1999A&A...345..915T

We have measured the equivalent width W_K of the Ki resonance line at 7699Å for a large sample of low activity late-type stars observed with high spectral resolution and we have verified that the relation W_K vs. Teff is monotonically decreasing, for both dwarf and giant stars. This behaviour is different from that of the NaI D lines for stars of the same type, which showed that the relation W_Na vs. Teff has a maximum for Teff∼4000K, which is better defined for giants than for dwarfs (Tripicchio et al., 1997A&A...327..681T). The fit of the observed KI equivalent widths by means of a NLTE spectral line synthesis using conventional background opacity shows that, for dwarf stars, the adopted models overestimate the observed W_K for temperatures ≲4000K. This result is similar to that discussed for the NaI D lines in our previous paper. On the other hand, for giant stars with T_eff≲3800K these models in general underestimate W_K. The discrepancies between observed and computed W_K and W_Na for cool stars are much stronger than the variations due to uncertainties in either atmospheric model or line parameters, like effective temperature and surface gravity, or Van der Waals broadening. For M dwarf stars, the most convincing explanation for the disagreement is the lack of atomic and molecular line opacity in the adopted models. In fact, a NLTE spectral synthesis including an additional background opacity reproduces with a good level of accuracy the equivalent widths, as well as the general shape of the profiles for both the NaI D and KI lines, in a subsample of early-M dwarfs.