Big X-ray Flares from Baby Stars

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Pre-main-sequence (PMS) stars are known to produce powerful X-ray flares, which resemble magnetic reconnection solar flares scaled by factors up to 10^4. However, there are numerous puzzles, including the structure of X-ray emitting coronae and magnetospheres, the effects of protoplanetary disks, and the effects of stellar rotation. To investigate these issues in detail, we examine 216 of the brightest flares from 161 PMS stars observed in the Chandra Orion Ultradeep Project (COUP). These constitute the largest homogeneous data set of PMS, or indeed stellar flares at any stellar age, ever acquired. Our effort is based on a new flare spectral analysis technique that avoids nonlinear parametric modeling. Several results are obtained. First, COUP flares are among the most powerful, longest, and hottest, corresponding to the largest known stellar coronal structures. Second, the most important finding is that X-ray coronal extents in fast rotating disk-free stars can significantly exceed the Keplerian corotation radius, whereas X-ray loop sizes in disky and accreting systems do not exceed the corotation radius. The former indicates that magnetic structures are capable of withstanding the effects of centrifugal forces; while the latter is consistent with models of star-disk magnetic interaction in which the inner disk truncates and confines the PMS stellar magnetosphere. Third, the sub-class of super-hot flares with peak plasma temperature exceeding 100 MK appear preferentially in accreting systems; and flares from accreting systems are tentatively found to have shorter durations. These results may be consequences of the distortion and destabilization of the stellar magnetosphere by the interacting disk. Finally, we find no evidence that any flare types, even slow-rise top-flat flares, are produced in star-disk magnetic loops. All are consistent with enhanced solar "long-duration events" with both footprints anchored in the stellar surface.

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