X-ray detection of the ionizing stars in ultracompact HII regions

We present observations of the W3 complex of massive star formation regions using the Chandra X-Ray Observatory. In the W3 core region, our observations resolve the emission previously observed with ASCA into 101 compact sources. The integrated spectrum is consistent with that observed earlier by ASCA and shows significant emission at energies larger than 2.5 keV. Comparing our Chandra data with existing near-infrared images, we find X-ray counterparts for W3 IRS 2, W3 IRS 2a, and W3 IRS 3a that are believed to be the ionizing stars for the H II regions W3A and W3B. We also detect X-ray emission coincident with the massive protostar candidate W3 IRS 5. Comparing our Chandra data with existing radio continuum data, we find that for radio continuum components A, B, C, D, G, and H, one or several X-ray sources are located at the peak radio position and/or the geometric center of the H II region. We postulate that the X-ray sources are the young massive stars that are also responsible for the ionization of the compact and ultracompact H II regions in the W3 core. Our observations show that very young massive stars are emitters of relatively hard X-rays and that they can be detected with Chandra even in a high-density environment.