Determination of Potassium Abundances for Giant and Dwarf Stars in the
  Galactic Disk
  An extensive study on the potassium abundances of late-type stars was carried
out by applying the non-LTE spectrum-fitting analysis to the K I resonance line
at 7698.96A to a large sample of 160 FGK dwarfs and 328 late-G /early-K giants
(including 89 giants in the Kepler field with seismologically known ages)
belonging to the disk population (-1 < [Fe/H] < 0.5), which may provide
important observational constraint on the nucleosynthesis history of K in the
galactic disk. Special attention was paid to clarifying the observed behaviors
of [K/Fe] in terms of [Fe/H] along with stellar age, and to checking whether
giants and dwarfs yield consistent results with each other. The following
results were obtained. (1) A slightly increasing tendency of [K/Fe] with a
decrease in [Fe/H] (d[K/Fe]/d[Fe/H] ~ -0.1 to -0.15; a shallower slope than
reported by previous studies) was confirmed for FGK dwarfs, though thick-disk
stars tend to show larger [K/Fe] deviating from this gradient. (2) Almost
similar characteristics was observed also for apparently bright field giants
locating in the solar neighborhood (such as like dwarfs). (3) However, the
[K/Fe] vs. [Fe/H] relation for more distant {\it Kepler} giants shows larger
scatter and is systematically higher (by <~0.1dex) than that of dwarfs,
implying that chemical evolution of K is rather diversified depending on the
position in the Galaxy. (4) Regarding the age-dependence, a marginal trend of
increasing [K/Fe] with age is recognized for dwarfs, while any systematic
tendency is not observed for Kepler giants. These consequences may suggest that
evolution of [K/Fe] with time in the galactic disk does exist but proceeded
more gradually than previously thought, and its condition is appreciably
location-dependent.
