HSTSTIS Observations of PG 0946+301: The Highest Quality UV Spectrum of a BALQSO

We describe deep (40 orbits) HST/STIS observations of the BALQSO PG 0946+301and make them available to the community. These observations are the major partof a multi-wavelength campaign on this object aimed at determining theionization equilibrium and abundances (IEA) in broad absorption line (BAL)QSOs. We present simple template fits to the entire data set, which yield firmidentifications for more than two dozen BALs from 18 ions and give lower limitsfor the ionic column densities. We find that the outflow's metalicity isconsistent with being solar, while the abundance ratio of phosphorus to othermetals is at least ten times solar. These findings are based on diagnosticsthat are not sensitive to saturation and partial covering effects in the BALs,which considerably weakened previous claims for enhanced metalicity. Ampleevidence for these effects is seen in the spectrum. We also discuss severaloptions for extracting tighter IEA constraints in future analyses, and presentthe significant temporal changes which are detected between these spectra andthose taken by the HST/FOS in 1992.Comment: 32 pages, 7 figures, to appear in ApJ. See also companion paper by Arav, Korista and De Koo