Interstellar TiII in the Milky Way and Magellanic Clouds

Abstract

We discuss several sets of TiII 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) ~ 1.3-1.5kms^-1] TiII spectra of Galactic targets with corresponding high-resolution spectra of NaI, KI and CaII 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 ~ 3.4-4.5kms^-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 ~ 4.5-8.7kms^-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(H_tot), 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.

This paper is based in part on observations collected at the European Southern Observatory, Chile, under programmes 65.I-0526, 67.C-0281, 67.D-0238, 70.D-0164, 72.C-0682 and 74.D-0109. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555.

Visiting observer, European Southern Observatory and Kitt Peak National Observatory. ‡

E-mail: dwelty@astro.illinois.edu (DEW); Paul.crowther@sheffield.ac.uk (PAC)