Membership of the Orion Nebula population from the Chandra Orion Ultradeep Project

The Chandra Orion Ultradeep project ( COUP) observation described in a companion paper by Getman et al. provides an exceptionally deep X-ray survey of the Orion Nebula Cluster and associated embedded young stellar objects. Membership of the region is important for studies of the stellar IMF, cluster dynamics, and star formation. The COUP study detected 1616 X-ray sources. In this study we confirm cloud membership for 1315 stars, identify 16 probable foreground field stars having optical counterparts with discrepant proper motions, and classify the remaining 285 X-ray sources, of which 51 are lightly and 234 heavily obscured. The 51 lightly obscured sources without known counterparts fall into three groups: (i) 16 are likely new members of the Orion Nebula Cluster; (ii) 2 with unusually soft and nonflaring X-ray emission appear to be associated with nebular shocks, and may be new examples of X-rays produced at the bow shocks of Herbig-Haro outflows; (iii) the remaining 33 are very weak uncertain sources, possibly spurious. Out of 234 heavily absorbed sources without optical or near-infrared counterparts 75 COUP sources are likely new embedded cloud members ( with membership for 42 confirmed by powerful X-ray flares), and the remaining 159 are likely extragalactic active galactic nuclei seen through the molecular cloud, as argued by a careful simulation of the extragalactic background population. Finally, a few new binary companions to Orion stars may have been found, but most cases of proximate COUP sources can be attributed to chance superpositions in this crowded field.