Westerlund 2
Encyclopedia
Westerlund 2 is a young massive obscured star cluster 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 contains a dozen of O stars, of which at least three are eclipsing binaries, many pre–main sequence stars and two Wolf-Rayet Star
s: the binary WR20a and the single star WR20b, all of spectral type WN6ha. They are probably not actually Wolf-Rayet stars, i.e. they are core hydrogen burning stars, just like the sun. Due to their large mass loss rate they appear to be Wolf Rayet stars.
It was shown recently that the core of the cluster contains several examples of rare very hot stars (Rauw et al. 2007, A&A, 463, 981). Just outside the cluster a massive eclipsing binary WR20a is found at 30 arcseconds (about 1.1 pc in projection), the bright yellow spot just on the lower right side of the cluster center.
As its name indicates, the Westerlund 2 cluster was discovered by Bengt Westerlund in the sixties (Westerlund 1961, PASP, 73, 51 and 1968, ApJ, 154, L67 see http://vizier.u-strasbg.fr/cgi-bin/Dic-Simbad?Cl%20Westerlund ) but its stellar content was assessed only in the last years (Moffat et al. 1991, AJ, 102, 642; Rauw et al. 2007, A&A, 463, 981). WR20a is also known since the sixties (The 1966, CoBos, 35, 1) but its binarity was only discovered in 2004 by a Belgian team.
of our sun
(Rauw et al. 2004, 420, L9, Rauw et al. 2005, A&A, 432, 985 and Bonanos et al. 2004) . It not clear why this system is located away from the center of the cluster. It is possible that the system was formed in the core, but that it got ejected by dynamical interactions.
Every 3.6 days the two star in this system revolve around each other. Although the stars are in very tight orbit, both stars in the system are detached (see e.g. Rauw et al. 2007, A&A, 463, 981 and Bonanos et al. 2004). It is to be expected that within a million years the two will expand and come into contact. A large Nitrogen abundance has been measured on the surface of the stars (Rauw et al. 2005, A&A, 432, 985), about six times the Nitrogen abundance of measured in the sun. This nitrogen is probably produced in deeper layers of the star and mixed towards the surface by rotational mixing.
A collision between the two winds of the systems has been detected in the visible as well as in X-rays (Rauw et al. 2005, A&A, 432, 985; Naze et al. 2008, A&A, 483, 171). The X-ray emitting region is quite extended since it does not suffer of any eclipse.
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
s: the binary WR20a and the single star WR20b, all of spectral type WN6ha. They are probably not actually Wolf-Rayet stars, i.e. they are core hydrogen burning stars, just like the sun. Due to their large mass loss rate they appear to be Wolf Rayet stars.
It was shown recently that the core of the cluster contains several examples of rare very hot stars (Rauw et al. 2007, A&A, 463, 981). Just outside the cluster a massive eclipsing binary WR20a is found at 30 arcseconds (about 1.1 pc in projection), the bright yellow spot just on the lower right side of the cluster center.
As its name indicates, the Westerlund 2 cluster was discovered by Bengt Westerlund in the sixties (Westerlund 1961, PASP, 73, 51 and 1968, ApJ, 154, L67 see http://vizier.u-strasbg.fr/cgi-bin/Dic-Simbad?Cl%20Westerlund ) but its stellar content was assessed only in the last years (Moffat et al. 1991, AJ, 102, 642; Rauw et al. 2007, A&A, 463, 981). WR20a is also known since the sixties (The 1966, CoBos, 35, 1) but its binarity was only discovered in 2004 by a Belgian team.
WR20a
WR20a was discovered in 2004 to be one of the most massive binary systems known, for which the masses of the components are accurately measured. Each star in the system has about eighty times the massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of our sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(Rauw et al. 2004, 420, L9, Rauw et al. 2005, A&A, 432, 985 and Bonanos et al. 2004) . It not clear why this system is located away from the center of the cluster. It is possible that the system was formed in the core, but that it got ejected by dynamical interactions.
Every 3.6 days the two star in this system revolve around each other. Although the stars are in very tight orbit, both stars in the system are detached (see e.g. Rauw et al. 2007, A&A, 463, 981 and Bonanos et al. 2004). It is to be expected that within a million years the two will expand and come into contact. A large Nitrogen abundance has been measured on the surface of the stars (Rauw et al. 2005, A&A, 432, 985), about six times the Nitrogen abundance of measured in the sun. This nitrogen is probably produced in deeper layers of the star and mixed towards the surface by rotational mixing.
A collision between the two winds of the systems has been detected in the visible as well as in X-rays (Rauw et al. 2005, A&A, 432, 985; Naze et al. 2008, A&A, 483, 171). The X-ray emitting region is quite extended since it does not suffer of any eclipse.
WR20b
WR20b seems to be single star, slightly fainter than the faintest star of WR20a, though its X-ray emission is quite peculiar (see Naze et al. 2008, A&A, 483, 171)External links
- Astronomy Picture of the DayAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
- Young Star Cluster Westerlund 2 26 June 2010