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Description
This catalog presents some of the results from 3 deep ROSAT High Resolution Imager (HRI) observations of the Orion Nebula star-forming region. The fields covered by the X-ray images contain over 1500 catalogued stars in a roughly 0.8 square degree region centered on the Trapezium. In all, 389 distinct X-ray sources were detected, at least two-thirds of which were associated with a single proper-motion cluster member. X-ray emission was detected from stars of all spectral types, from the massive O- and B-type components of the Trapezium to the coolest, low-mass pre-main-sequence (PMS) stars. In the research paper in which these data were presented (Gagne et al. 1995), the authors focussed primarily on X-ray emission from the late-type PMS stars. Of the ~100 late-type cluster members with measured spectral types, approximately three-fourths were detected; the authors derived X-ray luminosity upper limits for the remaining stars. They found that coronal X-ray emission appeared to turn on at around a spectral type of F6, with the upper envelope of activity increasing with decreasing effective temperature. The current database is a representation of Table 6 from Gagne et al. (1995) (notice that the data given in Tables 2 and 3 of this reference is included in the HEASARC database ORIONXRAY, q.v.) which lists X-ray and other data for 175 Orion stars for which spectral types, spectroscopic rotational velocities, and/or spot-modulated photometric rotational periods are available. The X-ray data (either detections or upper limits) are given in the form of X-ray luminosities log Lx and X-ray to bolometric luminosity ratios (log Lx/Lbol). The conversion factor from HRI counts to log Lx was derived for each star based on (i) an assumed 1 keV Raymond and Smith thermal spectrum, (ii) a distance of 440 pc, and (iii) a column density of 2 x 10<sup>21</sup> cm<sup>-2</sup> per magnitude of visual extinction A(V), where accurate A(V) values are used when available, or otherwise a moderate absorption of 0.25 magnitudes is assumed. Notice that, for stars not identified as candidate optical counterparts in Tables 2 and 3 of Gagne et al. (the HEASARC database ORIONXRAY), the derived upper limit to the X-ray luminosity usually corresponds to the 3 sigma upper limit to the observed count rate. For a handful of stars in the Trapezium region where a star was eliminated as a candidate optical counterpart, despite being within the source search circle, because a more likely counterpart had a smaller position offset, the upper limit corresponds to either half the observed X-ray source count rate or to 3 sigma, whichever is the highest. Such cases are indicated in this database by the presence of the string "NN" in the parameter 'Note'. This is a service provided by NASA HEASARC .
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