High‐Resolution<i>Chandra</i>X‐Ray Imaging and Spectroscopy of the σ Orionis Cluster

Abstract

We present results of a 90 ks Chandra X-ray observation of the young Orionis cluster (age 3Myr) obtained with the HETGS.We use the high-resolution grating spectrum andmoderate-resolution CCD spectrum of themassive central star Ori AB (O9:5 Vþ B0:5 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 Ori AB, the magnetic B star Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- orM-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located 300 north of Ori AB. Its X-ray properties and elongated mid-IR mor-phology suggest that it is an embedded low-mass T Tauri star whose disk /envelope is being photoevaporated by Ori AB.We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emis-sion lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM 264 km s1, 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