On the X‐Ray Emission from Massive Star Clusters and Their Evolving Superbubbles

Abstract

Here we discuss the X-ray emission properties from the hot thermalized plasma that results from the collisions of individual stellar winds and supernovae ejecta within rich and compact star clusters. We propose a simple analytical way of estimating the X-ray emission generated by super star clusters and derive an expression that indicates how this X-ray emission depends on the main cluster parameters. Our model predicts that the X-ray luminosity from the star cluster region is highly dependent on the star cluster wind terminal speed, a quantity related to the temperature of the thermalized ejecta. We have also compared the X-ray luminosity from the super stellar cluster (SSC) plasma with the luminosity of the interstellar bubbles generated from the mechanical interaction of the high-velocity star cluster winds with the interstellar medium (ISM). We found that the hard (2.0-8.0 keV) X-ray emission is usually dominated by the hotter SSC plasma, whereas the soft (0.3-2.0 keV) component is dominated by the bubble plasma. This implies that compact and massive star clusters should be detected as pointlike, hard X-ray sources embedded into extended regions of soft diffuse X-ray emission. We also compared our results with predictions from the population synthesis models that take into consideration binary systems and found that in the case of young, massive, and compact super star clusters the X-ray emission from the thermalized star cluster plasma may be comparable to or even larger than that expected from the high-mass X-ray binary (HMXB) population.