Non-LTE Effects in Rubidium Lines in Cool Stars

The formation of the rubidium resonance lines is considered by taking into account the effects of departures from local thermodynamic equilibrium (LTE). A rubidium model atom has been constructed using 29 Rb I levels and the Rb II ground level. Non-LTE calculations have been performed for a grid of model atmospheres with $T_{\textrm{eff}}$ from 3500 to 6500 K, $\log g$ from 1.0 to 5.0, [Fe/H] from ${-}1.0$ to ${+}0.5$, $V_t=1.0$ km s${}^{-1}$, and a relative rubidium abundance $\textrm{[Rb/Fe]}=0.0$. It is shown that disregarding the non-LTE effects can lead to significant errors in the abundance of this element. The non-LTE corrections for dwarf stars with effective temperatures below 4000 K depend critically on the inclusion of collisional interactions with hydrogen atoms. The differences in rubidium abundance when using quantum-mechanical calculations and Drawin's theoretical approximation to take into account the collision rates of atoms with hydrogen atoms can reach 0.17 dex. The rubidium abundance has been determined from its lines in the solar spectrum, $\textrm{(Rb/H)}=2.35± 0.05$, which virtually coincides with the rubidium abundance deduced from the analysis of meteorites, $\textrm{(Rb/H)}=2.36± 0.03$.