2008ApJ...673..354F

An ∼800 arcmin² mosaic image of the W3 star-forming complex obtained with the Chandra X-Ray Observatory gives a valuable new view of the spatial structure of its young stellar populations. The Chandra image reveals ∼1300 faint X-ray sources, most of which are pre-main-sequence (PMS) stars in the cloud. Some, but not all, of the high-mass stars producing hypercompact and ultracompact H II (UC H II) regions are also seen, as reported in a previous study. The Chandra images reveal three dramatically different embedded stellar populations. The W3 Main cluster extends over 7 pc with ∼900 X-ray stars in a nearly spherical distribution centered on the well-studied UC H II regions and high-mass protostars. The cluster surrounding the prototypical UC H II region W3(OH) shows a much smaller (≤0.6 pc), asymmetrical, and clumpy distribution of ∼50 PMS stars. The massive star ionizing the W3 North H II region is completely isolated without any accompanying PMS stars. In W3 Main, the inferred ages of the widely distributed PMS stars are significantly older than the inferred ages of the central OB stars illuminating the UC H II regions. We suggest that different formation mechanisms are necessary to explain the diversity of the W3 stellar populations: cluster-wide gravitational collapse with delayed OB star formation in W3 Main, collect-and-collapse triggering by shock fronts in W3(OH), and a runaway O star or isolated massive star formation in W3 North.