HST STIS observations of PG 0946+301:: The highest quality UV spectrum of a BALQSO

We describe deep (40 orbits) Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph observations of the broad absorption line (BAL) quasi-stellar object (QSO) PG 0946+301 and make them available to the community. These observations are the major part of a multiwavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEAs) in BALQSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metallicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least 10 times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metallicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses and present the significant temporal changes that are detected between these spectra and those taken by the HST Faint Object Spectrograph in 1992.