2016MNRAS.461.1000S

We construct a model atom for Ti i-ii using more than 3600 measured and predicted energy levels of Ti i and 1800 energy levels of Ti ii, and quantum mechanical photoionization cross-sections. Non-local thermodynamical equilibrium (NLTE) line formation for Ti i and Ti ii is treated through a wide range of spectral types from A to K, including metal-poor stars with [Fe/H] down to -2.6 dex. NLTE leads to weakened Ti i lines and positive abundance corrections. The magnitude of NLTE corrections is smaller compared to the literature data for FGK atmospheres. NLTE leads to strengthened Ti ii lines and negative NLTE abundance corrections. For the first time, we have performed NLTE calculations for Ti i-ii in the 6500 <= T_eff <= 13 000 K range. For four A-type stars, we derived in LTE an abundance discrepancy of up to 0.22 dex between Ti i and Ti ii, which vanishes in NLTE. For four other A-B stars, with only Ti ii lines observed, NLTE leads to a decrease of line-to-line scatter. An efficiency of inelastic Ti i + H i collisions was estimated from an analysis of Ti i and Ti ii lines in 17 cool stars with -2.6 <= [Fe/H] <= 0.0. Consistent NLTE abundances from Ti i and Ti ii were obtained by applying classical Drawinian rates for the stars with log g >= 4.1, and neglecting inelastic collisions with H i for the very metal-poor (VMP) giant HD 122563. For the VMP turn-off stars ([Fe/H] <= -2 and log g <= 4.1), we obtained the positive abundance difference Ti i-ii already in LTE, which increases in NLTE. Accurate collisional data for Ti i and Ti ii are necessary to help solve this problem.