ar X iv : a st ro - p h / 05 07 69 3 v 1 2 9 Ju l 2 00 5 X - raying the super star clusters in the Galactic center

The Galactic center harbors some of the most massive star clusters known in the Galaxy: the Arches and the Quintuplet. Based on the Chandra observations of these clusters (PI: Wang) which recently became public, I discuss the X-ray emission from the massive stars in these clusters. Confirming the general trend for Wolf-Rayet (WR) stars being X-ray dim, none of them is detected in the Quintuplet cluster. The most massive star known in the Galaxy, the Pistol star, is also not detected, invoking questions regarding the proposed binary nature of this object. X-ray emission in the Arches cluster is dominated by three stellar point sources. All three sources as well as the cluster’s diffuse radiation show strong emission at 6.4–6.7 keV, indicating the presence of fluorescenting cool material. The Arches point sources may be identified as colliding wind binaries, albeit other possibilities cannot be ruled out. The X-ray emission from young stellar clusters, such as the Arches and the Quintuplet, is tightly coupled with the massive stars evolution. When a massive star evolves from O type to WN type, and finally to WC type, it keeps its bolometric luminosity nearly constant while its X-ray luminosity is likely to decline (Oskinova 2005). This trend is confirmed by the study of WR stars in the Galaxy and the LMC (Guerrero et al. in prep., also these proceedings). In the 1–3 Myr old Arches cluster (Figer et al. 2002, F02), the majority of massive stars are of O type. The most massive stars are the most X-ray luminous ones due to the logLX = logLbol − 7 correlation. The stellar wind input into the intracluster medium is expected to result in diffuse cluster wind emission. The level of cluster wind emission scales with stellar mass-loss rates and wind kinetic energy (Chevalier & Clegg 1985). Both quantities rise sharply when stars evolve to the WR phase, characterized by conspicuously powerful stellar winds. Therefore, in the 3–5 Myr old Quintuplet cluster (Figer et al. 1999, FMM99) where a significant fraction of massive stars has evolved to the WR stage the level of cluster wind emission is expected to rise. The same time the stellar X-ray emission becomes much less prominent with most massive stars evolving fast to the WC stage and becoming X-ray dim. The Arches and the Quintuplet were recently observed with Chandra in a nearly 100 ksec exposure. Figure 1 shows the ACIS-I image of the Arches cluster. Three bright prominent