Westerlund 1
Westerlund 1 ( Wd 1 abbreviated - also called Ara cluster) is a compact, young open cluster in the constellation altar and is approximately 3.5 to 5 kpc from Earth. In fact, Wd 1 is the most massive known open cluster in the local group. He was discovered by Bengt Westerlund in 1961, but was from the ground, due to the strong extinction in its direction, are only poorly understood. Only with the availability of infrared and X-ray telescopes in space, he could be investigated further.
The cluster contains a large number of rare, developed, massive stars. Among them are six yellow hyper giant, four red supergiants, 24 Wolf -Rayet star, a LBV, many OB supergiants and an unusual SGB [e ] star, which could be the result of a recent merger star. In addition, observations have revealed the unusual Röntgenpulsar CXOU J164710.2 - 455216 in the X-ray range. This slowly rotating neutron star must have formed from a progenitor star with a large mass. It is believed that Wd 1 was formed in a single outburst of star formation, since the stars have a very similar age and a similar composition.
Westerlund 1 is not only home to some of the most massive and the least understood stars, but he is also very useful to understand what is happening in extragalactic super star clusters.
Observation
The brightest main sequence star (O7 -8) in Wd 1 have an apparent visual magnitude of 20.5 mag. Therefore, Wd 1 is in the optical range mainly from luminous Nachhauptreihensternen ( apparent magnitudes in the V band from 14.5 to 18 likes, absolute magnitudes -7 to -10 mag) dominates along with stars of luminosity classes Ib and II. Due to the extremely strong interstellar reddening in the direction of Wd 1, it is very difficult to make observations in the blue or ultraviolet spectral range. Wd 1 is observed, therefore, mainly in the red or infrared spectral range. Stars in Wd 1 are usually named with the label that has given them Westerlund. For Wolf- Rayet stars, a separate naming convention.
At the wavelengths of the X-ray range Wd 1 shows diffuse emission from the interstellar gas and point-source emissions of 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 SGB [e ] star W9, the double star W30a and the Wolf -Rayet stars WR A and WR B. About 50 more point sources have counterparts in the optical spectral range.
In the radio sector are the SGB [e ] star W9 and the red supergiant W20 and W26 strong sources. Most of the cool hypergiants and a couple of OB supergiants are also sources that could be detected.
Age and Development
The age of Wd 1 is estimated at 4-5 million years due to the masses of evolved stars. The simultaneous occurrence of Wolf -Rayet stars and red and yellow supergiants in Wd 1 limit the possible period for the emergence of Wd 1, since the theory is that red supergiants are formed only after about 4 million years ago ( very massive stars not go through a red supergiant phase) and the population of Wolf -Rayet stars after 5 million years ago strongly decreases. The estimated age is broadly consistent with the infrared observations of Wd 1, which show the occurrence of later O- main sequence stars in Wd 1. Studies of stars of intermediate mass range indicate a slightly lower age of 3.5 million years.
If the stars have formed in Wd 1 corresponding to a typical initial stellar mass function, then Wd 1 has a significant number of very massive stars have possessed (comparable to the younger Arches star cluster ). Current estimates of the age of Wd 1 are larger than the lifetime of these stars and the models of stellar evolution suggest that it must have already given 50 to 150 supernovae, which corresponds to a rate of one supernova in 10,000 years. However, only a definitive supernova remnant has been found ( CXOU J164710.2 - 455216 ). The absence of other compact objects and X-ray binaries is puzzling. Possible explanations for this observation were, for example, in supernova explosions destroyed a binary star system, black holes with low accretion and binary star systems in which both components are compact objects are proposed. A logical solution to this problem has not yet been found.
As the stars in Westerlund 1 have the same age, the same composition and the same distance is the star cluster an ideal object in order to understand the evolution of massive stars. Current star formation models can not explain, for example, the distribution of Wolf -Rayet star types in Wd 1.
Proportion of double stars
There is some evidence for a high proportion of binaries among high-mass stars in Wd 1 Some massive binary stars were photometrically directly or with the radial velocity method found. Many other double stars have been detected by secondary characteristics (eg strong X-ray brightness, non-thermal radio spectra or an excess of infrared radiation), which are typical of binary stars collide their stellar winds, or Wolf- Rayet stars which produce dust. Uncertain estimates suggest that 70 % of Wolf- Rayet stars and over 40% of the OB supergiants are double stars.
Distance and location
Wd 1 is too far away to perform a direct distance determination by means of the parallax. The distance can but based on the absolute brightness of stars and estimates of extinction are estimated in the direction of the cluster, for example. A determination of the distance across the yellow hypergiants and the Wolf- Rayet stars delivers in both cases, a value of about 5 kpc. A provision on the main sequence stars, however, suggests a value of 3.6 kpc. These estimates place Wd 1 at the outer edge of the galactic bar. This could be important for understanding how such a massive star clusters could be formed.
A measurement of some Wolf- Rayet stars in the radio sets kpc a lower limit for the distance given 2.