Evolution of X-ray emission from young massive stellar clusters
L. M. Oskinova
The evolution of young massive stellar cluster is modeled in order to explain the striking difference observed in the levels of X-ray emission between clusters of different age. It is shown that the level and character of soft (0.2-10 keV) X-ray emission change drastically with cluster age and are tightly linked with stellar evolution. Assuming instantaneous burst of star formation and standard initial mass function, the total X-ray luminosity of all low-mass stars 0.1- 3 Msun is higher than the total luminosity of the cluster's high-mass stars (8 - 100 Msun). The same time, massive stars are the brightest X-ray point sources, although they become much dimmer when they evolve to Wolf-Rayet (WR) star phase. A supernova remnant emission may be a dominant X-ray source in an old enough cluster, but only for a relatively short time of a few thousand years. The diffuse X-ray emission originates from the intercluster wind heated by the kinetic energy of stellar winds and supernova explosions. When massive stars reach the WR phase, the level of diffuse emission rises due to the powerful WR stellar winds. Subsequent SN explosions pump the level of diffuse emission even higher. The clusters older then \approx 2 Myr may have no bright stellar point sources, but a relatively high level of diffuse emission. The above scenario provides a plausible explanation of the X-ray observations of the Arches and the Quintuplet cluster in the Galaxy as well as of a sample of LMC clusters.
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