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TOPIC: Westerlund 2

Blobrana

L

Posts: 131433

Date: Jul 11 06:21 2007

RE: Westerlund 2	Permalink

Title: On the Distance and Molecular Environment of Westerlund 2 and HESS J1023-575 Authors: T. M. Dame The extended TeV gamma-ray source HESS J1023-575 is coincident with the massive, young stellar cluster Westerlund 2 (Wd2) and its surrounding HII region RCW 49. On the basis of an analysis of the CO emission and 21 cm absorption along the line of sight to Wd2, it is argued that this cluster, and by assumption the TeV source as well, must be associated with a giant molecular cloud in the far side of the Carina arm with a mass of 7.5 x 10^5 Mo. Analysis of the spatial and velocity structure of the cloud reveals clear evidence of interaction with Wd2. The cloud's kinematic distance of 6.0 ± 1.0 kpc is shown to be consistent with distances inferred both from the radius-linewidth relation of molecular clouds and from the foreground gas column derived from 230 X-ray sources in Wd2. Read more (286kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Feb 6 22:33 2007

Three new types of object found	Permalink

Three new types of object have been discovered in our galaxy: huge gamma-ray clouds, dense X-ray engines almost hidden in cocoons of dust, and bubbles blown by the wind from giant stars. All three discoveries were reported on Monday at a conference at Stanford University in California, US. Two come courtesy of the High Energy Stereoscopic System (HESS), an array of four telescopes in Namibia that detects photons of extremely high energy. When one of these gamma rays hits the atmosphere, it produces a flash of blue light that HESS can pick up. HESS has seen such gamma rays coming from Westerlund 2, a cluster of massive, bright stars about 25,000 light years away. The emission is too diffuse to be coming directly from the stars. "The gamma-ray source is much larger than the cluster itself," says team member Olaf Reimer of Stanford. Instead, he thinks the emission is generated by winds from several massive stars within the cluster. Called Wolf-Rayet stars, they are so bright their light blasts gas off of them and out into space. The total energy in this stellar wind over the course of each star's brief life is similar to that of a supernova explosion. Read more __________________

Blobrana

L

Posts: 131433

Date: Dec 22 13:03 2006

Westerlund 2	Permalink

Title: Early-type stars in the core of the young open cluster Westerlund2 Authors: G. Rauw (1), J. Manfroid (1), E. Gosset (1), Y. Naze (1), H. Sana (1,2), M. De Becker (1), C. Foellmi (2,3), A.F.J. Moffat (4) (1= IAGL, ULg, Belgium; 2=ESO-Chile; 3=Obs. de Grenoble, France; 4= Univ. de Montreal, Canada) Aims. The properties of the early-type stars in the core of the Westerlund2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR20a as well as the HII complex RCW49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. Results. The first light curves of the eclipsing binary WR20a in B and V filters are analysed and a distance of 8kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0pm1.4kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund2 does not provide enough ionising photons to account for the radio emission of the RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49. Read more (277kb, PDF) __________________

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