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: http://star.arm.ac.uk/~ambn/abstract173.html
Дата изменения: Fri May 31 18:25:33 1996
Дата индексирования: Tue Oct 2 00:08:48 2012
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Поисковые слова: mercuryprogram
In this new series we present observations and modelling of spectral lines
formed in main sequence star chromospheres (from G0 to M8). in this paper,
we discuss the Hydrogen line formations in very active dMe stellar chromospheres. We present the conclusions drawn from our modelling
approach to the AU Mic (dM2e) spectrum. Our final model atmosphere is
described in the next paper in this series.
Here, we investigate the effects of some stellar (T$_{eff}$ v sin i) and
atmospheric (turbulence, temperature structure) parameters on the line
formation. We found that observations are best reproduced by a chromospheric
structure including a constant temperature gradient (in a log(M) scale) in the
chromosphere and transition region. We show that a very thin transition
region is required to account for the observed Ly$\alpha$ to H$\alpha$ surface
flux ratio. Then, to drive the Balmer lines into emission and reproduce the
H$\alpha$ and H$\beta$ self-reversal and FWHM, the chromospheric gradient and
transition region pressure must be quite high for the most active stars.
We put forward a complete set of important constraints on the possible
structures of such active region atmospheres from 5,000 K to 50,000 K. Notably
we show that: (i) the chromosphere/transition region temperature break zone must
be located at about 8,200 $\pm$ $\sim$200 K, implying that there is rather
little plasma above this temperature, (ii) the temperature break zone is
central to the formation of Lymna and Balmer lines, it should be smooth and
contained in a small column mass domain, (iii) the transition region must
be very thin and at a high column lass, {\it log (M)} $\sim$ -3, (iv) the
temperature break demarcates the temperature domains of formation of Lyman and
other Hydrogen series (v) backwarming due to the Lyman and Balmer radiation
fields occurs at the top of the chromosphere, (vi) turbulence and rotational
broadening have little effect on the line profiles and fluxes, (vii) rotational
broadening should not however be neglected because H$\alpha$ self-reversal is
a major constraint to the modelling, (viii) the profiles are weakly dependant
on the stellar effective temperature whose uncertainties can be neglected in
first approximation. Finally, we further compare our results to observations
and propose some general properties that could be tested with future observations.