Distances and Metallicities of High- and Intermediate-velocity Clouds

A table is presented that summarizes published absorption line measurements for the highand intermediate velocity clouds (HVCs and IVCs). New values are derived for N(H I) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the H I data are described in Paper II). Distances to stellar probes are revisited and calculated consistently, in order to derive distance brackets or limits for many of the clouds, taking care to properly interpret non-detections. The main conclusions are the following. 1) Absolute abundances have been measured using lines of S II, N I and O I, with the following resulting values: ∼0.1 solar for one HVC (complex C), ∼0.3 solar for the Magellanic Stream, ∼0.5 solar for a southern IVC, and ∼ solar for two northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the range 0.5−2 solar are found for three more IVCs. 2) Depletion patterns in IVCs are like those in warm disk or halo gas. 3) Most distance limits are based on strong UV lines of C II, Si II and Mg II, a few on Ca II. Distance limits for major HVCs are >5 kpc, while distance brackets for several IVCs are in the range 0.5−2kpc. 4) Mass limits for major IVCs are 0.5−8×10M⊙, but for major HVCs they are >10M⊙. 5) The Ca II/H I ratio varies by up to a factor 2−5 within a single cloud, somewhat more between clouds. 6) The NaI/H I ratio varies by a factor >10 within a cloud, and even more between clouds. Thus, Ca II can be useful for determining both lower and upper distance limits, but Na I only yields upper limits. Subject headings: ISM: clouds, ISM: structure, Galaxy: halo, radio lines: ISM, ultraviolet: ISM,