Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...302..861h&link_type=abstract
Astronomy and Astrophysics, v.302, p.861
Astronomy and Astrophysics
Astrophysics
31
Radiative Transfer, Stars: Abundances, Stars: Chromospheres, Stars: Magnetic Fields, Stars: Late-Type
Scientific paper
In magnetically active stars, non-thermal 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 photospheric lines", we investigate the influence of the chromosphere on the Lithium I lines in M dwarfs. We choose this element because LiI 6708A is often used as an age indicator. Solving the NLTE-radiation transfer problem for a grid of model atmospheres that mimic the effect of magnetic activity (from basal to saturated chromospheres), we show that the Lithium lines are in fact activity sensitive. This dependence commences at rather high activity levels, i.e. when Halpha_ 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 non-photospheric 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 yield a similar effect. Therefore, great care must be exercised in the use of Li 6708A in both abundance and age calculations for active stars. We found that the 6708A to 6104A line ratio is less sensitive to magnetic activity than the line equivalent widths and should be a better age indicator.
Doyle Gerry J.
Houdebine E. R.
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