Quiescent and flare analysis for the chromospherically active star Gl 355 (LQ Hya)

We discuss ROSAT and ASCA observations of the young active star Gl 355. Dur ing the ROSAT observation a strong are was detected with a peak flux more t han an order of magnitude larger than the quiescent level. Spectral analysi s of the data allows us to study the temperature and emission measure distr ibution, and the coronal metal abundance, for the quiescent phase and, in t he case of ROSAT, also during the evolution of the are. The global coronal metallicity Z/Z(.) similar to 0.1 derived from both ROSAT and ASCA data is much lower than solar and presumably also much lower than the photospheric abundance expected for this very young star. The temperature structure of t he quiescent corona was about the same during the various observations, wit h a cooler component at T-1 similar to 7 MK and a hotter component (to whic h only ASCA was sensitive) at T-2 similar to 20 MK. During the are, the low temperature component remained approximately constant and equal to the qui escent value, while the high-temperature component was the only one that va ried. We have modeled the are with the hydrodynamic-decay sustained-heating approach of Reale at al. (1997) and we have derived a loop semi-length of the order of similar to 1.5 stellar radii, i.e. much larger than the dimens ions of flares on the Sun, but comparable with the typical dimensions infer red for other stellar flares. We have compared the derived loop size with t hat estimated with a simpler (but physically inconsistent) approach, findin g that for this, as well for several other stellar flares, the two methods give comparable loop sizes. Possible causes and consequences of this result are discussed.