Distances and metallicities of high- and intermediate-velocity clouds

A table is presented that summarizes published absorption line measurements for the high- and 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 nondetections. 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 : similar to0.1 solar for one HVC (complex C), similar to0.3 solar for the Magellanic Stream, similar to0.5 solar for a southern IVC, and similar to 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 greater than 5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. (4) Mass limits for major IVCs are 0.5-8 x 10(5) M., but for major HVCs they are more than 10(6) M.. (5) The Ca II /H I ratio varies by up to a factor 2E5 within a single cloud, somewhat more between clouds. (6) The Na I /H I ratio varies by a factor of more than 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.