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

TL;DR

This study investigates the ionization balance of titanium in the atmospheres of the Sun and late-type stars, demonstrating that NLTE modeling with adjusted collision rates resolves longstanding discrepancies in titanium abundance measurements.

Contribution

It provides a detailed NLTE model for Ti I and Ti II in stellar atmospheres, improving abundance determinations and clarifying the reliability of Ti II lines for chemical evolution studies.

Findings

NLTE effects are well modeled for the Sun, aligning Ti I and Ti II abundances.

Discrepancies in metal-poor stars highlight limitations of current NLTE models.

Ti II lines are reliable for chemical evolution studies, unlike Ti I lines.

Abstract

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 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.