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We present results of a 90 ksec Chandra X-ray observation of the young sigmaOrionis cluster (age ~3 Myr) obtained with the High Energy Transmission GratingSpectrometer. We use the high resolution grating spectrum and moderateresolution CCD spectrum of the massive central star sigma Ori AB (O9.5V +B0.5V) to test wind shock theories of X-ray emission and also analyze the highspatial resolution zero-order ACIS-S image of the central cluster region.Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but fivehave near-IR or optical counterparts and about one-fourth are variable. Notablehigh-mass stellar detections are sigma Ori AB, the magnetic B star sigma Ori E,and the B5V binary HD 37525. Most of the other detections have propertiesconsistent with lower mass K or M-type stars. We present the first X-rayspectrum of the unusual infrared source IRS1 located 3.3 arc-sec north of sigmaOri AB, which is likely an embedded T Tauri star whose disk/envelope is beingphotoevaporated by sigma Ori AB. We focus on the radiative wind shockinterpretation of the soft luminous X-ray emission from sigma Ori AB, but alsoconsider possible alternatives including magnetically-confined wind shocks andcolliding wind shocks. Its emission lines show no significant asymmetries orcentroid shifts and are moderately broadened to HWHM ~ 264 km/s, or one-fourththe terminal wind speed. Forbidden lines in He-like ions are formallyundetected, implying strong UV suppression. The Mg XI triplet forms in the windacceleration zone within one stellar radius above the surface. These X-rayproperties are consistent in several respects with the predictions of radiativewind shock theory for an optically thin wind, but explaining the narrow linewidths presents a challenge to the theory.

High-Resolution Chandra X-ray Imaging and Spectroscopy of the Sigma Orionis Cluster