing , 2006. - Vol. 636 , no. 1 . - P. L41 . - DOI : 10.1086 / 499776 . - . - arXiv : astro-ph / 0509408 .
- ↑ Westerlund, BE Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA (Eng.) // Astronomy and Astrophysics : journal. - EDP Sciences 1987. - Vol. 70 , no. 3 . - P. 311-324 . - ISSN 0365-0138 . - .
- ↑ 1 2 Crowther, Paul A .; Hadfield, LJ; Clark, JS; Negueruela, I .; Vacca, WD et al. A census of the Wolf – Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy // Monthly Notices of the Royal Astronomical Society : journal. - Oxford University Press , 2006. - Vol. 372 , no. 3 . - P. 1407-1424 . - DOI : 10.1111 / j.1365-2966.2006.10952.x . - . - arXiv : astro-ph / 0608356 .
- ↑ Bonanos, Alceste Z. Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1 (Eng.) // The Astronomical Journal : journal. - IOP Publishing 2007. - Vol. 133 , no. 6 . - P. 2696-2708 . - DOI : 10.1086 / 518093 . - . - arXiv : astro-ph / 0702614 .
- ↑ 1 2 Ritchie, BW; Clark, JS; Negueruela, I .; Crowther, PA et al. A VLT / FLAMES survey for massive binaries in Westerlund 1: I. first observations of luminous evolved stars (Eng.) // Pre-Print: journal. - 2009. - Vol. 507 , no. 3 . - P. 1585 . - DOI : 10.1051 / 0004-6361 / 200912686 . - . - arXiv : 0909.3815 .