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We have made three ROSAT HRI observations of the Orion Nebula region, ranging in exposure time from $\sim$10 to $\sim$50 ksecs. Over 300 X-ray sources have been detected, including stars whose spectral types range from the hot, massive O stars to the coolest PMS stars. In this paper we discuss how the data have helped us address the problem of the late B, early A stars' X-ray emission.
About 15 of 20 O6-B5 stars and $\sim$6 of 25 B6-A5 stars which fell within our fields of view were detected. The fraction of detections for the former group was limited by the sensitivity threshold, since the L$_x$/L$_{Bol}$ ratios for the O6-B5 stars are consistent with those found in all other studies of X-ray emission from these stars. This emission is thought to arise from radiation-driven shocks in the strong stellar winds (Lucy 1982).
Numerous X-ray studies of the B6-A5 stars have now been done (Walter et al. 1988, Caillault \& Zoonematkermani 1989, Strom et al. 1990, Schmitt et al. 1993) that seem to indicate that these stars do emit X-rays, albeit at a low level. X-ray emission from low-mass companions (from coronae generated via a magnetic- dynamo [cf Rosner et al. 1985]) is certainly a possible explanation. However, there is little positive evidence to suggest that the X-ray detected B6-A5 stars are indeed binaries. Other possible explanations for their emission may be that shell burning of residual deuterium drives the requisite convection for dynamo action (Palla \& Stahler 1990) or that the emission is attributable in some other way to their extreme youth, e.g., they may be Herbig Ae/Be stars, whose activity is manifested at other wavelengths. We present our ROSAT data and examine whether these models adequately explain the observed X-ray emission.
This research was supported by NASA Grants NAG5-1608 \& NAG5-1610 to the University of Georgia.
