The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate-Velocity Arch (IV Arch) in the direction of the quasar PG 1259+593 (l = 120.°6, b = +58.°1). Absorption lines from C II, N I, N II, O I, Al II, Si II, P II, S II, Ar I, Fe II, and Fe III are used to study the atomic abundances in these two halo clouds at VLSR ~ -130 km s-1 (Complex C) and -55 km s-1 (IV Arch). The O I/H I ratio provides the best measure of the overall metallicity in the diffuse interstellar medium because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 times solar, consistent with the idea that Complex C represents the infall of low-metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 times solar, which indicates a Galactic origin. We report the detection of an intermediate-velocity absorption component at +60 km s-1 that is not seen in H I 21 cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate and high-velocity gas in the Milky Way halo.