Ionization balance of Ti in the photospheres of the Sun and four late-type stars

In this paper we investigate statistical equilibrium of Ti in the atmospheres of late-type stars. The Ti i/Ti ii level populations are computed with available experimental atomic data, except for photoionization and collision-induced transition rates, for which we have to rely on theoretical approximations. For the Sun, the non-local thermodynamic equilibrium (NLTE) line formation with adjusted H i inelastic collision rates and mafags-os model atmosphere solve the long-standing discrepancy between Ti i and Ti ii lines. The NLTE abundances determined from both ionization stages agree within 0.01 dex with each other and with the Ti abundance in C i meteorites. The Ti NLTE model does not perform similarly well for the metal-poor stars, overestimating NLTE effects in the atmospheres of dwarfs, but underestimating overionization for giants. Investigating different sources of errors, we find that only [Ti/Fe] ratios based on Ti ii and Fe ii lines can be safely used in studies of Galactic chemical evolution. To avoid spurious abundance trends with metallicity and dwarf/giant discrepancies, it is strongly recommended to disregard Ti i lines in abundance analyses, as well as in determination of surface gravities.