  We have re-analyzed using a hydrodynamic model large flaring events on three
different categories of pre-main sequence (PMS) stars: the young stellar object
(YSO) YLW 15, the classical T Tauri star (CTTS) LkHalpha92, the weak-line T
Tauri star (WTTS) V773 Tau, and the WTTS HD 283572 (the first three objects
were observed by ASCA, the last by ROSAT; all have been previously reported in
the literature). The first three flares were previously analyzed on the basis
of the quasi-static model mostly used up to now, consistently yielding large
loops (L >= R*) and no evidence of sustained heating. Our hydrodynamic modeling
approach, however, shows that the size of the flaring regions must be much
smaller (L <=R*) and moreover this method shows in all cases evidence of
vigorous sustained heating during the flare decay, so that the decay of the
observed light curve actually reflects the temporal profile of the heating
rather than that of the free decay of the heated loop(s). The events on the
protostar YLW 15 have durations comparable to the stellar rotation period, so
that their limited size and their lack of self-eclipses give evidence 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 region between
the star and the accretion disk. In general, the present analysis shows that
flaring coronae on PMS stars have a structure similar to the coronae on older
active stars.
