A Survey of FUSE and HST Sightlines through High-Velocity Cloud Complex C

Abstract

Using archival Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) data, we have assembled a survey of eight sightlines through high-velocity cloud Complex C. Abundances of the observed ion species vary significantly for these sightlines, indicating that Complex C is not well characterized by a single metallicity. Reliable metallicities based on [OI/HI] range from 0.1-0.25 solar. Metallicities based on [SII/HI] range from 0.1-0.6 solar, but the trend of decreasing abundance with HI column density indicates that photoionization corrections may affect the conversion to [S/H]. We present models of the dependence of the ionization correction on HI 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 over-abundance of O, S, and Si relative to N and Fe. The enhancement of these alpha-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 supernovae-driven winds or tidal stripping, before the ISM could be enriched by N and Fe.