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Analysis and modeling of high temporal resolution spectroscopic observations of ﬂares on AD Leonis

We report the results of a high temporal resolution spectroscopic monitoring of the ﬂare star AD Leo. During 4 nights, more than 600 spectra were taken in the optical range using the Isaac Newton Telescope (INT) and the Intermediate Dispersion Spectrograph (IDS). We observed a large number of short and weak ﬂares occurring very frequently (ﬂare activity > 0.71 h − 1 ). This is consistent with the very important role that ﬂares can play in stellar coronal heating. The detected ﬂares are non white-light ﬂares and, although most solar ﬂares are of this kind, very few such events have been observed previously in stars. The behaviour of di ﬀ erent chromospheric lines (Balmer series from H α to H 11 , Ca ii H & K, Na i D 1 and D 2 , He i 4026 Å and He i D 3 ) was studied in detail for a total of 14 ﬂares. We estimated the physical parameters of the ﬂaring plasma by using a procedure that assumes a simpliﬁed slab model of ﬂares. All the obtained physical parameters are consistent with previously derived values for stellar ﬂares, and the areas – less than 2.3% of the stellar surface – are comparable with the size inferred for other solar and stellar ﬂares. We studied the relationships between the physical parameters and the area, duration, maximum ﬂux and energy released during the detected ﬂares.