Rubidium abundances of galactic disk stars

Spectroscopic determinations of rubidium abundances were made 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 the aim of establishing the behavior 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 T_eff dependent trend at ≳ 5,500 K; 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 T_eff ≲ 5,500 K and adopt the solar system (meteoritic) value as the reference Rb abundance. The [Rb/Fe] versus [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 compares favorably with the theoretical prediction of chemical evolution models. Accordingly, this study could not confirm the anomalous behavior of [Rb/Fe] ratio (tending to be subsolar but steeply increasing towards supersolar metallicity) recently reported for M dwarf stars of -0.3 ≲ [Fe/H] ≲ +0.3.