4.1. Inter-combination Lines

Shields (1976, also Baldwin & Netzer 1978, Davidson & Netzer 1979, Osmer 1980, Uomoto 1984) used several ratios involving UV inter-combination (semi-forbidden) lines, such as NIII] 1750 / CIII] 1909, NIII]/OIII] 1664 and NIV] 1486 / CIV, to study the relative nitrogen abundances. The lines are often weak and difficult to measure, but these studies generally concluded that N/O and N/C are roughly solar or higher - consistent with solar or higher overall metallicities. That work fell out of favor by the mid-1980s as it became clear that BELR densities probably exceed the critical densities of the inter-combination lines (~ 3 x 10⁹ to ~ 10¹¹ cm^-3). Thus collisional deexcitation, which was not previously accounted for, can be important. K.T Korista, G.J. Ferland and I have just begun a theoretical study to reexamine the usefulness of these ratios for abundance work. We suspect that at least some of the ratios (e.g. NIII] / OIII]) will prove to be useful abundance measures because the lines have similar critical densities and the collisional effects should roughly cancel out.