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D. Pooley, W. Lewin (MIT), S. Portegies Zwart (Amsterdam)
The most massive stars in a cluster tend to sink to the cluster center on a dynamical friction, or mass segregation, time scale. For young star clusters, this time scale is considerably shorter than the half-mass relaxation time scale. As a result, the most massive stars and black holes (BHs) in star clusters with an age between a few tens and hundred million years will collect in the cluster center, where they can form binaries with each other. As a consequence of this, we expect clusters in this age range to contain many colliding-wind binaries (young massive star + young massive star), BH-BH binaries, and high-mass X-ray binaries (BH + massive star). Both the colliding-wind binaries and high-mass X-ray binaries are observable in the X-rays.
We have obtained Chandra observations of two star clusters in the Large Magellanic Cloud (NGC 1850A and B) which are at an age when many of these binaries should have formed and should still be present in the clusters. Surprisingly, we detect no X-ray sources in either cluster. We discuss the ramifications of this result on our understanding of cluster evolution and on the formation of intermediate-mass black holes in star clusters as well as prospects for future observations.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.