Interstellar Ti ii in the Milky Way and Magellanic Clouds star

We discuss several sets of Ti II absorption-line data, which probe a variety of interstellar environments in our Galaxy and in the Magellanic Clouds. Comparisons of high-resolution [full width at half-maximum (FWHM) ∼ 1.3–1.5 km s−1] Ti II spectra of Galactic targets with corresponding high-resolution spectra of Na I, K I and Ca II reveal both similarities and differences in the detailed structure of the absorption-line profiles – reflecting component-to-component differences in the ionization and depletion behaviour of those species. Moderate-resolution (FWHM ∼ 3.4–4.5 km s−1) spectra of more heavily reddened Galactic stars provide more extensive information on the titanium depletion in colder, denser clouds – where more than 99.9 per cent of the Ti may be in the dust phase. Moderate-resolution (FWHM ∼ 4.5–8.7 km s−1) spectra of stars in the Magellanic Clouds suggest that the titanium depletion is generally much less severe in the Large and Small Magellanic Clouds than in our Galaxy [for a given N(Htot), E(B−V), or molecular fraction f(H2)]– providing additional evidence for differences in depletion patterns in those two lower-metallicity galaxies. We briefly discuss possible implications of these results for the interpretation of gas-phase abundances in quasi-stellar object absorption-line systems and of variations in the D/H ratio in the local Galactic interstellar medium.