X-rays from clusters

X-ray emission

Cool, low-mass convective stars (like the sun) emit X-rays from a hot corona at several million degrees. The corona is trapped and heated by a strong magnetic field which is generated by an internal dynamo mechanism. The dynamo is thought to be more efficient in fast rotating stars (the sun is slow), so young open clusters often contain many X-ray sources, because the low-mass stars have not yet had time to spin down from their birth rotation rates, which can be quite high.

X-rays from NGC 2516

As an example, consider the young open cluster NGC 2516, which has an age of only 150 million years. The picture at the top of the page shows a 55 ks exposure with the ROSAT X-ray telescope position sensitive proportional counter (PSPC). To the left is a contour overlay of the X-ray emission on top of an optical image of the cluster. The majority of X-ray sources are coincident with faint low-mass stars. There are only a few coincidences between the X-ray sources and the optically bright sources. This is because the bright stars of the cluster are more massive and have no convection zone and thus no internal dynamo. In fact we believe that where there is an X-ray source coincident with a bright star, it is because it is in a close binary with a fainter, low-mass star, that has a convection zone and a hot corona.
Rob Jeffries rdj@astro.keele.ac.uk
Department of Physics
University of Keele
Keele
Staffordshire ST5 5BG, UK
29th October 1995