Stellar X-Ray Emission in the Orion Nebula Region

Results from two extensive X-ray imaging surveys of the Orion Nebula are presented. In 6 Einstein High Resolution Imager (HRI) and 17 Imaging Proportional Counter (IPC) images of a roughly 4.5 square degree region, 245 distinct X-ray sources have been detected. In three ROSAT HRI images of a 0.8 square degree region, 389 distinct X-ray sources have been detected, >~ 2/3 of which are associated with a single proper-motion cluster member. The hot main-sequence O6-B5 stars detected in Orion have X-ray activity levels comparable to field O and B stars. X-ray emission has also been detected in the direction of a handful of main sequence late-B and early-A stars. Since the mechanisms producing X-rays in late-type coronae and early-type winds cannot operate in the late-B and early-A type atmospheres, it is argued that the observed X-rays are probably produced in the coronae of unseen late-type binary companions. When plotted in an X-ray luminosity versus bolometric luminosity diagram, late-type PMS stars lie below a ``saturation" line corresponding to L_X / L_bol ~10^-3. The late-G, K, and M-type stars exhibit a nearly two orders-of-magnitude spread in X-ray luminosity and in L_X / L_bol at a given effective temperature. Plots of X-ray activity versus v sin i rotational velocity and rotational period appear to show no clear dependance of activity on rotation. However, because of biases and uncertainties in the X-ray and optical samples, the data are not conclusive on this point. ROSAT light curves of the detected X-ray sources have revealed at least 10 strong X-ray flares with energies in excess of 3 × 10³⁵ ergs.