High‐ResolutionChandraX‐Ray Imaging and Spectroscopy of the σ Orionis Cluster

Abstract

We present results of a 90 ks Chandra X-ray observation of the young σ Orionis cluster (age ∼ 3 Myr ) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star σ Ori AB (O 9.5 V + B 0.5 V ) 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 σ Ori AB, the magnetic B star σ Ori E, and the B5 V 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 IRS 1, located ≈3'' north of σ Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by σ Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from σ 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−1, 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.