Non-LTE line formation for heavy elements in four very metal-poor stars

Aims.Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor (VMP) stars ($-2.15 \ge [$Fe/H$] \ge -2.66$). For two of them, HD 84937 and HD 122563, the fraction of the odd isotopes of Ba derived for the first time. Methods.Determination of an effective temperature, surface gravity, and element abundances was based on non-local thermodynamic equilibrium (non-LTE) line formation and analysis of high-resolution ($R \sim60\,000$ and 90 000) high signal-to-noise (${\it S/N} \ge 200$) observed spectra. A model atom for $\ion{H}{i}$ is presented. An effective temperature was obtained from the Balmer Hα and Hβ line wing fits. The surface gravity was calculated from the Hipparco