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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 413; no. 3; pp. 2184 - 2198
• Main Author: Bergemann, Maria
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01-05-2011; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; Atmosphere; Earth, ocean, space; Exact sciences and technology; line: formation; line: profiles; radiative transfer; Star & galaxy formation; Stars & galaxies; stars: abundances; Sun; Sun: abundances; Titanium; line: formation; Sun: abundances; stars: abundances; line: profiles; radiative transfer; Late type stars; Line shape; Atmosphere model; Radiative transfer; Chemical evolution; Metal-poor stars; Galactic evolution; Population II; Line formation; Stellar abundance; Thermodynamic non equilibrium; Milky Way; Sun; Statistical equilibrium; Meteorites; Inelastic collision; Photoionization; Ionization; Metallicity; Surface gravity; LTE
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2011.18295.x

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.