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The stars examined here were primarily culled from two lists of X--ray sources in Orion. The objects in the lists were detected by pointed observations using the ROSAT X--ray satellite. The observations, made by Walter (1993, 1994, 1995), cover two regions in Orion. One region is six degrees in north--south aspect and ten degrees east--west, centered on the belt of Orion. The second region is two degrees in extent east--west and ten degrees north--south with Orionis being near the northern edge. X--ray satellites have been proven to be extremely efficient in finding PMS stars which would have gone unnoticed by other search techniques such as objective prism imaging (Walter 1986). Near the dark clouds of the Taurus--Auriga star forming region, it has been estimated that the nTTs (detected primarily via their X--ray flux) outnumber cTTs by a ratio of nearly 10 to 1 (Walter et al. 1988). More recent work in Taurus-Auriga gives similar (a ratio of 9:1) results and suggests an age dependence of the ratio (Neuhäuser et al. 1995).
The ROSAT point source lists provided over 800 X--ray sources with fluxes at least three above the background located in Orion north of M42. To limit this to a more tractable number, I chose to concentrate on the regions densest in X--ray sources. In this way, the observing program could be very efficient by observing many X--ray sources in every exposure. By selecting the best regions to monitor, I could measure rotational periods for the highest possible number of PMS stars. In the end, two main regions were chosen for monitoring: stars near Orionis and stars northwest of the belt of Orion. The group of sources associated with Orionis are within the OB1b association. It was chosen since it represented an extreme of compactness with over 50 sources in the central 25 arcminutes. The total area of this region is about 900 minutes of arc. This region was observed by ROSAT during 2 pointed observations. The first of these was 25 Kseconds of exposure time using the Position Sensitive Proportional Counter (PSPC). Supporting ribs obscure part of the PSPC's nominal 1 radius field of view. Problems caused by the obstruction are small, since the central field of view is clear and the ribs move relative to the sky due to spacecraft wobble. A larger problem with the PSPC is scattered light from the bright central X--ray source ( Orionis), which raises the local background and makes detection of faint sources difficult. A followup 15 Ksecond exposure was obtained by M. Freyberg (1994) using the High Resolution Imager (HRI), which had an unobstructed view of 25 arcminutes centered on Orionis and higher resolution. These data were processed using SASS processing and analyzed using IDL/RX software (Walter 1993). Source extractions were performed using the SEX procedure. Sixty--three sources were identified within 25 of Orionis.
While both PSPC and HRI data were available, the HRI was the primary source of targets, because it was much more complete near Orionis. The HRI detected 18 sources that PSPC missed. Sixteen of these sources are within 630 of Orionis, so they were probably missed due to scattered light. The PSPC identified 22 sources within 15 of Orionis in addition to the sources listed in Table 1, giving a total of 85 X--ray sources in the Orionis region. It is not surprising that the PSPC would detect additional sources since it is about 5 times more sensitive than the HRI. Of the 22 sources, 8 had no obvious optical counterpart and one was the bright star SAO 132412. This left 13 sources additional sources to be monitored. All the X--ray sources near Orionis with optical counterparts are listed in Table 1.
A second region to the northwest of the belt in the OB1a was also chosen because it was expected that this group would represent somewhat more evolved stars.
This area was initially observed by the PSPC using a 7 Ksecond exposure. The data were analyzed in a similar manner to those near Orionis. One PSPC field covers over 3000 arcminutes of field. This is too much to monitor with the same time resolution as the Orionis field. The area of analysis was limited to the 1350 arcminutes which was densest in X--ray sources. These sources are listed in Table 2