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Observation and modelling of main sequence star
chromospheres. IV. Ultraviolet excess emission
in active M dwarfs
E.R. Houdebine M. Mathioudakis, J.G. Doyle and B.H.
Foing
The variation in the continuum intensity (50­5000nm) of late­type M dwarfs
is investigated via two grids of model atmospheres with different temperature
minima. It is shown that the (E)UV intensity is mostly dependent on the
transition region pressure, although the temperature minimum also plays an
important role. We also observe a significant frequency redistribution of the
photospheric flux when changing the minimum temperature, and a black­body
type of emission from the lower chromosphere. We examine the formation of the
continuum and point out that, although some differences appear for very low or
very high activity levels, in general the global picture is much alike the Sun. We
show that the UV continua are very good diagnostics of cool dwarf atmospheres,
from the temperature minimum to the trasition region. Our calculations give a
good overview of the domains where physical parameters and spectral signatures
are most likely observed.
We compute the UBV(RIJKL) J broad band fluxes for our models and con­
clude that the chromospheric contribution should be detectable in the U band
and possibly also in the B band. We compare our calculations to recent high res­
olution observations for selected stars in a narrow spectral range ((R­I) џ =0.875\Sigma0.05);
we show that Hff line profiles behave as expected, with a tight correlation be­
tween the line width and equivalent width. Hff emission line stars show an
excess in U­B color but not in B­V. They are also more luminous than their less
active absorption line counterparts, which indicates that active dwarfs have not
yet reached the main sequence and are intermediate between T Tauri stars and
main sequence stars. The anomalously large proportion of active stars towards
late spectral types is attributed to the very slow contraction phase for low mass
stars.
We calculate the fluxes in the Extreme Ultra­Violet Explorer Sn/SiO band
(500­740 љ A) and show that they are compatible with observed upper limits. We
further compare our results for the upper activity range to pre­main sequence
models and observations. They strongly support the case for a chromospheric
contribution to Hff and the blue/UV excess for those objects (T Tauris, maked
T Tauris, YSOs).
An important conclusion is that at high pressures, corresponding to active
dMe stellar atmospheres, the chromosphere becomes a very efficient radiator at
continuum wavelenghts. From log(m)ё­5 (column mass), radiative losses in the
continuum rise exponentially and faster than in Hydrogen spectral lines because
of their larger optical depths. As a consequence, H I spectral lines have a small
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or negligible contribution to the total H I (lines and continua) and white light
radiative budget.
We show that the hydrogen series dominate the radiative cooling in the
spectral lines (form 40% to 90%) for Hff emission line stars. The cooling in
the (E)UV continuum overwhelms the total radiative budget and is much larger
than that in outstanding chromospheric and transition region lines (e.g. Ca II,
Mg II, H I Lyman and Balmer). We propose this as a possible contribution for
the apparent saturation observed in some spectral lines, therefore questioning
the suggestion of saturation in magnetic activity levels.
Key words: Stars: magnetic activity ­ Stars: late­type dwarfs ­ Stars: pre­
main sequence ­ Chromospheric modelling ­ Chromospheric and coronal heating
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