Optical observations of three Galactic halo stars: evidence for cloudlets in intermediate- and high-velocity interstellar clouds

Very high resolution (R ∼ 160000) absorption line measurements of the interstellar Ca ii K and Na i D lines and medium resolution (R ∼ 50000) measurements of the interstellar Ti ii (3384 Å) line towards three Galactic halo stars are presented. The data have signal-to-noise ratios of ∼ 90–240. The sightlines studied clearly show several intermediate-high velocity interstellar clouds with local standard of rest velocities in the range ∼ −40 to −110 km s−1. Two different methods have been used to analyze these data. Line profile fitting allows us to model multiple components for the different high-velocity clouds separated by only a few km s−1. The close proximity of the lines in velocity space indicates that the cloudlets are related. Conversion of the absorption profiles to apparent optical depth profiles also allows us to examine the column densities and their ratios as a function of velocity. Variations in N(Ca ii)/N(Na i), N(Ca ii)/N(Ti ii) and N(Ti ii)/N(Na i) with cloud velocity are discussed.