Distances and Metallicities of HVCS

We review the methods for determining the distances and metallicities of high-velocity clouds. A short summary is given of several indirect methods, but we concentrate on the absorption-line method, which is based on the detection and non-detection of interstellar absorption lines in the spectra of background and foreground stars, as well as of extragalactic targets. The requirements and problems for determining both distances and metallicities are discussed, and summaries are given of current results. These include a distance bracket of 8 to 10 kpc for complex A, lower limits of ˜5 kpc for complexes C and H, and an upper limit of ˜4 kpc for part of complex M. We also find that several HVCs have low metallicity (˜0.1 to 0.2 solar for complexes A, C, WD, and cloud WW 84), while the Magellanic Stream shows near-Magellanic values, and some other HVCs (complexes M, WB, and HVC 100-7+110) have near-solar metallicity. These results clearly show that the HVC phenomenon is diverse