Probing the Na I D and K I 7699 resonance lines sensitivity to background opacity in late-type stars *
A. Tripicchio 1,
M.T. Gomez 1,
G. Severino 1,
E. Covino 1,
R.J. GarcíaLópez 2 and
L. Terranegra 1
Received 16 November 1998 / Accepted 19 March 1999
We have measured the equivalent width of the K I resonance line at 7699 Å for a large sample of low activity late-type stars observed with high spectral resolution and we have verified that the relation vs. 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 vs. has a maximum for 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 line synthesis using conventional background opacity shows that, for dwarf stars, the adopted models overestimate the observed for temperatures K. This result is similar to that discussed for the NaI D lines in our previous paper. On the other hand, for giant stars with K these models in general underestimate .
The discrepancies between observed and computed and 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.
Key words: stars: late-type stars: atmospheres line: formation radiative transfer line: profiles
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© European Southern Observatory (ESO) 1999
Online publication: April 28, 1999