Distances and Metallicities of HVCS

Abstract

We review the methods for determining the distances and metal-licities of high-velocity clouds. A short summary is given of several indi-rect 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 extragalac-tic 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 Mag-ellanic Stream shows near-Magellanic values, and some other HVCs (com-plexes M, WB, and HVC 100−7+110) have near-solar metallicity. These results clearly show that the HVC phenomenon is diverse.