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Description
This table contains (some of) the results from an X-ray imaging survey of the young cluster NGC 2264, carried out with the European Photon Imaging Cameras (EPIC) on board the XMM-Newton spacecraft. XMM-Newton EPIC observations were made separately of the northern and southern portions of NGC 2264 on 2001 March 20 and 2002 March 17 - 18, respectively. Details concerning the two pointings are summarized in Table 1 of the reference paper. The nominal integration time was 42 ks for both observations. The three EPIC cameras were operated in full window mode. To prevent contamination of the X-ray images by the XUV and EUV emission of the optically and UV-bright sources in the field of view, the thick filter was used, which imposes a strong cut-off in the response at the lower energies. The X-ray data are merged with extant optical and near-infrared photometry, spectral classifications, H-alpha emission strengths, and rotation periods to examine the interrelationships between coronal and chromospheric activity, rotation, stellar mass, and internal structure for a statistically significant sample of pre-main-sequence stars. Out of the 316 distinct point-like sources that were detected at >= 3-sigma levels in one or more of six EPIC images, a total of 300 distinct X-ray sources can be identified with optical or near-infrared counterparts. The sources are concentrated within three regions of the cluster: in the vicinity of S Mon, within the large emission/reflection nebulosity southwest of S Mon, and along the broad ridge of molecular gas that extends from the Cone Nebula to the NGC 2264 IRS 2 field. From the extinction-corrected color-magnitude diagram of the cluster, ages and masses for the optically identified X-ray sources are derived. A median age of ~ 2.5 Myr and an apparent age dispersion of ~ 5 Myr are suggested by pre-main-sequence evolutionary models. The X-ray luminosity of the detected sources appears well-correlated with bolometric luminosity, although there is considerable scatter in the relationship. Stellar mass contributes significantly to this dispersion, while isochronal age and rotation do not. X-ray luminosity and mass are well correlated such that L<sub>X</sub> ~ (M/M<sub>solar</sub>)<sup>1.5</sup>, which is similar to the relationship found within the younger Orion Nebula Cluster. No strong evidence is found for a correlation between E(H-K), the near-infrared color excess, and the fractional X-ray luminosity, which suggests that optically thick dust disks have little direct influence on the observed X-ray activity levels. Among the X-ray-detected weak-line T Tauri stars, the fractional X-ray luminosity, L<sub>X</sub>/L<sub>bol</sub>, is moderately well correlated with the fractional H-alpha luminosity, L<sub>H(alpha)</sub>/L<sub>bol</sub>, but only at the 2-sigma level of significance. The cumulative distribution functions for the X-ray luminosities of the X-ray-detected classical and weak-line T Tauri stars within the cluster are comparable, assuming the demarcation between the two classes is at an H-alpha equivalent width of 10 Angstroms. However, if the non-detections in X-rays for the entire sample of H-alpha emitters known within the cluster are taken into account, then the cumulative distribution functions of these two groups are clearly different, such that classical T Tauri stars are underdetected by at least a factor of 2 relative to the weak-line T Tauri stars. Examining a small subsample of X-ray-detected stars that are probable accretors based on the presence of strong H-alpha emission and near-infrared excess, the authors conclude that definitive non-accretors are ~ 1.6 times more X-ray luminous than their accreting counterparts. In agreement with earlier published findings for the Orion Nebula Cluster, the authors find a slight positive correlation (valid at the 2-sigma confidence level) between L<sub>X</sub>/L<sub>bol</sub> and the rotation period in NGC 2264 stars. The lack of a strong anti-correlation between X-ray activity and rotation period in the stellar population of NGC 2264 suggests that either the deeply convective T Tauri stars are rotationally saturated or that the physical mechanism responsible for generating magnetic fields in pre-main-sequence stars is distinct from the one that operates in evolved main-sequence stars. This table was created by the HEASARC in September 2007 based on the electronic version of Table 2 from the reference paper which was obtained from the electronic AJ site. This is a service provided by NASA HEASARC .
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