Abstract

We present an analysis of interstellar Na I (λ air = 3302.37 and 3302.98 Å), Ti II (λ air = 3383.76 Å) and Can II K (λ air = 3933.66 Å) absorption features for 74 sightlines towards O- and B-type stars in the Galactic disc. The data were obtained from the Ultraviolet and Visual Echelle Spectrograph Paranal Observatory Project, at a spectral resolution of 3.75 km s -1 and with mean signal-to-noise ratios per pixel of 260, 300 and 430 for the Na I, Ti II and Ca II observations, respectively. Interstellar features were detected in all but one of the Ti II sightlines and all of the Ca II sightlines. The dependence of the column density of these three species with distance, height relative to the Galactic plane, H I column density, reddening and depletion relative to the solar abundance has been investigated. We also examine the accuracy of using the Na I column density as an indicator of that for H I. In general, we find similar strong correlations for both Ti and Ca, and weaker correlations for Na. Our results confirm the general belief that Ti and Ca occur in the same regions of the interstellar medium (ISM) and also that the Ti II/Ca II ratio is constant over all parameters. We hence conclude that the absorption properties of Ti and Ca are essentially constant under the general ISM conditions of the Galactic disc. © 2006 RAS.