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Optical photometric data were taken of fields encompassing all these sources during a total of seven telescope runs. The facilities used were the Kitt Peak National Observatory (KPNO), the Cerro Tololo Inter--American Observatory (CTIO), the Lowell Observatory, the Florence and George Wise Observatory (WISE) and the Mount Stony Brook Observatory (MTSB; see Appendix for details.) With the exception of the CTIO run and part of the KPNO run, photometric conditions were not encountered. The CCD detectors used were Photometrics 512 and 1K arrays, two Tektroniks 2K arrays and a Santa Barbara Instrument Group ST--6 camera. Johnson U, B, V and Kron--Cousins R and I band filters were used. These details are summarized in Table 3.
The initial goal of the photometric observations is to follow up the X--ray sources, assign them colors in a standard system, so that the observed objects can be placed on a color--magnitude diagram. This can be used to verify that the X--ray sources are indeed PMS stars. In many cases, more than one star falls within the X--ray error circle (nominally about 15). The photometric data are very useful in selecting out the probable X--ray source from among the several possibilities. The optical colors can be combined with infrared and spectral data so that the stars can be placed on the H--R diagram. Once on the H--R diagram, they can be fit to evolutionary tracks so that ages and masses can be measured for individual sources.
Raw CCD frames were debiased and flat fielded using the CCDRED package in IRAF. In addition, for the data taken with the Photometrics detectors, a dark correction was applied between the debiasing and the flatfielding. Aperture photometry was performed using the APPHOT package in IRAF. The apertures used varied from run to run due to changes in the seeing and focus at the individual sites. The apertures averaged about four arcseconds in radius. Sky background was measured in an annulus with a five arcsecond inner radius and a seven arcsecond outer radius. The product of this phase of the reductions was an instrumental magnitude for every star of interest.
The instrumental magnitudes were converted to the standard Johnson (U, B, V; 1963) Kron--Cousins(R, I; Cousins 1980) system using the IRAF/PHOTCAL package. Data used for these observations were taken from only the three best photometric nights: 8 December 1992, 29 January 1996 and 2 February 1996. For target stars, there were usually two observations made in all filters, except for the U band in which only one observation was made due to time constraints. Calibration stars were taken from Landolt (1992). Standard star field observations were made approximately once every 90 minutes. Airmasses of standard stars varied between 1.1 and 2.4. Airmasses of target stars were between 1.1 and 1.3 for calibrated observations. Since the Landolt fields are very dense in standard stars, there were more than 100 standard measurements made in each filter on each night. Internal errors were less than 0.01 magnitudes. Sky variability limited the overall accuracy of the data. Overall errors were about 0.02 magnitude in the V band at 16th magnitude in a two--minute exposure. Errors were somewhat higher in the B and U bands.
The measured colors and magnitudes for all X--ray sources observed in Orion OB1b (stars nears Orionis) and OB1a (stars NW of the belt) are listed in Tables 4 through 7. The errors given in these tables are the deviation of the standards to the fit at the magnitude of the target star. If more than one observation was made, the standard deviations of the mean of the measurements is listed instead. In most cases, this latter value is much larger than the deviation of the fit and gives an indication of the intrinsic variability of the star.
In Figures 1 and 2, the V magnitude is plotted against the R-I color for stars near Orionis and those northwest of the belt respectively. A line representing the location of the main sequence at 380 pc has been added to the figure. The bulk of the X--ray sources lie above this line. These are probably PMS stars (assuming that they are at 380 pc). Other objects in this plot are most likely background or foreground objects which were within the error ellipse of the X--ray source. Overall, about 85% of the X--ray sources with measurable V, R and I band fluxes appear to be PMS stars. In about ten cases, there were multiple targets with V>16 inside the error circle. In these cases, both objects are plotted on the H-R diagram. Although the extinctions have not been calculated for the X--ray sources shown in these figures, conclusions about the PMS nature of these sources is not effected since the reddening vector runs roughly parallel to the main sequence.