Astronomy and Astrophysics, volume 345, 915-924 (1999/5-3)
Probing the NaI D and KI λ7699 resonance lines sensitivity to background opacity in late-type stars.
TRIPICCHIO A., GOMEZ M.T., SEVERINO G., COVINO E., GARCIA LOPEZ R.J. and TERRANEGRA L.
Abstract (from CDS):
We have measured the equivalent width WK
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 WK
eff 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 WNa
eff has a maximum for T
eff∼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 WK
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 Teff
≲3800K these models in general underestimate WK
. The discrepancies between observed and computed WK
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.
stars: late-type - stars: atmospheres - line: formation - radiative transfer - line: profiles
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