Probing the NaI D and KI lambda 7699 resonance lines sensitivity to background opacity in late-type stars

We have measured the equivalent width WK Of the K I resonance line at 7699 Angstrom for a large sample of low activity late-type stars observed with h igh spectral resolution and we have verified that the relation W-K vs. T-ef f is monotonically decreasing, for both dwarf and giant stars. This behavio ur is different from that of the Na I D lines for stars of the same type, w hich showed that the relation W-Na vs. T-eff has a maximum for T-eff simila r to 4000 K, which is better defined for giants than for dwarfs (Tripicchio et al. 1997). The fit of the observed KI equivalent widths by means of a NLTE spectral li ne synthesis using conventional background opacity shows that, for dwarf st ars, the adopted models overestimate the observed WK for temperatures less than or similar to 4000 K. This result is similar to that discussed for the Na I D lines in our previous paper. On the other hand, for giant stars wit h T-eff less than or similar to 3800 K these models in general underestimat e WK. The discrepancies between observed and computed WK and WNa for cool stars a re much stronger than the variations due to uncertainties in either atmosph eric model or line parameters, like effective temperature and surface gravi ty, or Van der Waals broadening. For M dwarf stars, the most convincing exp lanation for the disagreement is the lack of atomic and molecular line opac ity 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 th e Na I D and K I lines, in a subsample of early-hi dwarfs.