THE HST QUASAR ABSORPTION-LINE KEY PROJECT .4. HST FAINT OBJECT SPECTROGRAPH AND GROUND-BASED OBSERVATIONS OF THE UNUSUAL LOW-REDSHIFT BROAD ABSORPTION-LINE QUASI-STELLAR OBJECT PG-0043+039

HST FOS observations have shown that the spectrum of the low-redshift (z(em) congruent-to 0.384) QSO PG 0043 + 039 exhibits weak broad absor ption lines (BALs). The BALs were discovered during the course of UV s pectrophotometry made for the HST Quasar Absorption Line Key Project. The HST data are analyzed along with ground-based optical and IUE spec trophotometry. The object is found to have a number of atypical proper ties relative to normal non-BAL QSOs. For example, the object is one o f the strongest Fe II emitters known, narrow-line [O III] and [O II] e mission is not detected, and the C IV broad emission line is exceeding ly weak. The observed continuum is atypical in the sense that it is mu ch weaker than that of a normal optically selected QSO at rest wavelen gths less than or similar to 2200 angstrom. Intrinsic reddening of E(B - V) congruent-to 0.11 mag by dust similar to that found in the SMC a t the redshift of PG 0043 + 039 conservatively accounts for the observ ed continuum shape moderately well. These observed characteristics are typical of low-ionization BAL QSOs, but convincing evidence for BALs due to low-ionization transitions of Mg II, Al III, Al II, or C II doe s not exist. Therefore, this object may be a misaligned BAL QSO having many of the characteristics of low-ionization BAL QSOs with the sight line passing through a putative dusty region, but evidently missing c louds of high enough column density to produce observable low-ionizati on BALs. If the intrinsic dust-extinction model is correct, the observ ations suggest that the dust is not confined to the presumably higher density, low-ionization BAL clouds, but that it has drifted to nearby high-ionization BAL regions. We also consider other possible mechanism s for producing the shape of the continuous energy distribution which cannot be ruled out. We compare the Fe II emission in PG 0043 + 039 wi th that in another Key Project QSO, NGC 2841-UB 3, which has optical F e II emission comparable in strength to that in PG 0043 + 039, but has anomalously weak UV Fe II emission. In addition, from an analysis of UV and optical spectrophotometric data at 5 epochs over approximately 11 yr, there is tentative evidence that PG 0043 + 039 has varied in br ightness by as much as 1.1 mag during this time interval. Two differen t interpretations of PG 0043 + 039 and the low-ionization BAL QSOs are considered: One is that the low-ionization BAL QSOs comprise a distin ct population of QSOs that have large BAL region covering factor, no a ppreciable narrow emission-line region, and other atypical properties. An alternative possibility is that PG 0043 + 039 has a small BAL regi on covering factor, which is consistent with previously proposed model s for most BAL QSOs, but that nonspherical geometries coupled with mec hanisms for anisotropic emission give rise to aspect-angle dependent Q SO spectral properties, such as weak narrow-line [O III] emission. Whi le the actual value of the BAL region covering factor in PG 0043 + 039 and low-ionization BAL QSOs is unclear, given the absence of a specif ic model which would give rise to anisotropic continuum or line emissi on of the type required, the evidence presented here would seem to ind icate that the BAL region covering factor in a subclass of QSOs is lar ger than resonance line scattering models generally indicate. Various model scenarios for explaining the weak narrow-line [O III] emission a re considered, but there is no definitive explanation.