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The upper chromosphere and lower transition re­
gion of dMe stars
A.C. Lanzafame
Models for the upper chromosphere and the lower transition region of dM0e­
dM2e stars are presented. The modelling procedure is based on a modification
of the semi­empirical approach useful for evaluating the effects of prescribed
transport processes to the energy balance. The statistical equilibrium equa­
tions include terms due to hydrogen ambipolar diffusion in order to investigate
its importance in the ionisation and energy balance. Several spectroscopic diag­
nostics have been used such as the hydrogen Hљ A and Lyљ A lines, Si II resonance
multiplets near 1814 љ A and 1264 љ A and the Mg II h& k lines. The analysis of
the Hљ A profile in the dMle star V1005 Ori (=Gl 182) indicates that the tem­
perature gradient in the lower transition region is smaller than solar. Since
the gas pressure is higher than solar, these conditions imply the validity of a
weekly non­uniform approximation (e.g., the Spitzer­HЁarm relation for the ther­
mal conductivity is valid in this case). Ambipolar diffusion is found ineffective
for the ionisation balance and therefore for the energy transport in the lower
transition region of dMe stars. This implies that, unlike the solar case, a sig­
nificant local dissipation of mechanical heating must be present to sustain the
observed radiative emission from the lower transition region.
Key words: Stars: atmospheres ­ Stars: chromospheres ­ Stars: late­type ­
Radiative transfer ­ Line: profiles
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