Stellar X-ray Emission in the Orion Nebula Region

We present results from two comprehensive imaging X-ray surveys of the Orion Nebula star-forming complex. We have analyzed three deep ROSAT High Resolution Imager (HRI) pointings of a 0.8 square degree region around the Trapezium. In addition, we have completed our analysis of 17 Einstein Imaging Proportional Counter and 6 Einstein HRI fields covering roughly 4.5 square degrees, also centered on the Trapezium. In total, more than 450 distinct sources of emission brighter than L_X ~ 10(30) erg s(-1) have been detected; ~ 320 of these have been identified with certain or probable cluster members. The vast majority of the X-ray sources are associated with intermediate and low-mass pre-main sequence (PMS) cluster members. No correlation between X-ray activity and rotation is evident among the handful of stars with measured rotational velocity and/or period. The low-mass PMS stars show signs of saturated X-ray activity, with relative X-ray luminosity L_X/L_bol ~ 10(-3) . We compare these results with those obtained in other star-forming regions such as rho Ophiuchi, Taurus-Aurigae, and Chamaeleon I. Nearly all the O and B-type stars observed by ROSAT were detected with L_X/L_bol in the range 10(-7.3) to 10(-5.3) . Also detected by ROSAT are 5 sources whose only candidate optical counterparts are main-sequence B6-A5 stars. These stars are thought to be incapable of producing detectable X-ray emission. Since only ~ 20% of the observed B6-A5 stars are detected, we argue that the observed emission is probably not produced by the B6-A5 stars but, rather, in the coronae of unseen late-type companions. An X-ray light-curve analysis has been performed on both the Einstein and ROSAT data sets, revealing a total of ~ 25 flaring events. Over the course of the Einstein and ROSAT observations, emission from a number of sources was measured many times, allowing us to examine long-term variability on the time scale of a few months between individual Einstein or ROSAT exposures and on the time scale of a decade between the two missions. Research support has been provided by NASA through grants NAGW-2698, NAG5-1608, and NAG5-1610.