X-ray flares in orion young stars. I. Flare characteristics

Pre-main-sequence (PMS) stars are known to produce powerful X-ray flares, which resemble magnetic reconnection solar flares scaled by factors up to 10⁴. 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. We provide a catalog of derived properties and an electronic atlas for this unique collection of flares. The current study (Paper I) examines the flare morphologies, and provides a general comparison of COUP flare characteristics with those of other active X-ray stars and the Sun. Paper II will concentrate on relationships between flare behavior, protoplanetary disks, and other stellar properties. Several results are obtained. First, COUP flares are among the most powerful, longest, and hottest, corresponding to the largest known coronal structures. Second, no significant statistical differences in peak flare luminosity or temperature distributions are found among different morphological flare classes, suggesting a common underlying mechanism for all flares. Third, comparison with the general solar scaling laws indicates that COUP flares may not adequately fit proposed power-temperature and duration-temperature solar-stellar fits. Fourth, COUP superhot flares are found to be brighter but shorter than cooler COUP flares. Finally, the majority of bright COUP flares can be viewed as enhanced analogs of the rare solar "long-duration events."