A Survey ofFar Ultraviolet Spectroscopic ExplorerandHubble Space TelescopeSight Lines through High‐Velocity Cloud Complex C

Using archival Far Ultraviolet Spectroscopic Explorer ( FUSE ) and Hubble Space Telescope ( HST ) data, we have assembled a survey of eight sight lines through high-velocity cloud Complex C. Abundances of the observed ion species vary significantly for these sight lines, indicating that Complex C is not well characterized by a single metallicity. Reliable metallicities based on [O I/H I] range from 0.1 to 0.25 Z ☉ . Metallicities based on [S II/H I] range from 0.1 to 0.6 Z ☉ , but the trend of decreasing abundance with H I column density indicates that photoionization corrections may affect the conversion to [S/H]. We present models of the dependence of the ionization correction on H I column density; these ionization corrections are significant when converting ion abundances to elemental abundances for S, Si, and Fe. The measured abundances in this survey indicate that parts of the cloud have a higher metallicity than previously thought and that Complex C may represent a mixture of "Galactic fountain" gas with infalling low-metallicity gas. We find that [S/O] and [Si/O] have a solar ratio, suggesting little dust depletion. Further, the measured abundances suggest an overabundance of O, S, and Si relative to N and Fe. The enhancement of these α-elements suggests that the bulk of the metals in Complex C were produced by Type II supernovae and then removed from the star-forming region, possibly via supernova-driven winds or tidal stripping, before the ISM could be enriched by N and Fe.