2003A&A...397.1019H

We investigate the spectral signatures of stellar flares in the wavelength range 3600Å to 4500Å and in broad band photometry. We study the phenomenology of the spectral signatures and we found that flares are best described by two main phases; an impulsive phase and a gradual phase, for which the physical properties are different. Important spectral differences between flares lead us to distinguish four main classes: (i) solar-like chromospheric or two-ribbon flares, (ii) white-light flares, (iii) combined white-light flares with distinct impulsive and gradual phases, and (iv) non solar-like flares (usually occuring on RS CVn type stars). We show how this classification corresponds to substantial differences in the physical properties of the flare components. We compiled all available spectroscopic data for stellar flares. We found several new empirical correlations between the time lags in the spectral lines (rise and decay times). We found for instance that during the gradual phase, the rise time in the H_γ line is well correlated to the rise time in the Caii K line, and that the Caii K line is 1.63 times slower to rise than the H_γ line. Similar correlations were found between the rise and decay times in these lines. These correlations are evidence that there is a dominant mechanism commanding the temporal flux evolutions during the gradual phase of flares. This mechanism applies on time scales ranging from one minute to more than a hundred minutes. We found correlations between the time lags and the maximum fluxes in the Johnson U-band and the spectral lines. These correlations show that the larger the flare the longer it takes to evolve. We show that the maximum flux in the Johnson U-band correlates well, but not linearly with the maximum flux in the H_γ line during the impulsive phase, and over five orders of magnitude. We argue how this correlation can provide constraints on the currently available models. The spectral line maximum fluxes during the gradual phase also correlate with the Johnson U-band flux, which demonstrates that somehow the impulsive and gradual phases are physically linked. The correlation between the H_γ and Caii K line fluxes during the gradual phase and the lack of dependence of the flux ratio of those lines on flare magnitude reveal that larger flares are not ``hotter'' than smaller flares during the gradual phase. We examine the behaviour of the spectral line widths: While the Caii K line width shows essentially no detectable variation, the Balmer line widths show a complex dependency on the white light intensity. We distinguish three energy domains where the Balmer line widths exhibit a different behaviour: (i) for small flares, the widths remain rather constant as a function of white light intensity, which suggests that in this energy range the U-band is not a good diagnostic of the total energy release and that dense ``kernels'' do not dominate the Balmer emission, (ii) for medium size flares, the widths rapidly increase with the U-band flux up to about 15Å, which indicates that emission in the Balmer lines is then dominated by kernel emission, and (iii) for large flares the widths decrease with increasing U-band flux, which suggests that the Balmer line emission is then increasingly dominated by a radiative pumping process between the white light and the Balmer lines. Finally, we remark that the Balmer line fluxes are well correlated to the white light flux during the impulsive phase of flares, and not only at flare maximum. We obtain a power law correlation between the Balmer line and U-band fluxes that is evidence for either a common source (``kernels'') or strongly related flare components.