ИСТИНА

A comprehensive model atom of Ti I-II was constructed using predicted high-excitation levels and accurate photoionisation cross-sections for the first time. It was tested with well-studied B-K stars of different luminosity (log g) and metallicity (-4 < [Fe=H] < 0.5). Consistent NLTE abundances from lines of Ti I and Ti II were found for the main sequence B-F stars, while in LTE the abundance difference Ti I-Ti II = -0.2 dex. One of the main uncertainties in NLTE results for cool stars is the effectiveness of inelastic collisions with hydrogen atoms (SH). We show that the Ti I/Ti II ionisation equilibrium is achieved within 0.06 dex for each of 15 F-G dwarfs in the -2.7 < [Fe/H] < 0.3 metallicity range, when applying SH = 0.5 in the statistical equilibrium calculations. For seven turn-off (TO) and subgiant (SG) stars with [Fe/H] < -2, the deviations from LTE are significant for both Ti I and Ti II lines and NLTE leads to discrepancy in abundances between Ti I and Ti II lines. This can be due to inaccurate treatment of collisions with hydrogen atoms in very metal-poor atmospheres. Accurate collisional data for Ti I and Ti II would be extremely helpful. We calculated the NLTE corrections for lines of Ti I and Ti II in the grid of model atmospheres with 4000 K < Teff < 5000 K, 0.5 < log g < 2.5, -4 < [Fe/H] < 0. They were applied to check atmospheric parameters and determine Ti abundances of stars in Sculptor and Bootes dwarf galaxies.