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Observation and modelling of main sequence star
chromospheres. IV. The chromospheric contribu­
tion to Li I lines in active dwarfs
E.R. Houdebine and J.G. Doyle
In magnetically active stars, non­thermal heating and magnetic fields result in
inhomogeneous outer atmospheres (plages) and photospheres (dark and bright
spots. So as to gauge the importance of these phenomena on ``normal photo­
spheric lines'', we investigate the influence of the chromospheric on the Lithium
I lines in M dwarfs. We choose this element because Li I 6708 љ Ais often used as
an age indicator.
Solving the NLTE­radiation transfer problem for a grid of model atmospheres
that mimic the effect of magnetic activity (from basal to saturated chromo­
spheres), we show that the Lithium lines are in fact activity sensitive. This
dependence commences at rather high activity levels, i.e. when Hff goes into
emission. As in the solar case, this chromospheric plage effect weakens the line
equivalent widths and may cancel the photospheric spot effect on disc integrated
measurements.
The ionisation balance and line formation remain photoionisation controlled
throughout the known pressure range in M star chromospheres. Changes in
the line profiles and equivalent widths are due to the non­photospheric UV
continuum that further ionises neutral lithium and depletes the line optical
depths, whereas in solar plages collisional ionisation is the driving mechanism.
In K dwarfs, both would compete to yeild a similar effect. Therefore, great care
must be exercised in the use of Li 6708 љ Ain both abundance and age calculations
for active stars. We found that the 6708 љ Ato 6104 љ Aline ratio is less sensitive to
magnetic activity than the line equivalent widths and should be a better age
indicator.
Key words: radiative transfer ­ stars: lithium abundance ­ stars: chromo­
spheres ­ stars: magnetic activity ­ stars: late­type
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