Dynamics of flares on late type dMe stars

Abstract

Abstract.We investigate the spectral signatures of stellar flares in the wavelength range 3600 Å to 4500 Å 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 Ca  K line, and that the Ca  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 Ca 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 Ca  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 15 Å, which in...

Multiwavelength observations of a giant flare on CN Leonis

“…This behaviour is compatible with what is normally observed for the Ca ii H and K lines, which decay more gradually than Balmer and Helium lines (e.g. Houdebine et al 1993), consistent with an interpretation of a faster evolution of high-temperature lines in the chromosphere (Houdebine 2003). The optical spectral data allowed us to trace the chromospheric reaction to the flare (with one He ii transition line covered).…”

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

“…Therefore, the gas that is heated and evaporated into the newly formed loop after magnetic reconnection (Cargill & Priest 1983;Forbes & Malherbe 1986) cools and reaches the formation temperature of the Balmer series before the one of the Ca ii H & K lines. Houdebine (2003) found that the rise and decay times in the Ca ii K and Hγ lines obey good relationships, which implies that there is a well-defined underlying mechanism responsible for the flux time profiles in these lines. Our results show that the duration of flares tends to be larger when the EWRQ max of the Balmer lines is greater.…”

Fluctuations and Flares in the Ultraviolet Line Emission of Cool Stars: Implications for Exoplanet Transit Observations

“…Within a single flare, multiple peaks and changes in the decay rate are possible. Some researchers have identified as flares events in which the stellar flux rises and fades more gradually (Houdebine 2003;Tovmassian et al 2003).…”