Modeling the long duration rise phase of a flaredetected on the M star TWA 11B

摘要

We present preliminary results from the analysis of a long duration flare that wasobserved in a serendipitous XMM-Newton detection of the M star CD-39 7717B (TWA 11B),member of the young stellar association TW Hya. Only the rise phase (with a duration of ti 35 ks)and possibly the flare peak are present in the light-curve. The decay phase was not monitored.The fluorescent iron emission line at 6.4 keV was observed during this event. As far as we areconcerned, since TWA 11B seems to have no disk, this is only the third detection of Fe photosphericfluorescence. During the flare, the X-ray flux increased a factor of 4. Taking the light curve andthe evolution of the hardness ratio into account, we interpret the rise phase as resulting from theignition of a first group of loops (part 1) which triggered a subsequent two-ribbon flare (part 2). Part1 is analysed using the Reale's (2007) model, since it has been assumed to be dominated by a singleloop. For part 2, the diagnostic method for two-ribbon flares developed by Kopp & Poletto (1984)is applied. Loop semi-lengths of about 2.5 — 3.0 stellar radii are obtained. These large structureswere previously detected only in very young objects, this is the first time that are detected in a moreevolved8 Myr) star.