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

Abstract

In this paper we investigate statistical equilibrium of Ti in the atmospheres of late-type stars. The Tii/Tiii 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 Hi inelastic collision rates and mafags-os model atmosphere solve the long-standing discrepancy between Tii and Tiii lines. The NLTE abundances determined from both ionization stages agree within 0.01dex with each other and with the Ti abundance in Ci 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 Tiii and Feii 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 Tii lines in abundance analyses, as well as in determination of surface gravities. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.