FAR-ULTRAVIOLET SPECTRA OF BROAD ABSORPTION-LINE QSOS AND CONSTRAINTSON MODELS FOR THE IONIZATION STRUCTURE AND METALLICITY OF THE BAL-REGION

Hubble Space Telescope Faint Object Spectrograph G270H spectra of four moderate- to high-redshift broad absorption line (BAL) QSOs are prese nted. In addition, evidence is discussed that indicates that the gas t hat gives rise to the BALs has nonuniform ionization and enhanced abun dances. In the context of a photoionization model, ionization paramete r fluctuations of at least similar to 32-16 are needed to explain the observed column densities of different ions of the same element. In su ch a model, the gas must have very enhanced metal-to-hydrogen abundanc es relative to solar composition. However, the actual metal-to-hydroge n abundance enhancements are difficult to constrain because they are s o model-dependent. For example, in a photoionization model the shape o f the photoionizing continuum has a significant influence on the deriv ed metal-to-hydrogen enhancement, but for normally adopted shapes the enhancement is very large, i.e., an enhancement of similar to 120-230 times solar for nitrogen. If collisional ionization is important, the need for metal-to-hydrogen abundance enhancements would be severely re duced. The derived relative metal abundances are somewhat more robust because the relevant metal-line transitions correspond to similar ioni zation potentials. In a photoionization model, nitrogen enhancements r elative to oxygen and carbon of similar to 4-3 and 9:-10: times solar values, respectively, are indicated. If normal stellar nucleosynthesis is important, the results may be indicative of star formation with a relatively flat initial mass function, with the nitrogen overabundance produced by secondary processing in massive stars. However, large enh ancements of elements like phosphorus in some objects suggest that the enrichment scenario may be more complex. In addition, the results dis cussed here place constraints on the details of a photoionization mode l for the BAL-region gas. These constraints are unphysical enough that they may indicate that central-source photoionization is not dominant .