Rubidium abundances of galactic disk stars
  Spectroscopic determinations of Rubidium abundances were conducted by
applying the spectrum fitting method to the Rb I 7800 line for an extensive
sample of ~500 late-type dwarfs as well as giants (including Hyades cluster
stars) belonging to the galactic disk population, with an aim of establishing
the behaviour of [Rb/Fe] ratio for disk stars in the metallicity range of
-0.6<[Fe/H]<+0.3. An inspection of the resulting Rb abundances for Hyades
dwarfs revealed that they show a systematic Teff-dependent trend at >5500K;
this means that the results for mid-G to F stars (including the Sun) are not
reliable (i.e., more or less overestimated), which might be due to some
imperfect treatment of surface convection in classical model atmospheres. As
such, it was decided to confine only to late-G and K stars at Teff<5500K and
adopt the solar-system (meteoritic) value as the reference Rb abundance. The
[Rb/Fe] vs.[Fe/H] relations derived for field dwarfs and giants turned out to
be consistent with each other, showing a gradual increase of [Rb/Fe] with a
decrease in [Fe/H] (with d[Rb/Fe]/d[Fe/H] gradient of ~-0.4 around the solar
metallicity), which is favourably compared with the theoretical prediction of
chemical evolution models. Accordingly, this study could not confirm the
anomalous behaviour of [Rb/Fe] ratio (tending to be subsolar but steeply
increasing toward supersolar metallicity) recently reported for M dwarf stars
of -0.3<[Fe/H]<+0.3.
