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

TL;DR

This study uses high-resolution Chandra X-ray observations to analyze the young sigma Orionis cluster, focusing on stellar X-ray emissions, wind shock theories, and the properties of detected sources including sigma Ori AB.

Contribution

First detailed X-ray spectral analysis of sigma Ori AB and the surrounding cluster, testing wind shock models and identifying new X-ray sources including an embedded T Tauri star.

Findings

Detected 42 X-ray sources with most having IR or optical counterparts.

X-ray emission from sigma Ori AB is consistent with radiative wind shock theory.

Identified an embedded T Tauri star likely undergoing photoevaporation.

Abstract

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.