THE NA I LAMBDA LAMBDA 5890,5896 RESONANCE DOUBLET AS CHROMOSPHERIC DIAGNOSTICS IN M DWARFS

The Nai D lines at 5890/5896 A are very prominent features in the spectrum of late-type stars. Nevertheless, little attention has been devoted to the potential use of those lines as chromospheric diagnostics. As a case study, we explore the dependence on chromospheric activity of the D lines in a star with Teff = 3700 K, logg =4 :7 and solar metallicity. The re- sults are compared with the better studied hydrogen spectrum. We nd that the D lines seem to be a promising diagnostic of the lower-middle chromosphere, that can complement the in- formation given by lines like H. We also nd that, for detailed quantitativestudies,itisnecessarytoincludeapropertreatment ofthebackgroundopacities.Lessimportant,instead,istheneed ofcarefultreatmentoftransitionsinducedbycollisionswithhy- drogen atoms. Finally, our calculations make it clear that, for the most active stars, the level of coronal emission should also be taken into account.