Westerlund 1

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Open star cluster
Westerlund 1
Photo of the MPG / ESO 2.2 m telescope
Photo of the MPG / ESO 2.2 m telescope
AladinLite
Constellation altar
Position
equinox : J2000.0
Right ascension 16 h 47 m 04.0 s
declination -45 ° 51 ′ 04.9 ″
Appearance
Physical data

Affiliation Milky Way
distance  12000-16000 Lj
(3500-5000 pc )
Age 4 to 5 million years
history
Discovered by Bengt Westerlund
Discovery time 1961
Catalog names
 C 1644-457 •  ESO  277-12 •

Westerlund 1 (abbreviated Wd 1 , also called Ara Cluster ) is a compact, young open star cluster in the constellation Altar and is about 3.5 to 5 kpc from Earth. In fact, Wd 1 is the most massive known open star cluster in the local group . It was discovered by Bengt Westerlund in 1961, but could not be explored very much from the ground due to the strong extinction in its direction. Only with the availability of infrared and X-ray telescopes in space could it be examined more closely.

The cluster contains a large number of rare, evolved, massive stars. Among them are six yellow hypergiants , four red supergiants , 24 Wolf-Rayet stars , an LBV , many OB supergiants, and an unusual sgB [e] star that could be the result of a recent star merger . With W26 , Westerlund 1 contains one of the largest stars known to date. In addition, observations in the X-ray range have revealed the unusual X-ray pulsar CXOU J164710.2-455216 . This slowly rotating neutron star must have formed from a previous star with a large mass. It is believed that Wd 1 was formed in a single burst of star formation because the stars are very similar in age and composition.

Not only is Westerlund 1 home to some of the most massive and least understood stars, but it is also very useful for understanding what happens in extragalactic super star clusters .

observation

The brightest main sequence stars (O7-8) in Wd 1 have an apparent visual magnitude of around 20.5 mag. Therefore, Wd 1 in the optical range is mainly dominated by luminous post-main sequence stars (apparent magnitudes in the V-band 14.5 to 18 mag, absolute magnitudes −7 to −10 mag) together with stars of luminosity classes Ib and II. Due to the extremely strong interstellar discoloration in the direction of Wd 1, it is very difficult to make observations in the blue or ultraviolet spectral range. Wd 1 is therefore mainly observed in the red or infrared spectral range. Stars in Wd 1 are usually named with the name Westerlund gave them. A separate naming convention is used for the Wolf-Rayet stars.

At the wavelengths of the X-ray range, Wd 1 shows diffuse emissions from the interstellar gas and point emissions from massive post-main-sequence stars and low-mass pre-main-sequence stars. The Magnetar CXOU J164710.2-455216 is the brightest point source, followed by the so-called [e] star W9, the double star W30a and the Wolf-Rayet stars WR A and WR B. Around 50 other point sources have counterparts in the optical spectral range.

In the radio sector , the so-called [e] star W9 and the red supergiants W20 and W26 are strong sources. Most of the cool hypergiants and a few OB supergiants are also sources that could be detected.

Age and Development

Photo from the Hubble Space Telescope

The age of Wd 1 is estimated to be 4-5 million years due to the mass of the developed stars. The simultaneous occurrence of Wolf-Rayet stars and red and yellow supergiants in Wd 1 limits the possible period for the formation of Wd 1, since the theory says that red supergiants only form after about 4 million years (very massive stars do not go through a red supergiant phase) and the population of Wolf-Rayet stars decreases sharply after 5 million years. The estimated age is broadly consistent with the infrared observations of Wd 1, which show the occurrence of late O main sequence stars in Wd 1. Investigations on stars in the medium mass range indicate a somewhat lower age of 3.5 million years.

If the stars in Wd 1 have formed according to a typical original stellar mass function , then Wd 1 must have had a considerable number of very massive stars (comparable to the younger Arches star cluster ). Current estimates of the age of Wd 1 are greater than the lifespan of these stars, and the models of stellar evolution suggest that there must have already been 50 to 150 supernovae , which is the rate of one supernova in 10,000 years. So far, however, only one definitive supernova remnant has been found ( CXOU J164710.2-455216 ). The absence of other compact objects and X-ray binary stars is puzzling. Possible explanations for this observation have been suggested for example binary star systems destroyed in supernova explosions, black holes with low accretion and binary star systems in which both components are compact objects. A conclusive solution to this problem has not yet been found.

Since the stars in Westerlund 1 have the same age, composition and distance, the star cluster is an ideal object to understand the evolution of massive stars. Current star formation models , for example, cannot yet explain the distribution of Wolf-Rayet star types in Wd 1.

Proportion of double stars

There are some indications for a high proportion of binary star systems among the stars of high mass in Wd 1. Some massive binary stars were found directly photometrically or with the radial velocity method . Many other binary stars have been detected by secondary features (e.g. strong X-ray brightness, nonthermal radio spectra, or an excess of infrared radiation) that are typical of binary stars whose star winds collide or Wolf-Rayet stars that produce dust. Uncertain estimates assume that 70% of Wolf-Rayet stars and over 40% of OB supergiants are double stars.

Distance and location

Wd 1 is too far away to determine the distance directly using the parallax method. The distance can, however, be estimated using the absolute brightness of the stars and estimates of the extinction in the direction of the star cluster. A determination of the distance via the yellow hypergiants and the Wolf-Rayet stars gives a value of around 5 kpc in both cases . A determination of the main sequence stars suggests a value of around 3.6 kpc . These estimates place Wd 1 at the outer edge of the galactic bar. This could be important in understanding how such a massive star cluster could form.

A measurement of a few Wolf-Rayet stars in the radio range sets a lower limit for the distance at 2 kpc .

Web links

Commons : Westerlund 1  - Collection of images, videos and audio files

Individual evidence

  1. ^ B. Westerlund: A Heavily Reddened Cluster in Ara . In: Astronomical Journal . 70, 1961, p. 57. bibcode : 1961AJ ..... 66T..57W . doi : 10.1086 / 108585 .
  2. ↑ a b c J.S. Clark, et al. : On the massive stellar population of the super star cluster Westerlund 1 . In: Astronomy & Astrophysics . 434, No. 3, 2005, pp. 949-969. arxiv : astro-ph / 0504342 . bibcode : 2005A & A ... 434..949C . doi : 10.1051 / 0004-6361: 20042413 .
  3. ^ Westerlund 1: Neutron Star Discovered Where a Black Hole Was Expected . In: Harvard-Smithsonian Center for Astrophysics . November 2, 2005. Retrieved October 16, 2011.
  4. ↑ Michael P. Muno, et al. : A Neutron Star with a Massive Progenitor in Westerlund 1 . In: Astrophysical Journal Letters . 636, No. 1, 2006, p. L41. arxiv : astro-ph / 0509408 . bibcode : 2006ApJ ... 636L..41M . doi : 10.1086 / 499776 .
  5. ^ BE Westerlund: Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA . In: Astronomy and Astrophysics . 70, No. 3, 1987, pp. 311-324. ISSN  0365-0138 . bibcode : 1987A & AS ... 70..311W .
  6. ↑ a b c Paul A. Crowther, et al. : A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy . In: Monthly Notices of the Royal Astronomical Society . 372, No. 3, 2006, pp. 1407-1424. arxiv : astro-ph / 0608356 . bibcode : 2006MNRAS.372.1407C . doi : 10.1111 / j.1365-2966.2006.10952.x .
  7. ↑ a b W. Brandner, et al. : Intermediate to low-mass stellar content of Westerlund 1 . In: Astronomy & Astrophysics . 478, No. 1, 2008, pp. 137-149. arxiv : 0711.1624 . bibcode : 2008A & A ... 478..137B . doi : 10.1051 / 0004-6361: 20077579 .
  8. ↑ Ignacio Negueruela, et al. : Westerlund 1 as a Template for Massive Star Evolution . In: Proceedings of the International Astronomical Union . 3, 2007, pp. 301-306. arxiv : 0802.4168 . doi : 10.1017 / S1743921308020620 .
  9. ↑ Alceste Z. Bonanos: Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1 . In: Astronomical Journal . 133, No. 6, 2007, pp. 2696-2708. arxiv : astro-ph / 0702614 . bibcode : 2007AJ .... 133.2696B . doi : 10.1086 / 518093 .
  10. ↑ a b B. W. Ritchie, et al. : A VLT / FLAMES survey for massive binaries in Westerlund 1: I. first observations of luminous evolved stars . In: Pre-Print . 2009. arxiv : 0909.3815 . bibcode : 2009A & A ... 507.1585R . doi : 10.1051 / 0004-6361 / 200912686 .