Issue |
A&A
Volume 397, Number 3, January III 2003
|
|
---|---|---|
Page(s) | 1019 - 1034 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20021537 | |
Published online | 21 January 2003 |
Dynamics of flares on late type dMe stars
IV. Constraints from spectrophotometry in the visible
Department of Physics (Theoretical Physics), Oxford University, 1 Keble Road, Oxford OX1 3NP, UK
Corresponding author: erh@thphys.ox.ac.uk
Received:
12
November
2001
Accepted:
1
August
2002
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 line is well correlated to the rise time
in the Ca ii K line, and that the Ca ii K line is 1.63 times
slower to rise than the H
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
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
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 Ca ii 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 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.
Key words: stars: flare / stars: activity / stars: late-type
© ESO, 2003
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