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

We describe deep (40 orbits) Hubble Space Telescope (HST) Space Telescope I maging Spectrograph observations of the broad absorption line (BAL) quasi-s tellar object (QSO) PG 0946+301 and make them available to the community. T hese 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 g ive lower limits for the ionic column densities. We find that the outflow's metallicity is consistent with being solar, while the abundance ratio of p hosphorus to other metals is at least 10 times solar. These findings are ba sed on diagnostics that are not sensitive to saturation and partial coverin g effects in the BALs, which considerably weakened previous claims for enha nced 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 detec ted between these spectra and those taken by the HST Faint Object Spectrogr aph in 1992.