X-ray flares, coronae and disks in Orion young stars
Konstantin Getman (Penn State University) , E. Feigelson (PSU), P. Broos (PSU), G. Garmire (PSU)
Pre-main sequence (PMS) stars are known to produce powerful X-ray flares which resemble magnetic reconnection solar flares scaled by factors upto 10^4. Several puzzles are present: the stability of implied magnetic loops 10 times the stellar radius; possible magnetic loops extending to the protoplanetary disk; and the origin of slow-rise flares. To investigate these issues in detail, we examine >200 of the brightest flares from >160 COUP stars which constitute the largest known homogeneous dataset of PMS flares ever acquired. We use an innovative method to trace the evolution of the flare plasma from an adaptively smoothed X-ray median energy of flare counts, and standard solar flare models to derive sizes of flaring coronal structures. We classify COUP flares into several morphological groups including "typical" rapid-rise-slow-decay and "slow-rise-and/or-top-flat" (SRTF). Rise and decay times range from hours to >1day, peak luminosities span 10^30.5-10^32.9erg/s range, and peak plasma temperatures - 20->100MK. For 800f the flares, inferred coronal loop sizes span 0.5-6Rstar while the remainder have even larger loops. Two main results are obtained. First, we find evidence that the coronal extent of PMS stars with inner circumstellar disks does not exceed the Keplerian corotation radius. In contrast, 300f stars without inner disks have coronas extending upto two corotation radii. This supports the model of Jardine et al. (2006) for coronal stripping by circumstellar disks. However, there is no relationship found between flare morphology and an inner disk indicative that star-disk field lines produce distinctive flare types. Second, our analyses indicate that the rapidly accreting PMS stars lack long-lasting and morphologically complex flare events such as SRTFs, events which could involve multiple flares from the entire coronal magnetosphere. We speculate that the progression of magnetic reconnection in multipolar magnetospheres ceases when it reaches an accretion hotspot with mass loaded coronal loops.