Coronal structure geometries on pre-main sequence stars

Using a hydrodynamic model we have re-analyzed large flaring events on thre e different categories of pre-main sequence (PMS) stars: the young stellar object (YSO) YLW 15, the classical T Tauri star (CTTS) LkH alpha 92, the we ak-line T Tauri star (WTTS) V773 Tau, and the WTTS HD 283572 (the first thr ee objects were observed by ASCA, the last by ROSAT; all observations have been previously reported in the literature). The first three flares were pr eviously analyzed on the basis of a quasi-static model mostly used up to no w, consistently yielding large loops (L greater than or similar to R-*) and no evidence of sustained heating. Our hydrodynamic modeling approach, howe ver, shows that the size of the flaring regions must be much smaller (L les s than or similar to R-*) and moreover this method shows in all cases evide nce of vigorous sustained heating during the are decay, so that the decay o f the observed light curve actually reflects the temporal profile of the he ating rather than that of the free decay of the heated loop(s). The events on the protostar YLW15 have durations comparable to the stellar rotation pe riod, so that their limited size and their lack of self-eclipses give evide nce of a polar location on the star. This is in contrast with the recently advanced hypothesis that these flares are due to long loops spanning the re gion between the star and the accretion disk. In general, the present analy sis shows that flaring coronae on PMS stars have a structure similar to the coronae on older active stars.