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K. G. Stassun (Vanderbilt Univ.), D. R. Ardila (Johns Hopkins Univ.), M. Barsony (San Francisco State Univ.), G. Basri (U.C. Berkeley), R. D. Mathieu (Univ. Wisconsin)
We present an analysis of archival Chandra 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. We find that these stars have LX/Lbol near, but below, the ``saturation" value of 10-3, and that X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing LX/Lbol 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 LX. 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 LX. 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. We also present preliminary results from a simultaneous optical/X-ray monitoring study of variability in the ONC, where we find very little evidence for correlated variability, indicating that X-ray events are not temporally related to accretion events. These findings suggest that X-rays from PMS stars have their origins primarily in chromospheres/coronae, not accretion.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.