  We present results of a 90 ksec Chandra X-ray observation of the young sigma
Orionis cluster (age ~3 Myr) obtained with the High Energy Transmission Grating
Spectrometer. We use the high resolution grating spectrum and moderate
resolution 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 high
spatial 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 five
have near-IR or optical counterparts and about one-fourth are variable. Notable
high-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 properties
consistent with lower mass K or M-type stars. We present the first X-ray
spectrum of the unusual infrared source IRS1 located 3.3 arc-sec north of sigma
Ori AB, which is likely an embedded T Tauri star whose disk/envelope is being
photoevaporated by sigma Ori AB. We focus on the radiative wind shock
interpretation of the soft luminous X-ray emission from sigma Ori AB, but also
consider possible alternatives including magnetically-confined wind shocks and
colliding wind shocks. Its emission lines show no significant asymmetries or
centroid shifts and are moderately broadened to HWHM ~ 264 km/s, or one-fourth
the terminal wind speed. Forbidden lines in He-like ions are formally
undetected, implying strong UV suppression. The Mg XI triplet forms in the wind
acceleration zone within one stellar radius above the surface. These X-ray
properties are consistent in several respects with the predictions of radiative
wind shock theory for an optically thin wind, but explaining the narrow line
widths presents a challenge to the theory.
