2004AJ....127.3537S

We reanalyze all archival Chandra/ACIS observations of the Orion Nebula Cluster (ONC) to study the X-ray properties of a large sample of pre-main-sequence (PMS) stars with optically determined rotation periods. Our goal is to elucidate the origins of X-rays in PMS stars by seeking out connections between the X-rays and the mechanisms most likely driving their production–rotation and accretion. Stars in our sample have L_X/L_bolnear, but below, the ``saturation'' value of 10<sup>–3</sup>. In addition, in this sample X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing L<sub>X</sub>/L<sub>bol</sub>with more rapid rotation. These findings suggest that stars with optical rotation periods are in the ``super-saturated'' regime of the rotation-activity relationship, consistent with their Rossby numbers. However, we also find that stars with optical rotation periods are significantly biased to high L_X. This is not the result of magnitude bias in the optical rotation-period sample, but rather of the diminishingly small amplitude of optical variations in stars with low L_X. Evidently, there exists in the ONC a population of stars whose rotation periods are unknown and that possess lower average X-ray luminosities than those of stars with known rotation periods. These stars may sample the linear regime of the rotation-activity relationship. Accretion also manifests itself in X-rays, though in a somewhat counterintuitive fashion: while stars with spectroscopic signatures of accretion show harder X-ray spectra than nonaccretors, they show lower X-ray luminosities and no enhancement of X-ray variability. We interpret these findings in terms of a common origin for the X-ray emission observed from both accreting and nonaccreting stars, with the X-rays from accreting stars simply being attenuated by magnetospheric accretion columns. This suggests that X-rays from PMS stars have their origins primarily in chromospheres, not accretion.